US20050094153A1 - Metrology system using optical phase - Google Patents
Metrology system using optical phase Download PDFInfo
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- US20050094153A1 US20050094153A1 US11/016,025 US1602504A US2005094153A1 US 20050094153 A1 US20050094153 A1 US 20050094153A1 US 1602504 A US1602504 A US 1602504A US 2005094153 A1 US2005094153 A1 US 2005094153A1
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70483—Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
- G03F7/70605—Workpiece metrology
- G03F7/70616—Monitoring the printed patterns
- G03F7/70633—Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching
Definitions
- This invention relates in general to metrology systems for measuring periodic structures such as overlay targets employed in photolithography in a research or production environment, and, in particular, to a metrology system employing optical phase for detecting misalignment of such structures.
- Overlay error measurement requires specially designed marks to be strategically placed at various locations, normally in the street area between dies, on the wafers for each process.
- the alignment of the two overlay targets from two consecutive processes is measured for a number of locations on the wafer and the overlay error map across the wafer is analyzed to provide feedback for the alignment control of lithography steppers.
- a key process control parameter in the manufacturing of integrated circuits is the measurement of overlay target alignment between successive layers on a semiconductor wafer. If the two overlay targets are misaligned relative to each other, the electronic devices fabricated will malfunction and the semiconductor wafer will need to be reworked or discarded.
- conventional overlay targets are box-in-box targets and bar-in-bar targets.
- the box-in-box target typically has a 10 ⁇ m inner box and a 20 ⁇ m outer box.
- the outer box is printed on the substrate (or previous process layer) and the inner box is resist printed on the current layer. Overlay error is reported as the mis-position of the inner mark with respect to the outer mark.
- a bar-in-bar target also has a 10 ⁇ m inner target on the current layers and a 20 ⁇ m outer target on the previous layers. However, the box edge is replaced with a narrow bar 2 ⁇ m wide.
- the box-in-box targets are more compact; however, the bar-in-bar targets provide better measurement performance.
- Overlay targets may comprise grating structures on top of the wafer or etched into the surface of the wafer.
- one overlay target may be formed by etching into the wafer while another adjacent overlay target may be a photoresist layer at a higher elevation over the wafer.
- This invention is based on the observation that by utilizing optical phase detection, high sensitivity for detecting misalignment of periodic structures can be achieved.
- two periodic structures such as overlay targets are placed side-by-side so that they are periodic substantially along the same direction, where portions of both structures are illuminated by coherent radiation.
- the size(s) of the beam(s) illuminating portions of the structures are large enough to generate diffraction signals by the structures. These diffraction signals are caused to interfere leading to the detection of optical phase which is a measurement of the misalignment between the, structures.
- the misalignment may then be used to control lithographic instruments such as a lithographic stepper or to determine whether or not the patterns of the structures are correctly placed and will yield functional devices.
- the phase sensitive detection is less sensitive to environmental factors such as vibration and thermal drifts. Since the system employs larger spot illumination, the optics of the system are less sensitive to focus accuracy.
- the system is compact and readily integratable with process equipment. Due to the enhanced sensitivity compared to conventional systems, the system is able to detect misalignment of periodic structures that are low contrast.
- FIG. 1 is a top view of two overlay targets placed next to each other that are illuminated by two corresponding radiation beams to illustrate the invention.
- FIG. 2 is a perspective view of two overlay targets placed side-by-side and a Wollaston prism useful for illustrating the invention.
- FIGS. 3A and 3B are side views from different angles of an overlay target metrology system illustrating an embodiment of the invention.
- FIG. 3C is a perspective view of the system of FIGS. 3A and 3B .
- FIG. 4 is a flow diagram illustrating cross-sectional light distributions at various positions in the paths of radiation beams in the system of FIGS. 3A and 3B .
- FIG. 5 is a schematic view of the Wollaston prism of FIGS. 3A and 3B illustrating its function in dividing a beam of radiation into two beams.
- FIG. 6 is a graphical plot of output signals from the detectors of FIGS. 3A and 3B to illustrate a method for obtaining offset or misalignment information between the overlay targets from the detector outputs.
- FIG. 7A is a schematic view of a conventional Wollaston prism dividing an input polarized beam into two polarized beams useful for illustrating the invention.
- FIG. 7B is a schematic view of a modified Wollaston prism dividing an input polarized beam into two, useful for illustrating the invention.
- FIG. 8A is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate a preferred embodiment of the invention.
- FIG. 8B is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate an embodiment of the invention similar to that of FIG. 8A in many respects.
- FIG. 9A is a cross-sectional view of a metrology system for measuring misalignment of overlay targets to illustrate another embodiment of the invention.
- FIG. 9B is a graphical plot of the output signals from the metrology system of FIG. 9A .
- FIG. 9C is a perspective view of the system of FIGS. 9A and 91 .
- FIG. 10 is a top view of two overlay targets placed adjacent to each other illuminated by a single beam of radiation to illustrate yet another embodiment of the invention.
- FIGS. 11A and 11B are two side views from different angles of a metrology system where a single beam of radiation is employed to illuminate two overlay targets for detecting misalignment illustrating yet another embodiment of the invention.
- FIG. 12 is a flow diagram illustrating the cross-sectional light distributions at various positions of radiation beams in the system of FIGS. 11A and 11B .
- FIG. 13 is a flow diagram illustrating the cross-sectional light distributions at various positions of radiation beams in the system of FIGS. 3A and 3B where the two overlay targets are periodic in two orthogonal directions.
- FIG. 14 is a block diagram illustrating a lithographical instrument in combination with a metrology system for measuring misalignment of overlay targets to illustrate still another embodiment of the invention.
- FIG. 1 is a top view of two periodic structures 22 , 24 such as overlay targets, placed side-by-side, where both structures are illuminated by a beam of radiation having a round cross-section as shown in FIG. 1 .
- both structures 22 , 24 have the same period A, and are preferably aligned so that they are periodic substantially along the same line of direction X in the XY coordinate system.
- the two illuminated spots 26 , 28 are preferably aligned with the centers of the spots substantially aligned along the Y axis. This can be achieved, for example, by aligning the two spots with two markers on one of the overlay target structures to correctly position the two spots.
- spot 26 along the X axis is such that its dimension along the X axis is at least equal to or greater than the period ⁇ (more typically equal to or greater than several periods ⁇ 's) in order for a diffracted signal to be generated by illuminating structure 22 at spot 26 .
- spot 26 would span a number of grating lines 22 a of structure 22 .
- spot 28 can be said of spot 28 with respect to the period A and other features of structure 24 .
- FIG. 2 A metrology system 30 for generating the two illuminated spots 26 , 28 on the two corresponding structures is illustrated in FIG. 2 .
- a beam of radiation (not shown) may be split by means of a Wollaston prism 32 to illuminate the two structures 22 and 24 .
- the Wollaston prism is located above the two structures at a predetermined z height above the structures.
- FIGS. 3A and 3B are side views of the metrology system 30 of FIG. 2 in directions along the arrows 3 A, 3 B in FIG. 2 , respectively.
- FIG. 3C is a perspective view of system 30 .
- a beam of radiation 34 is split by prism 32 into two beams 34 a , 34 b in the Y direction. These two beams are collected by lens 36 towards two overlay target structures 22 , 24 which are placed side-by-side above a substrate 38 such as a silicon wafer.
- Beam 34 a illuminates spot 26 on structure 22 and beam 34 b illuminates spot 28 of structure 24 .
- Beam 34 a is diffracted by structure 22 .
- the zeroth-order diffraction retraces the original path of beam 34 a and through prism 32 .
- the first order diffraction 40 a -and 40 a +from structure 22 of beam 34 a are collected by lens 36 towards prism 32 and reflected by mirrors 44 and 46 towards two detectors 52 , 54 as shown in FIG. 3B .
- the zeroth-order diffracted signal from structure 24 of beam 34 b retraces the original path of beam 34 b .
- the first order diffracted signals 42 b + and 42 b ⁇ are collected by lens 36 through prism 32 to the two detectors 52 , 54 .
- the relative positions of the various beams of radiation in FIGS. 3A and 3B are shown more clearly in the flow diagram of FIG. 4 .
- the outputs of detectors 52 , 54 are S ⁇ 1 and S +1 , respectively, as shown in FIG. 3B .
- the input beam 34 is applied to Wollaston prism 32 which splits or divides the beam into two beams 34 a , 34 b when it reaches the objective 36 .
- the two beams 34 a , 34 b illuminate, respectively, spots 26 , 28 of structures 22 , 24 .
- the zeroth-order diffracted signal from the two structures have been omitted from FIG. 4 .
- the positive and negative diffracted signals 40 a + and 40 a ⁇ are collected by lens 36 .
- the positive and negative first diffraction signal 42 b + and 42 b ⁇ are collected by lens 36 towards prism 32 .
- Prism 32 combines beams 40 a + and 42 b + from the two spots 26 , 28 into one beam towards detector 54 .
- the input beam 34 is coherent so that beams 34 a , 34 b are coherent after passing prism 32 . Therefore, the positive first order diffraction 40 a + and 42 b + are also coherent. They would, therefore, interfere when combined by prism 32 and at detector 54 .
- the negative first order diffracted signals 40 a ⁇ and 42 b ⁇ are combined by prism 32 and interfere at the prism and at the detector 52 .
- phase difference between the outputs of the two detectors 52 , 54 is determined. This phase difference indicates the phase difference between beams 40 a +, 42 b + and that between the pair of beams 40 a ⁇ and 42 b ⁇ , and provides information concerning misalignment between the two structures 22 , 24 .
- Equations 1 and 2 The two output signals S +1 and S ⁇ 1 are shown in Equations 1 and 2 below: S +1 ⁇ +1 a + ⁇ +1 b +2 ⁇ square root ⁇ square root over ( ⁇ +1 a ⁇ +1 b ) ⁇ cos( ⁇ +1 + ⁇ w + ⁇ z + ⁇ x ) ⁇ (1) S ⁇ 1 ⁇ ⁇ 1 a + ⁇ ⁇ 1 b +2 ⁇ square root ⁇ square root over ( ⁇ ⁇ 1 a ⁇ ⁇ 1 b ) ⁇ cos( ⁇ ⁇ 1 + ⁇ w + ⁇ z ⁇ x ) ⁇ (2)
- ⁇ +1 a is the diffraction efficiency of the diffracted signal 40 a + from interaction between structure 22 and beam 34 a at spot 26
- ⁇ +1 b the diffraction efficiency of the +1 diffracted signal 42 b + from interaction between structure 24 and beam 34 b at spot 28
- ⁇ ⁇ 1 a the diffraction efficiency of the diffracted signal 40
- the two phase differences ⁇ +1 and ⁇ ⁇ 1 are identical for +1 and ⁇ 1 orders if the grating profile is symmetric.
- ⁇ x is the phase difference caused by any misalignment ⁇ x between the two periodic structures 22 , 24 as set forth in equation (3) below.
- the amount of this phase shift is readily determined from the shifting theorem of Fourier transform.
- the grating with period ⁇ in frequency domain has a spatial frequency at 1/ ⁇ .
- a phase shift ⁇ x in space translates into 2 ⁇ (1/ ⁇ ) ⁇ phase shift in frequency domain, direcily predicted by shifting theorem.
- ⁇ x is the phase difference between the two detector outputs caused by the height difference ⁇ z between the two structures 22 , 24 as shown in equation (4) below.
- ⁇ w is the phase difference between the two detector outputs caused by phase shift induced by the Wollaston prism 32 as shown in equation (5) below.
- ⁇ x 2 ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ ⁇ x ( 3 )
- ⁇ z 2 ⁇ ⁇ ⁇ ⁇ ⁇ n ⁇ ⁇ ⁇ ⁇ ⁇ z ( 4 )
- ⁇ w ⁇ 2 ⁇ ⁇ ⁇ ⁇ ⁇ ( n o - n e ) ⁇ ⁇ ⁇ h ( 5 )
- ⁇ is the grating period of structures 22 , 24 ; n the average index of refraction of material 39 ; n o , n e the indices of refraction for the ordinary and extraordinary rays of the prism 32 ; ⁇ the wavelength of beam 34 , and ⁇ h is defined below.
- FIG. 5 is a schematic view of the Wollaston prism 32 of FIGS. 3A, 3B illustrating its function in dividing a beam of radiation into two beams and the phase shift introduced by the prism.
- beam 34 is split by prism 32 into two beams 34 a , 34 b with an angle ⁇ in between them, where ⁇ is given by equation (6) below.
- the phase difference caused by prism 32 between the two detector outputs S +1 and S ⁇ 1 is given by equation (7) below, where n o and n e are the refractive indices of prism 32 in the ordinary and in the extraordinary directions of the prism.
- ⁇ h in equation (5) above is the optical path length difference between h1 (the optical path length of beam 34 in prism 32 before reaching the optical interface 32 a ) and 112 (the optical path length of beams 34 a and 34 b through the prism after the interface 32 a ).
- h1 the optical path length of beam 34 in prism 32 before reaching the optical interface 32 a
- 112 the optical path length of beams 34 a and 34 b through the prism after the interface 32 a
- ⁇ 2 ⁇ ( n o ⁇ n e ) (6)
- ⁇ w 2 ⁇ ⁇ ⁇ ⁇ ⁇ 2 ⁇ ⁇ ⁇ ⁇ ( n o - n e )
- the phase term ⁇ w changes as a function of y, which, in turn, causes the two detector outputs S +1 and S ⁇ 1 to also change as a function of the displacement y of prism 32 along the y axis, as illustrated in FIG. 6 .
- the phase term ⁇ z is the same for both detector outputs. Therefore, the phase difference between the two detector outputs S +1 and S ⁇ 1 in FIG. 6 is caused only by the overlay target error in equation (3).
- Wollaston prisms are designed such that the optical axis 32 b (shown in a YZ plane as shown in FIG. 7A ) is parallel to the length L in one half and the optical axis 32 c (shown in an XZ plane as shown in FIG. 7A ) is perpendicular to the length in the other half of the prism, where the two halves are separated by optical interface 32 a , so that the two beams 34 a , 34 b will be either parallel polarized or orthogonally polarized with respect to the grating fingers of structures 22 , 24 .
- the two different polarizations of the two beams have different diffraction efficiencies.
- Another solution is to use a modified Wollaston prism 32 ′, in which the optical axis 32 b ′ (shown in a YZ plane as shown in FIG. 7B ) in the first half of the prism is at +45° with respect to the length L of the prism 32 ′ and the optic axis 32 c ′ (also shown in a YZ plane as shown in FIG. 7B ) is at ⁇ 45° to the length in the other half of prism 32 ′ on the other side of interface 32 a ′ as shown in FIG. 7B . If the incoming beam 34 is vertically polarized (as indicated by line 34 c ′ in FIG.
- FIG. 8A an alternative differential heterodyne system 100 requiring no moving parts is possible; this is illustrated in FIG. 8A as another embodiment.
- a coherent beam of radiation 102 is split into two beams 102 a and 102 b by polarizing beamsplitter 104 .
- Half wave plate 101 is employed to control the intensity ratio between E 1 and E 2 to compensate for the diffraction efficiency differences between the two overlay targets 22 , 24 .
- Beam 102 a is frequency shifted upwards by modulator 106 by frequency ⁇ 1 to become beam E 1
- beam 102 b is frequency shifted upwards by a modulator 108 by frequency ⁇ 2 to become beam E 2 .
- the two modulated beams are reflected by mirrors 110 and passed through respective wave plates 112 a , 112 b so that E 1 and E 2 can pass through and be reflected respectively by the polarizing beam splitter 114 .
- the two beams pass through beamsplitter 116 to a birefringent element such as a Wollaston prism 32 which separates the two beams E 1 and E 2 on account of their different polarizations.
- Beam E 1 is collected by lens 36 towards structure 24 and beam E is collected by the lens towards structure 22 .
- the +1 diffraction order E 1,+1 of beam E 1 from structure 24 is focused by lens 36 towards prism 32 .
- the +1 diffraction order E 2,+1 of beam E 2 from structure 22 is also focused by lens 36 and combined with the +1 diffraction order E 1,+1 by prism 32 into a single beam which is reflected by beamsplitter 116 .
- the polarizations of the fields E 1,+1 , E 2,+1 are orthogonal to each other.
- Analyzer 118 is placed at substantially 45° to the polarization directions of E 1,+1 , E 2,+1 so that the components passed by the analyzer have the same polarization and will interfere at detector 54 , after reflection by mirror 134 .
- the ⁇ 1 diffraction order signals E 1, ⁇ 1 , E 2, ⁇ 1 of the respective beams E 1 , E 2 , by structures 24 , 22 , respectively, are focused by lens 36 towards the same spot in prism 32 which combines the two beams into a single beam E 1, ⁇ 1 +E 2, ⁇ 1 .
- the components of E 1, ⁇ 1 , E 2, ⁇ 1 passed by the analyzer 118 have the same polarization and will interfere at detector 52 , after reflection by mirror 132 .
- the outputs of the detectors 52 , 54 are then provided to a phase detector 140 which detects a beat frequency ⁇ b which is equal to the difference between ⁇ 1 and ⁇ 2 in order to determine the misalignment or overlay target error ⁇ x.
- ⁇ +1 , ⁇ ⁇ 1 , ⁇ x , ⁇ z , and ⁇ w have the same meanings as those described above in the previous embodiment where the Wollaston prism is moved, and are set forth in equations (3)-(5) above.
- E 1
- E 2
- E 1,+1
- E 1, ⁇ 1
- E 2,+1
- E 2, ⁇ 1
- phase difference between S +1 and S ⁇ 1 is 2 ⁇ x .
- the overlay target error ⁇ x between the two structures 22 , 24 may be obtained from equations (3) and (9) above.
- the detector 140 may be set to detect at the beat or difference frequency ⁇ 1 ⁇ 2 to improve the signal-to-noise ratio of ⁇ x .
- FIG. 8B is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate another embodiment of the invention analogous to that in FIG. 8A .
- metrology system 100 ′ is similar to system 100 on FIG. 8A , except that anon-polarizing beamsplitter 114 ′ is used instead of the polarizing beamsplitter 114 of FIG. 8A .
- Waveplate 101 is again used for controlling the intensity in ratio between the two beams E, and E.
- the typical heterodyne phase different ⁇ x detected is of the order of 10 ⁇ 4 radians where the spacing ⁇ of structures 22 , 24 is of the order of 2 microns in equation (3) above. Therefore, with such or similar grating period of structures 22 , 24 , it is possible to detect misalignment Ax which can be as small as 30 picometers.
- FIGS. 9A and 9C illustrate another embodiment of the invention without any moving parts.
- FIG. 9B is a graphical plot of the output signals from the metrology system of FIGS. 9A, 9C .
- a portion of an input coherent beam of radiation 102 at frequency ⁇ 0 is reflected by mirror 202 and passes through an acoustooptic deflector (AOD) 204 .
- a portion 212 of the input beam at frequency ⁇ 0 passes through the AOD without being deflected or changed in frequency and is collected by lens 36 towards structure 24 .
- Another portion 214 of the input beam 102 is upshifted by frequency ⁇ and is deflected by the AOD 204 to provide an upshifted and deflected beam at frequency to ⁇ 0 + ⁇ which is also collected by lens 36 towards structure 22 .
- Beam 214 is diffracted by structure 22 and the +1 and ⁇ 1 orders of the diffraction are beams 214 + and 214 ⁇ , respectively.
- the two diffracted beams 214 + and 214 ⁇ are both at frequency ⁇ 0 + ⁇ .
- the two diffracted beams 214 + and 214 ⁇ are collected by lens 36 towards AOD 204 which passes without deflection or change in frequency a portion thereof towards mirror 208 .
- the undeflected beam 212 at frequency ⁇ 0 is diffracted by structure 24 into two diffracted beams 212 + and 212 ⁇ , both at frequency to ⁇ 0 .
- AOD 204 deflects and downshifts in frequency by ⁇ a portion of the two beams towards mirror 208 .
- the AOD 204 combines the deflected portion of beam 212 + and the undeflected portion of beam 214 + to form the +1 order mixed output signal S +1 so that the two portions would interfere and yield an output at beat frequency equal to 2 ⁇ .
- AOD 204 would combine the deflected portion of beam 212 ⁇ and the undeflected portion of beam 214 ⁇ as the ⁇ 1 order mixed signal output S ⁇ 1 at beat frequency 2 ⁇ .
- equations 16-18 may then be used to derive the overlay target area ⁇ x , where the beat frequency ⁇ b in equations 16 and 17 is equal to 2 ⁇ .
- FIG. 10 is a schematic view of the two structures 22 , 24 illuminated by a single beam to illustrate another embodiment of the invention.
- a single beam may be used instead of splitting single beam into two beams, for illuminating the two respective structures.
- FIGS. 11A, 11B are the front and side views of a system 300 employing a single beam for illuminating the two structures to illustrate yet another embodiment of the invention.
- this embodiment is analogous to that shown in FIGS. 2, 3A and 3 B, except that a single beam 302 is employed that illuminates both structures 22 , 24 .
- FIGS. 10 is a schematic view of the two structures 22 , 24 illuminated by a single beam to illustrate another embodiment of the invention.
- FIGS. 11A, 11B are the front and side views of a system 300 employing a single beam for illuminating the two structures to illustrate yet another embodiment of the invention.
- FIGS. 10, 11A , 11 B this embodiment is analogous to that shown in FIGS. 2, 3A and 3 B
- the single beam 302 illuminates a portion of structure 22 and a portion of structure 24 at their edges next to each other.
- the +1 and ⁇ 1 diffraction signals from the two structures are collected by lens 36 and combined and detected by two detectors (not shown) to provide the two outputs S +1 and S ⁇ 1 as in the embodiment of FIGS. 2, 3A and 3 B.
- a flow diagram illustrating the relative positions of the beams are shown in FIG. 12 .
- the +1 diffraction beams from structures 22 , 24 are combined into a single beam 340 and the ⁇ 1 diffraction beams from the two structures are combined into a single beam 342 .
- the two beams are detected by respective detectors 52 , 54 to provide the output S +1 and S ⁇ 1 as before.
- the alignment of the illuminating beam 302 is preferably controlled relative to the two structures to ensure that the relative intensity of the diffraction beams from both structures are held stable and substantially equal. This can be accomplished by precision beam positioning relative to the patterning on the wafer which can be augmented using an imaging system such as an optical microscope to visualize the beam location and make fine adjustments to the location of the beam 302 . Other methods include measuring the relative intensity of the diffraction components from the two structures in a feedback system to move the beam 302 to even and consistent alignment between the two structures. For such alignment, the wafer may be moved by means of cross-roller translation stages driven by micrometers. Alternatively, beam 302 may be moved by means of adjustable beam steering mirrors or galvanometer mirrors and the like. All such different alignment mechanisms are believed to be known to those skilled in the art.
- misalignment is detected along one direction, namely, the x direction.
- a second optical detection system is employed.
- Yet another embodiment envisions a two-dimensional alignment system as illustrated in FIG. 13 .
- a pair of two-dimensional gratings C and D is created and positioned on their respective processing layers in the same manner as the previously described structures 22 , 24 .
- the two illumination beam on the two gratings would each give rise to four principal diffraction components, two in each axis of the gratings C and D.
- the apparatus for the embodiment of FIG. 13 is the same as the previous embodiment illustrated in reference to FIGS. 2, 3A , 3 B and 4 .
- the two quartets of diffraction orders from gratings C and D are brought into alignment by the Wollaston prism 32 and four separate photodetectors 352 , 354 , 356 and 358 are used to separately measure the two sets of interference signals corresponding to the positive and negative diffraction orders in each of the two axes.
- the detector used in this configuration may be a quadrant photodiode.
- phase information proceeds as before and alignment information in two axes may be simultaneously derived.
- the misalignment along the y direction is determined by calibration based on two targets on the same layer to offset the DC phase shift induced by the Wollaston prism for the diffraction in the y direction.
- FIG. 14 is a block diagram illustrating a lithographic instrument 400 in combination with a metrology system 100 for measuring misalignment of overlay targets to illustrate still another embodiment of the invention.
- the misalignment between two overlay targets measured by the metrology system 100 of FIG. 8A may be supplied to lithographic instrument 400 for controlling a lithographic process, such as one performed by a lithographic stepper.
- the metrology system 100 may be integrated with instrument 400 to become an integrated system for the convenient operation of misalignment detection and lithographic process control.
- misalignment information may be used to determine probable device yield, and to make decision about future processing or rework of the devices.
Abstract
Misalignment error between two periodic structures such as two overlay targets placed side-by-side is measured. The two structures are illuminated by coherent radiation and the positive and negative diffraction beams of the input beam by the two structures are detected to discover the optical phase difference between the positive and negative diffraction beams. The misalignment between the two structures may then be ascertained from the phase difference.
Description
- This invention relates in general to metrology systems for measuring periodic structures such as overlay targets employed in photolithography in a research or production environment, and, in particular, to a metrology system employing optical phase for detecting misalignment of such structures.
- Overlay error measurement requires specially designed marks to be strategically placed at various locations, normally in the street area between dies, on the wafers for each process. The alignment of the two overlay targets from two consecutive processes is measured for a number of locations on the wafer and the overlay error map across the wafer is analyzed to provide feedback for the alignment control of lithography steppers.
- A key process control parameter in the manufacturing of integrated circuits is the measurement of overlay target alignment between successive layers on a semiconductor wafer. If the two overlay targets are misaligned relative to each other, the electronic devices fabricated will malfunction and the semiconductor wafer will need to be reworked or discarded.
- Typically, conventional overlay targets are box-in-box targets and bar-in-bar targets. The box-in-box target typically has a 10 μm inner box and a 20 μm outer box. The outer box is printed on the substrate (or previous process layer) and the inner box is resist printed on the current layer. Overlay error is reported as the mis-position of the inner mark with respect to the outer mark. A bar-in-bar target also has a 10 μm inner target on the current layers and a 20 μm outer target on the previous layers. However, the box edge is replaced with a
narrow bar 2 μm wide. The box-in-box targets are more compact; however, the bar-in-bar targets provide better measurement performance. Overlay targets may comprise grating structures on top of the wafer or etched into the surface of the wafer. For example, one overlay target may be formed by etching into the wafer while another adjacent overlay target may be a photoresist layer at a higher elevation over the wafer. - Conventional systems for detecting overlay target misalignment typically employs an electronic camera that images the “box-in-box target.” The accuracy of the conventional system is limited by the accuracy of the line profiles in the target, by aberrations in the illumination and imaging optics and by the image sampling in the camera. Such methods are complex and they require full imaging optics. Vibration isolation is also required and it may be difficult to integrate such systems into process equipment.
- An improvement to the conventional method is described in U.S. Pat. No. 6,023,338. This patent discloses a method where two overlay target structures are placed next to each other and two radiation beams are scanned in two separate paths across portions of both structures. The intensity of the radiation reflected along both paths are detected and processed to calculate any offset between the two structures.
- While the above-described improved method may be useful for some applications, it requires beams to be scanned across periodic structures such as overlay targets. It is desirable to develop an improved system with better performance and simplified scanning characteristics.
- This invention is based on the observation that by utilizing optical phase detection, high sensitivity for detecting misalignment of periodic structures can be achieved. Thus, two periodic structures such as overlay targets are placed side-by-side so that they are periodic substantially along the same direction, where portions of both structures are illuminated by coherent radiation. The size(s) of the beam(s) illuminating portions of the structures are large enough to generate diffraction signals by the structures. These diffraction signals are caused to interfere leading to the detection of optical phase which is a measurement of the misalignment between the, structures. The misalignment may then be used to control lithographic instruments such as a lithographic stepper or to determine whether or not the patterns of the structures are correctly placed and will yield functional devices.
- When the paths of radiation traveling between the radiation source, the structures and detectors are close together, the phase sensitive detection is less sensitive to environmental factors such as vibration and thermal drifts. Since the system employs larger spot illumination, the optics of the system are less sensitive to focus accuracy. The system is compact and readily integratable with process equipment. Due to the enhanced sensitivity compared to conventional systems, the system is able to detect misalignment of periodic structures that are low contrast.
-
FIG. 1 is a top view of two overlay targets placed next to each other that are illuminated by two corresponding radiation beams to illustrate the invention. -
FIG. 2 is a perspective view of two overlay targets placed side-by-side and a Wollaston prism useful for illustrating the invention. -
FIGS. 3A and 3B are side views from different angles of an overlay target metrology system illustrating an embodiment of the invention.FIG. 3C is a perspective view of the system ofFIGS. 3A and 3B . -
FIG. 4 is a flow diagram illustrating cross-sectional light distributions at various positions in the paths of radiation beams in the system ofFIGS. 3A and 3B . -
FIG. 5 is a schematic view of the Wollaston prism ofFIGS. 3A and 3B illustrating its function in dividing a beam of radiation into two beams. -
FIG. 6 is a graphical plot of output signals from the detectors ofFIGS. 3A and 3B to illustrate a method for obtaining offset or misalignment information between the overlay targets from the detector outputs. -
FIG. 7A is a schematic view of a conventional Wollaston prism dividing an input polarized beam into two polarized beams useful for illustrating the invention. -
FIG. 7B is a schematic view of a modified Wollaston prism dividing an input polarized beam into two, useful for illustrating the invention. -
FIG. 8A is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate a preferred embodiment of the invention. -
FIG. 8B is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate an embodiment of the invention similar to that ofFIG. 8A in many respects. -
FIG. 9A is a cross-sectional view of a metrology system for measuring misalignment of overlay targets to illustrate another embodiment of the invention. -
FIG. 9B is a graphical plot of the output signals from the metrology system ofFIG. 9A . -
FIG. 9C is a perspective view of the system ofFIGS. 9A and 91 . -
FIG. 10 is a top view of two overlay targets placed adjacent to each other illuminated by a single beam of radiation to illustrate yet another embodiment of the invention. -
FIGS. 11A and 11B are two side views from different angles of a metrology system where a single beam of radiation is employed to illuminate two overlay targets for detecting misalignment illustrating yet another embodiment of the invention. -
FIG. 12 is a flow diagram illustrating the cross-sectional light distributions at various positions of radiation beams in the system ofFIGS. 11A and 11B . -
FIG. 13 is a flow diagram illustrating the cross-sectional light distributions at various positions of radiation beams in the system ofFIGS. 3A and 3B where the two overlay targets are periodic in two orthogonal directions. -
FIG. 14 is a block diagram illustrating a lithographical instrument in combination with a metrology system for measuring misalignment of overlay targets to illustrate still another embodiment of the invention. - For simplicity of description, identical components are labeled by the same numerals in this application.
-
FIG. 1 is a top view of twoperiodic structures FIG. 1 . Preferably bothstructures spots spot 26 along the X axis is such that its dimension along the X axis is at least equal to or greater than the period Λ (more typically equal to or greater than several periods Λ's) in order for a diffracted signal to be generated by illuminatingstructure 22 atspot 26. Preferably, spot 26 would span a number ofgrating lines 22 a ofstructure 22. The same can be said ofspot 28 with respect to the period A and other features ofstructure 24. - A
metrology system 30 for generating the two illuminatedspots FIG. 2 . As shown inFIG. 2 , a beam of radiation (not shown) may be split by means of aWollaston prism 32 to illuminate the twostructures FIG. 2 , the Wollaston prism is located above the two structures at a predetermined z height above the structures. -
FIGS. 3A and 3B are side views of themetrology system 30 ofFIG. 2 in directions along thearrows FIG. 2 , respectively.FIG. 3C is a perspective view ofsystem 30. As shown in these figures, a beam ofradiation 34 is split byprism 32 into twobeams lens 36 towards twooverlay target structures substrate 38 such as a silicon wafer.Beam 34 ailluminates spot 26 onstructure 22 andbeam 34 b illuminatesspot 28 ofstructure 24.Beam 34 a is diffracted bystructure 22. The zeroth-order diffraction retraces the original path ofbeam 34 a and throughprism 32. The first order diffraction 40a -and 40 a+fromstructure 22 ofbeam 34 a are collected bylens 36 towardsprism 32 and reflected bymirrors detectors FIG. 3B . Similarly, the zeroth-order diffracted signal fromstructure 24 ofbeam 34 b retraces the original path ofbeam 34 b. The first order diffracted signals 42 b+ and 42 b− are collected bylens 36 throughprism 32 to the twodetectors FIGS. 3A and 3B are shown more clearly in the flow diagram ofFIG. 4 . The outputs ofdetectors FIG. 3B . As shown inFIG. 4 , theinput beam 34 is applied toWollaston prism 32 which splits or divides the beam into twobeams beams structures FIG. 4 . The positive and negative diffracted signals 40 a+ and 40 a− are collected bylens 36. Similarly, the positive and negative first diffraction signal 42 b+ and 42 b− are collected bylens 36 towardsprism 32.Prism 32 combines beams 40 a+ and 42 b+ from the twospots detector 54. Theinput beam 34 is coherent so thatbeams prism 32. Therefore, the positive first order diffraction 40 a+ and 42 b+ are also coherent. They would, therefore, interfere when combined byprism 32 and atdetector 54. Similarly, the negative first order diffractedsignals 40 a− and 42 b− are combined byprism 32 and interfere at the prism and at thedetector 52. The phase difference between the outputs of the twodetectors beams 40 a− and 42 b−, and provides information concerning misalignment between the twostructures Equations
S +1∝{η+1 a+η+1 b+2{square root}{square root over (η+1 aη+1 b)} cos(φ+1+φw+φz+φx)} (1)
S −1∝{η−1 a+η−1 b+2{square root}{square root over (η−1 aη−1 b)} cos(φ−1+φw+φz−φx)} (2)
In the equations (1) and (2) above, η+1 a is the diffraction efficiency of the diffracted signal 40 a+ from interaction betweenstructure 22 andbeam 34 a atspot 26, η+1 b, the diffraction efficiency of the +1 diffracted signal 42 b+ from interaction betweenstructure 24 andbeam 34 b atspot 28, η−1 a the diffraction efficiency of the diffractedsignal 40 a− from interaction betweenstructure 22 andbeam 34 a atspot 26, and η−1 b the diffraction efficiency of the −1 diffractedsignal 42 b− from interaction betweenstructure 24 andbeam 34 b atspot 28. The phase terms φ+1 and φ−1 are residual phase differences due to the grating property difference and are defined by the equations below:
φ+1=φ+1 a−φ+1 b;
φ−1=φ−1 a−φ−1 b
where φ+1 a and φ−1 a are the phase terms in the positive and negative diffracted signals, where such phase terms depend on the material and other properties ofstructure 22; and φ+1 b and φ−1 b, are the phase terms in the positive and negative diffracted signals, where such phase terms depend on the material and other properties of structure 24: The two phase differences φ+1 and φ−1 are identical for +1 and −1 orders if the grating profile is symmetric. The term φx is the phase difference caused by any misalignment Δx between the twoperiodic structures structures Wollaston prism 32 as shown in equation (5) below.
where Λ is the grating period ofstructures material 39; no, ne the indices of refraction for the ordinary and extraordinary rays of theprism 32; λ the wavelength ofbeam 34, and Δh is defined below. -
FIG. 5 is a schematic view of theWollaston prism 32 ofFIGS. 3A, 3B illustrating its function in dividing a beam of radiation into two beams and the phase shift introduced by the prism. As shown inFIG. 5 ,beam 34 is split byprism 32 into twobeams prism 32 between the two detector outputs S+1 and S−1 is given by equation (7) below, where no and ne are the refractive indices ofprism 32 in the ordinary and in the extraordinary directions of the prism. The term Δh in equation (5) above is the optical path length difference between h1 (the optical path length ofbeam 34 inprism 32 before reaching theoptical interface 32 a) and 112 (the optical path length ofbeams interface 32 a). Whenprism 32 is moved along the y direction relative tobeam 34,beam 34 will reach theinterface 32 a ofprism 32 at a different position, thereby changing the optical-path lengths h1, h2 and also changing the quantity Δh. Ah is proportional to the distance y traveled byprism 32 as set forth below:
Δh=2yα
where α is the angle theoptical interface 32 a makes with the sides of the prism through which thebeams FIG. 5 , so that equation (5) becomes equation (7) below.
θ=2α(n o −n e) (6)
where the origin of the Y axis is at the position where h1=h2. The separation δy between two spots on target is given by:
where f is the objective focal length. For small angle θ, an approximation of this formula becomes:
δy=fθ - Thus, by moving
prism 32 along the y axis and thereby changing the value of y, the phase term φw changes as a function of y, which, in turn, causes the two detector outputs S+1 and S−1 to also change as a function of the displacement y ofprism 32 along the y axis, as illustrated inFIG. 6 . As noted above, if the grating profiles ofstructures FIG. 6 is caused only by the overlay target error in equation (3). In other words, the phase difference Δφ between the two detector outputs is given by 2φx as indicated in equation (9) below:
Δφ=2φx (9)
wherein the phase difference between detector signals S+1 and S−1: twice overlay error. Therefore, by measuring the phase difference between the two detector outputs at different displacement values of y as indicated inFIG. 6 , the misalignment Ax between the twostructures structures processing device 50 inFIG. 3B , where the device may simply be a microprocessor, or programmable logic or any other suitable processing device which can compute the misalignment Δx from the two outputs S+1 and S−1. - As a practical matter, normally Wollaston prisms are designed such that the
optical axis 32 b (shown in a YZ plane as shown inFIG. 7A ) is parallel to the length L in one half and theoptical axis 32 c (shown in an XZ plane as shown inFIG. 7A ) is perpendicular to the length in the other half of the prism, where the two halves are separated byoptical interface 32 a, so that the twobeams structures system 30 for a given bandwidth. It is possible to overcome this problem by rotating the prism by 45°, so that thepolarization direction 34 c is at 45° to theoptical axis 32 b, and so that the polarizations of the twobeams FIG. 7A . However, that rotates the spot on the sample, and results in a signal even in the absence of any misalignment. The offset can be cancelled through calibration; however, this requires a target with larger footprint. - Another solution is to use a modified
Wollaston prism 32′, in which theoptical axis 32 b′ (shown in a YZ plane as shown inFIG. 7B ) in the first half of the prism is at +45° with respect to the length L of theprism 32′ and theoptic axis 32 c′ (also shown in a YZ plane as shown inFIG. 7B ) is at −45° to the length in the other half ofprism 32′ on the other side ofinterface 32 a′ as shown inFIG. 7B . If theincoming beam 34 is vertically polarized (as indicated byline 34 c′ inFIG. 7B ), it will be split by the prism into two substantially equal amplitude beams 34 a′, 34 b′, each at 45° polarization with respect to the fingers of thegratings - Instead of having to move the Wollaston prism as described above, an alternative differential
heterodyne system 100 requiring no moving parts is possible; this is illustrated inFIG. 8A as another embodiment. As shown inFIG. 8A , a coherent beam ofradiation 102 is split into twobeams beamsplitter 104.Half wave plate 101 is employed to control the intensity ratio between E1 and E2 to compensate for the diffraction efficiency differences between the twooverlay targets Beam 102 a is frequency shifted upwards bymodulator 106 by frequency ω1 to become beam E1 andbeam 102 b is frequency shifted upwards by amodulator 108 by frequency ω2 to become beam E2. The two modulated beams are reflected bymirrors 110 and passed throughrespective wave plates polarizing beam splitter 114. The two beams pass throughbeamsplitter 116 to a birefringent element such as aWollaston prism 32 which separates the two beams E1 and E2 on account of their different polarizations. Beam E1 is collected bylens 36 towardsstructure 24 and beam E is collected by the lens towardsstructure 22. - The +1 diffraction order E1,+1 of beam E1 from
structure 24 is focused bylens 36 towardsprism 32. The +1 diffraction order E2,+1 of beam E2 fromstructure 22 is also focused bylens 36 and combined with the +1 diffraction order E1,+1 byprism 32 into a single beam which is reflected bybeamsplitter 116. The polarizations of the fields E1,+1, E2,+1 are orthogonal to each other.Analyzer 118 is placed at substantially 45° to the polarization directions of E1,+1, E2,+1 so that the components passed by the analyzer have the same polarization and will interfere atdetector 54, after reflection bymirror 134. Similarly, the −1 diffraction order signals E1,−1, E2,−1 of the respective beams E1, E2, bystructures lens 36 towards the same spot inprism 32 which combines the two beams into a single beam E1,−1+E2,−1. Again the components of E1,−1, E2,−1 passed by theanalyzer 118 have the same polarization and will interfere atdetector 52, after reflection bymirror 132. The outputs of thedetectors phase detector 140 which detects a beat frequency ωb which is equal to the difference between ω1 and ω2 in order to determine the misalignment or overlay target error Δx. The equations of the various optical signals E1, E2, E1,+1, E2,+1, E1,−1, E2,−1 are set forth below. In the equations below, |E1|, |E2| are the amplitudes of the two beams E1 and E2 and ωo is the frequency ofinput beam 102. The terms θn1 and θn2 indicate the initial phase ofinput beam 102 and also the optical path length difference of the two beams betweenbeamsplitter 104 andpolarizing beamsplitter 114. The terms φ+1, φ−1, φx, φz, and φw have the same meanings as those described above in the previous embodiment where the Wollaston prism is moved, and are set forth in equations (3)-(5) above.
E 1 =|E 1 |exp{j(ω0 t+ω 1 t+θ n1)} (10)
E 2 =|E 2 |exp{j(ω0 t+ω 2 t+θ n2)} (11)
E 1,+1 =|E 1|{square root}{square root over (η+1 b)}exp{j(ω0 t+ω 1 t+θ n1+φ+1 b+φx)} (12)
E 1,−1 =|E 1|{square root}{square root over (η−1 b)}exp{j(ω0 t+ω 1 t+θ n1+φ−1 b−φx)} (13)
E 2,+1 =|E 2|{square root}{square root over (η+1 a)}exp{j(ω0 t+ω 2 t+θ n2+φ+1 a+φz+φw)} (14)
E 2,−1 =|E 2|{square root}{square root over (η−1 a)}exp{j(ω0 t+ω 2 t+θ n2+φ−1 a+φz+φw)} (15)
The quantities {square root}{square root over (η+1 b)}, {square root}{square root over (η+1 a)}, {square root}{square root over (η−1 b)}, {square root}{square root over (η−1 a)} are the diffraction efficiencies of the two structures for the +1 and −1 diffraction in the equations above. The output ofdetector 54 is given by equation (16) below and the output ofdetector 52 is given by equation 17 below.
S +1 ∝|E 1,+1 +E 2,+1|2 =|E 1|2η+1 b +|E 2|2η+1 a+2|E 1 ∥E 2|{square root}{square root over (η+1 aη +1 b )} cos[ωb t+(θn2−θn1)+(φ+1 a−φ+1 b)+φz+φw−φx] (16)
S −1 ∝|E 1,−1 +E 2,−1|2 =|E 1|2η−1 b +|E 2|2η−1 a+2|E 1 ∥E 2|{square root}{square root over (η−1 aη −1 b )} cos[ωb t+(θn2−θn1)+(φ−1 a−φ−1 b)+φz+φw−φx] (17)
S+1 and S−1 are sinusoidal functions at the beat frequency or difference frequency ωb=ω1−ω2 in the time domain. Again, where the twostructures
φ+1 a=φ−1 a
φ+1 b=φ−1 b (18) - Thus, from equations (16)-(18) above, it is noted that the phase difference between S+1 and S−1, is 2φx.
- Therefore, by detecting the phase difference between the two detector outputs, the overlay target error Δx between the two
structures - Advantageously, the
detector 140 may be set to detect at the beat or difference frequency ω1−ω2 to improve the signal-to-noise ratio of φx. - From a comparison of
FIG. 8A toFIGS. 3A, 3B , it will be evident-that the two diffracted and combined beams E1,+1+E2,+1 and E1,−1+E2,−1 as well as theircorresponding detectors -
FIG. 8B is a partially schematic and partially cross-sectional view of a dual heterodyne differential phase metrology system for measuring misalignment of two adjacent overlay targets to illustrate another embodiment of the invention analogous to that inFIG. 8A . As shown inFIG. 8B ,metrology system 100′ is similar tosystem 100 onFIG. 8A , except that anon-polarizingbeamsplitter 114′ is used instead of thepolarizing beamsplitter 114 ofFIG. 8A .Waveplate 101 is again used for controlling the intensity in ratio between the two beams E, and E. - The typical heterodyne phase different φx detected is of the order of 10−4 radians where the spacing Λ of
structures structures -
FIGS. 9A and 9C illustrate another embodiment of the invention without any moving parts.FIG. 9B is a graphical plot of the output signals from the metrology system ofFIGS. 9A, 9C . As shown inFIGS. 9A, 9C , a portion of an input coherent beam ofradiation 102 at frequency ω0 is reflected bymirror 202 and passes through an acoustooptic deflector (AOD) 204. Aportion 212 of the input beam at frequency ω0 passes through the AOD without being deflected or changed in frequency and is collected bylens 36 towardsstructure 24. Anotherportion 214 of theinput beam 102 is upshifted by frequency ω and is deflected by theAOD 204 to provide an upshifted and deflected beam at frequency to ω0+ω which is also collected bylens 36 towardsstructure 22. -
Beam 214 is diffracted bystructure 22 and the +1 and −1 orders of the diffraction are beams 214+ and 214−, respectively. The two diffracted beams 214+ and 214− are both at frequency ω0+ω. The two diffracted beams 214+ and 214− are collected bylens 36 towardsAOD 204 which passes without deflection or change in frequency a portion thereof towardsmirror 208. Theundeflected beam 212 at frequency ω0 is diffracted bystructure 24 into two diffracted beams 212+ and 212−, both at frequency to ω0. These two beams are collected bylens 36 towards theAOD 204 which deflects and downshifts in frequency by ω a portion of the two beams towardsmirror 208. TheAOD 204 combines the deflected portion of beam 212+ and the undeflected portion of beam 214+ to form the +1 order mixed output signal S+1 so that the two portions would interfere and yield an output at beat frequency equal to 2ω. Similarly,AOD 204 would combine the deflected portion ofbeam 212− and the undeflected portion ofbeam 214− as the −1 order mixed signal output S−1 at beat frequency 2ω. - As in the previous embodiment shown in
FIGS. 8A, 8B , any misalignment between the twostructures FIG. 9C . Thus, equations 16-18 may then be used to derive the overlay target area Δx, where the beat frequency ωb in equations 16 and 17 is equal to 2ω. -
FIG. 10 is a schematic view of the twostructures FIGS. 11A, 11B are the front and side views of asystem 300 employing a single beam for illuminating the two structures to illustrate yet another embodiment of the invention. As shown inFIGS. 10, 11A , 11B, this embodiment is analogous to that shown inFIGS. 2, 3A and 3B, except that asingle beam 302 is employed that illuminates bothstructures FIGS. 10 and 11 A, thesingle beam 302 illuminates a portion ofstructure 22 and a portion ofstructure 24 at their edges next to each other. The +1 and −1 diffraction signals from the two structures are collected bylens 36 and combined and detected by two detectors (not shown) to provide the two outputs S+1 and S−1 as in the embodiment ofFIGS. 2, 3A and 3B. A flow diagram illustrating the relative positions of the beams are shown inFIG. 12 . As shown inFIG. 12 the +1 diffraction beams fromstructures single beam 340 and the −1 diffraction beams from the two structures are combined into asingle beam 342. The two beams are detected byrespective detectors - One consideration in the embodiment of
FIGS. 10, 11A , 11B and 12 is that the alignment of the illuminatingbeam 302 is preferably controlled relative to the two structures to ensure that the relative intensity of the diffraction beams from both structures are held stable and substantially equal. This can be accomplished by precision beam positioning relative to the patterning on the wafer which can be augmented using an imaging system such as an optical microscope to visualize the beam location and make fine adjustments to the location of thebeam 302. Other methods include measuring the relative intensity of the diffraction components from the two structures in a feedback system to move thebeam 302 to even and consistent alignment between the two structures. For such alignment, the wafer may be moved by means of cross-roller translation stages driven by micrometers. Alternatively,beam 302 may be moved by means of adjustable beam steering mirrors or galvanometer mirrors and the like. All such different alignment mechanisms are believed to be known to those skilled in the art. - In the embodiments described above, misalignment is detected along one direction, namely, the x direction. In order to discover misalignment along an orthogonal direction such as the y direction, typically a second optical detection system is employed. Yet another embodiment envisions a two-dimensional alignment system as illustrated in
FIG. 13 . For this purpose a pair of two-dimensional gratings C and D is created and positioned on their respective processing layers in the same manner as the previously describedstructures FIGS. 2, 3A , 3B and 4, the two illumination beam on the two gratings would each give rise to four principal diffraction components, two in each axis of the gratings C and D. - Aside from the use of two-dimensional gratings C and D instead of one-
dimensional gratings FIG. 13 is the same as the previous embodiment illustrated in reference toFIGS. 2, 3A , 3B and 4. The two quartets of diffraction orders from gratings C and D are brought into alignment by theWollaston prism 32 and fourseparate photodetectors -
FIG. 14 is a block diagram illustrating alithographic instrument 400 in combination with ametrology system 100 for measuring misalignment of overlay targets to illustrate still another embodiment of the invention. As shown inFIG. 14 , the misalignment between two overlay targets measured by themetrology system 100 ofFIG. 8A may be supplied tolithographic instrument 400 for controlling a lithographic process, such as one performed by a lithographic stepper. Thus, themetrology system 100 may be integrated withinstrument 400 to become an integrated system for the convenient operation of misalignment detection and lithographic process control. Alternatively misalignment information may be used to determine probable device yield, and to make decision about future processing or rework of the devices. The process for performing such yield analysis and management is known to those skilled in the art so that no details need to be provided herein. This can also be done by supplying the misalignment of overlay targets information toinstrument 400, where the instrument is equipped with yield analysis and management capability residing in a microprocessor in theinstrument 400, in addition to lithographic control. Alternatively, such capability may reside in processing device orprocessor 50. - While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their-equivalents.
Claims (2)
1. A method for detecting misalignment of two periodic structures placed next to each other so that they are periodic substantially along a first line, comprising:
illuminating a portion of each of the two structures using radiation that is substantially coherent, each of said portions having a dimension along the first line larger than the period of the corresponding structure;
detecting diffracted radiation signals from the illuminated portions of the structures to provide at least one output signal; and
determining from the at least one output signal a misalignment between the structures.
2-43. (canceled)
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US6710876B1 (en) | 2004-03-23 |
US20040207849A1 (en) | 2004-10-21 |
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