CA2182358A1

CA2182358A1 - Multi-wavelength laser optic system for probe station and laser cutting

Info

Publication number: CA2182358A1
Application number: CA002182358A
Authority: CA
Inventors: Tony P. Leong; Edward S. North; Richard Linsley Herbst
Original assignee: Individual
Current assignee: New Wave Research Inc
Priority date: 1994-02-18
Filing date: 1995-02-09
Publication date: 1995-08-24
Also published as: EP0745015B1; EP0745015A1; US5963364A; US5703713A; US5611946A; EP0745015A4; US5811751A; JPH10500628A; WO1995022429A1; JP3026362B2; DE69534972T2; DE69534972D1

Abstract

A probe station which includes a base machine (10), a probe platen (17) mounted on the base machine (10), and a single passive air cooled Nd:YAG laser (100), mounted with a microscope (22). The single laser supplies an output beam to a first non-linear crystal (106) for generating the second harmonic of the fundamental output wavelength. A mirror (107) then directs the beam at a 90 degree angle through a polarizer (108) to repolarize the fundamental wavelength. The beam then passes to a second non-linear crystal (109) for generating the third and fourth harmonic of the fundamental wavelength. A reflecting mirror (111) then directs the beam to a variable attenuator (112) to select the appropriate output wavelength in the infrared (1064 nm), in the green (532 mm), or in the ultraviolet (355 or 266 mm).

Description

W095/22429 2 1 8 ~3 5 L~ PCT/US95/01731 MULTI-WAVELENGTH LASER OPTIC SYSTEM FOR PROBE STATION AND LASER CUTTING
B~C~GROUND OF TE~I~ INVF~TION
FiP1~1 of the Tnventinn The present invention relates to analytical probe stations, such as used in rA ~ fJ and testing procedures, laser cutters used standing alone or with the probe stations, arld to multi-wavelength laser systems suitable for use in this ~ UIIIII. IIL.
Dec~Aril-tinn of RPlAtP~l Art Analytical probe stations are in ~,videspread use in c..,.;...,.,.1.,. ~.,, ., and design facilities. When a design engineer or failure analyst has to debug a circuit, it is most often done with the aid of am analytic probe station.
The probe stations typically imclude a base machine with a platen for mounting probes, probes for positioning on the platen, a chuck or chucks on which to moumt a ~....;..""1.,.1", or other subject of the probe station, a Ua~,u~c bridge to support a IlliCIU~ UIJC" and a ~ mounted on the llfi~.lua~ulJc bridge. The probes contain Illi~,lUa~,U~i~, probe needles which are used to check signals and make IlI~C~lCllf ~ at various ~ocations in the integrated circuit.
The llfi~,lu~ul.c has a field of view on the subject of the probe, so that a scientist or engineer can probe a, ,~ . " device or other component under direct visllAAli7Atinn A lC~C~ Livc system is Icnown as the SUSS PM 5 laboratory prober . . ,A. ~ r - I ~ ~ Cdl by Karl Suss, Waterbury Center, Vermont. The laboratory prober is typically available with a var ety of IlliwuaL u~ includingrelatively low " ~ f ~ aLclcua.,u~ , systems to extremely high , .A~, . . r,. A. ;l ~l probing llfi~.~ua~ u~
I

wo ss/2242s ~2 1 8 2 315 8 PCTIUS95101731 Such probe stations are often used for analysis of integrated circuits, or otber devices like liquid crystal display (LCD) panels, which comprise a plurality of layers of material. For instance, an integrated circuit may be formed on a br ~ with one or more polysilicon layers, one or more oxide or isolation layers, and one or more metal layers.
To be able to probe an integrated circuit, the protective passivation layer must be removed. This may be done using an ultrasonic cutter, a sturdy probe tip(by scratching), a plasma or chemical etching process, a focused ion beam system, or a laser system. The laser is pulsed through the Illl~lU~.~U~C and can remove tbe passivation material to enable the engineer to probe the circuit. The laser may also be used to cut circuit lines to isolate or modify a circuit.
Similarly, in the " IA . I r~. 1, ,. ' "~ process for large-scale LCDs, shorts may appear at various locations. Since large LCDs are expensive, it is ' to repair these shorts. A laser is used to remove the short by focusmg enough energy density on the short material to vaporize it. LCDs typically use indium tin oxide (ITO) for the nearly invisible conducting lines on the LCD screen. C~rome is also used on the borders as conductive buses. A color filter is used in color LCDs. The color filter material may also have ", - - ,. . r~. 1, .. ;. ,0 defects which can be repaired.
ITO, chrome, and various color filter shorts may be repaired v~ith a laser system.
Accordingly, some probe stations in the prior art bave been coupled to læers. One typical system in the prior art is known as the Xenon Laser Cutter, Model No. SUSS XLG This system utilizes a pulsed xenon laser source which is directed to the device umder test tbrough special optics coupled with a high ".~ ";r,~ti~ u~,c. The~lc~u ~a~ Lll ofthissystemismthe green optical region, so it passes readily tbrough the Illi~,lu~ c optics. This single wavelength system is quite complex. The xenon laser must be mounted with the Illi~lUb~U~JC~ such that the output of the xenon laser is directed through the Illi~lU;~,U~lC optics. In a probe station CllVilUIIIll~,llL, which already involves a large amount of ,"~;. ". " I~ the addition of a laser system makes the station much wo 95/z24zg 2 1 8 2 3 5 8 PCTIUS95/01731 larger and more ~ In addition, the expense of such laser systems has been quite high.
One limitation of a laser cutter system for a probe station which generates a single ~av~ , is that the single ~ Lll may not be ,~ ,vl, for cutting off certain types of layers of material. For instance, ~.. ;. ~.,.1.. 1.,.~
typically have aluminum lines deposited on silicon wafers. There may be one, two, tbree, or four layers of metal lines separated by interlayer dielectrics. The whole device is then coated with a non-conductive passivation material to protect the circuit. The metal lines are typically aluminum, but may also be gold or titanium-tungsten. The passivation materials are typically silicon dioxide, silicon nitride, and polyimide.
In the ~ failure analysis market, the most universal wavelength is in the green region. Most metals absorb green laser energy very well and are usually very easily cut with one pulse. The green ~,V~ ;Lll may be generated with a xenon laser, or with a frequency doubled Nd:YAG system. Most passivation materials are transparent to visible light as well as green laser energy. To remove aLivll material which does not absorb green energy, one must heat up the underlying metal to a ~II~ .a~ which will "blow out" the passivation material.
This can usually be ~ . ' ' ' if the underlying metal is of sufficient mass which will not vaporize when the laser pulse hits. It becomes very difficult to remove certain passivation materials such as nitride and polyimide when the metal line is small, or if one is trying to access a metal layer underlying a top metal layer, or the underlying material is silicon or polysilicon.
Certain passivation materials, specifically nitride and polyimide, can be removed directly with ultraviolet energy. These materials absorb W energy directly and are slowly ablated using multiple low energy W laser pulses.
Unfortunately, silicon dioxide is not absorbmg of most W ~ ;LI~ (except for around 200 1,~ ) and must be removed indirectly using the heating method described above. Infrared laser energy is widely used in the flat panel wossl2242s ; ~ .~"~J..,~vi/~l ~
2 i82358 display repair market. Most of the materials used in this market absorb infrared~av~ llls. However, some materials, such as chromium, and some color filter materials are more absorbing of the green wavelength. In this market, all of thematerial in the target area must be removed, and the cuts are usually relativelylarge, i.e., 5-40 microrls.
The infrared wavelength also has application in the ~., .:, ..,..1....1... analysis field. Silicon is generally transparent to infrared energy. This allows one to remove metal lines with less damage to the underlying silicon with infrared thancan be ~...., . ,I.l;~h. ~1 witb green energy. Using green energy, the cut line can short to the substrate because of heating of the silicon. This happens less often withinfrared, making it am excellent ~ av.,l~ ;al to green for ,. .";~.,,,.l... l.., failure amalysis.
The following table r for some ~av~ a~ on common materials.
1064 r~m ~jnfrAred) 532 rlnn (~reer~) 355 nm ~ll~violet~
ITO ITO Nitride Chromium Chromium Polyimide Color filter Color filter Silicon Dioxide Nitride Polyimide (big cuts) SOG
Poly-silicon Gold Aluminum Tungsten However, the prior art has been unable to provide two ~av~ L~ of light from a single laser system. Thus, the probe station or laser cutter which is capable of supplying more than one wavelength of light has used two or more separate laser systems, becoming very large and unwieldy. For instance, one prior art system has combined an excimer laser which supplies ultraviolet with a doubled YAG laser ~ WO95/22429 21 82358 r~
on a single probe station, such as a . . ."~1,;. . ~ ;. ", ofthe model ~CM-308 EXCIMER
LASER CU~ER ATTACH~ENT with the laser of the model LCP GREE~ YAG
LASER CUTTER, both llla~ u~ by Florod Corp. of Gardena, California.
However, the excimer laser is a large bulky device, requiring ~ r~l tubular S wave guides to deliver the laser energy to the Illi~,lOa~ U~C optics. This system is - very expensive and uses up valuable laboratory space.
Accordingly, there is a need for a multi ..av~ l laser system for use with a probe station or laser cutter, which is CC~ small in siæ, and efficient.
SUMMARY OF THF INVEI~TION
The present invention provides a probe station which comprises a base machine, a chuck moumted on the base machine to hold a device under test (DU~, a probe platen mounted on the base mâchine on which to mount probes for the DUT, a Illh~lUa.~UIJC moumted on the base machine having a field of view on the DUT on the chuck, and a single laser, mounted with the Illh,lU~ UIJ~. The singlelaser supplies an output beam through the Illl~lUaW~/e optics on a beam line to the field of view of the Illi~,lua~,ulJ~. The laser includes optics to selectively gene}ate the output beam on the beam line in a plurality of ~av~ L Ia. A preferred systemincludes a solid state laser, a harmonic generator coupled with the solid state laser, and switchable optics for selecting the wavelength of the output beam from amongmore than two selectable ~a~ tlla in the infrared, visible and ultraviolet ranges.
Wavelengths, in this ~ o.l;.,....; may be selected from the r,.".~
~a~ ,LII of the laser and one of the harmonics, from among a plurality of harmorlics of the laser, or from among the r 1 ' ~ and a plurality of harmonics of the laser. 1~ addition, the laser system includes a variable attenuator which operates for the plurality of w~ llls which are selectable as outputs.
- The variable attenuator is based on a novel half-wave plate, which provides for woss/2242s 21 82!35~3 PCT/US95/01731 substantially half-wave phase retardation for the r,....~ and the second, third and fourth harmonics.
The laser, according to one aspect of the present invention, comprises a passively air cooled, electro-optically Q-switched, Nd:YAG laser. One or more non-linear optics is mounted in the beam path to generate at least one harmonic of the r, .. ,.1 .... Ilrl wavelength. This laser provides a compact, vibration-free system which supplies output in the infrared (1064 .,~ ), in the green (532 ), and in the ultraviolet (355 l . -- ,. . ,. ~ , or 266 ). These v-v_v~ s correspond to the r ~ ' ~ output ~-v_v.,lc.l~;Lll of the Nd:YAG
laser, the second harmonic, and either the third or fourth harmonic of the laser.
A variable attenuator is placed m the beam path for the plurality of ~v_v~ s by which a user can control the power of all of the output vva~ ~L1I~ . Finally, switchable optics are imcluded in the beam path to select the ~-v_v~ of the output beam from among the ~ v_~ and the plurality of harmonics. The switchable optics result in delivery of the output beam along the same beam line through the llu~,lu ,~,u~ 1 of the selected output.
When the r"...l_. ,... ,:_l output wavelength of the laser system is desired as am output selectable by the system, an additional problem occurs. In particular, the non-linear optics may cause walkoff of the r ~ ' I vv_v~ . In this ."l~o.l;"....; an optic is included in the beam line which ~...., ,. - . ~ ~ for the walkoff, so that when selected, the r~ vv~ is supplied on the beam line through the ,...,,IV~Cu~.
Further, the switchable optics, according to the preferred laser system, include a plurality of filters mounted on a mechanism for switching a selected one of the plurality of filters mto the beam path. As mentioned above, if the r, . . "1 . ". . " I wavelength is desired for output, the filter which is used to select for the r ", l -- " -,: -l wavelength is mounted at a pl~.l. t~ 1 angle to the beam path to ~,ulllr for the walkoff. It is critical that the plural VV_V~ hS in the IR, ~ W095/22429 21 82358 ~ 5t~
visible and W used with the probe station are de~ivered L~m~ Lly along a single beam line through the llfiL~IU~ optics and with controlled ,It~f-n~ inn The variable attenuator, according to the present inYention, provides for attenuation at all ofthe selectable ~vav.~ lls for the output beam. The variableâttenuator includes a half-wave plate in the beam path which is tuned to the plurality of ~vav~ . A polarizer is included, and a mechanism for rotating the wave plate to attenuate the plurality of vvav~ . The wave plate must be carefully designed to operate efficiently at all of the Vvav~ desired for possible outputs.
The laser system, according to the present invention, is compact, vibration-free, and relatively ill.,A~J~,~;ve. One çAnfigA,~r~tiA~n, according to the present invention of the laser system, comprises a passively air-cooled, electro-optically Q-s-v-vitched Nd:YAG laser, generating a beam at a r. . ~ vvav~ 14 Lll along a beam path. A first, non-linear crystal is mounted in the beam path to generate a secondharmonicofthe r,.. ~ vav~ h. Asecondnon-linearcrystalis mounted in the beam patn to generate at least one of a third and fourth harmonics ofthe r... 1 .,~ wavelength. Avariableattenuatorismountedmthebeampath forthe r.. , ll~ vvav~ thesecondharmonic~andatleastoneofthethird amd fourth harmonics. Finally, switchable optics in the beam path select the vvav~L~ of the output beam from among the r. ~ vav~ l, the second harmonic, and at least one of the third and fourth harmonics. Two or three wavelength models of the present invention are 6.25 inches wide, 12 mches high, and ~ inches deep. Thc system weighs only 8 pounds.
Because of the compact size, and vibration-free , ' the laser according to the present invention can be mounted on a llfl. I~ O~ to form a simple multi-vvav~ h, laser cutter, as an alternative to the probe station ~,..l.~.l;..~. .,1 discussed above.

WO 95122429 21 8 2 3 j 8 PCTIUS95/01731 The probe st~tion and laser system, according to the present invention, meet the exacting l~lUil~ of ~ . . .;. . . I", l, .. ",i",. " ",.. h ~ for design verification and evaluation, failure amalysis, and LCD repair ~ The solid state, vibration-free, air-cooled system combines convenient operation, small size, superior uniformity and stab;lity into a single instrument with unmatched I. r.,.".~ The multi . ~ h system provides optimum flexibility for a ramge of ~
Other aspects and advantages of the invention cam be seen upon review of the figures, the detailed A~errirtir~n~ and the claims which follow.
T~RTF.F DF.~t`T~lPTION OF T~TF. FIGURFC
Fig. 1 is a ~ p~ iiVI; view of an analyticaT probe station with a multi-~,va~ laser, according to tbe present invention.
Figs. 2A and 2B illustrate the layout of a multi . ~ ' laser, âccording to a first ~ . ~ " of the present invention.
Figs. 3A and 3B illustrate the r~ rhsmicm for movmg filters m and out of the beam path in the optical lâyouts of Figs. 2A, 2B and 4.
Fig. 4 illustrates the optical layout of an alternative laser design according to the present invention.
Fig. 5 is a perspective view of a laser cutter system consisting of a I..i.,lua.,ulJe with a multi . ~ laser moumted thereon according to the present invention.
DETAlT~Fn DE~CRIPTION
A detailed descriptiûn of preferred I '.~.1;,.. 1~ of the present invention is provided with reference to the figures, in which Fig. l illustrates a probe station according to the present invention with a multi . ~ Iaser mounted thereon.
Fig. I provides a simplified diagram of an analytical probe station according to the present invention. One important feature of an amalytical probe `2 1 823S~
station is compact size to preserve valuable laboratory space. However, the stations are complex machines which are adapted to handle a wide variety of p ro b i n g rr i ; ., n ~
The probe stations consists of a base machine 10, typically including a base frame, a lui~,lu~upe moumting bridge 11, a translation stage 12 for X and Y
- Al ~ cables for probe heads and the like (not shown), a ~ 13 for allowing rotation of a chuck, and other features known in the art. As illustrated in the figure, the base machine 10 includes a variety of controls, e.g., 14 and 15 used for A.~Amfi~ in~ the probe station for a particular ArrliAAtiAln Also included in the probe station a~e a chuck 16 with typically magnetic or vacuum powered; 1: - 1 " .. .t` for holdmg a subject ofthe probe on the chuck 16.
Adjacent the chuck is a platen 17 on which to position a plurality of probe heads 18,19. The probe heads 18 are coupled to probe arms 20, 21 which extend onto the subject of the probe.
A ~ u~,~ æ is mounted on the llu~lu~vl~ bridge 11. The ~
includes a plurality of objective lenses 23 as known in the a~t. Typically, the Illi~lU~,UIJ~ 22 is a ~ r~ IIU~IU~U~, such as a the Mitutoyo ~S-60 microscope, which is available through MTI (~nrrA~r~til~n in City of Industry, California.
According to the present invention, a multi .. v, ' _ ' laser 24 is mounted with the IlU~,lU~UIJe 22. The multi-wavelength laser 24 is compact, fitting right on the Illh,lUi~,UUC, and does not waste expensive laboratory resources. The multi-~vav~ laser 24 is coupled to a power supply 25 by which the parameters of the output beam are controlled. An electrical umbilical cord 26 is coupled between the power supply 25 and the multi-wavelength laser 24. The laser may be mounted on the camera port for the IIU-,IU~,U~J~, or on a dedicated laser port of some available Illi~lUDl,Up~,~. The beam is directed through the IlU.loSCO~uC and exits through the IIU~IU~I..UIJ~ objective into the field of view. The beam is focused into a small area ~' ' by the objective lens used and the si~e of a variable g W0 95/22429 . . . ~ PCT/US9~/01731 2i`823~
aperture in the laser head. High power objectives focus the beam into smaller areas than lower powered objectives amd create enough energy density to melt or vaporize mamy materials. A I OOX objective will generate four times the energy density of a SOX objective. A 20X objective produces only 16% of the energy density of a 50X objective.
Mi~1u~,up~,~ are generally designed for visible light and, as such, the energy of visible light lasers are most easily transmitted through the IIII-,IU~-~U,U~
optics. Many IIIi~,Iua~,u~t I . ~ - - r- .1. ., ~, ~ offer infrared versions of their microscope which transmit, in addition to visible light, near infrared energy. Some microscope ".--,.. r~ ., are also developing Ill;~lU~ UU~.~D which will transmit near Wenergy in addition to visible light.
According to the preferred ~".1.~,.1.. -~ the multi ~ v~ L~II laser head has dimensions of a~U~UI~ 12 inches high by 6.25 inches wide by 5 inches deep including a camera adaptor. It consists of a passively air-cooled, Nd:YAG
IS laser, optics in the beam path of the laser for generating a plurality of harmonics of the r" ", ~ wavelength, a variable attenuator operable for all the selectableoutput ~av~,L,~ and switchable optics which are used to select the output v~av~ l upon operation of a switch. The laser system is electro-optically Q-switched OEnd operates at lHz ~ .. Iy or a single shot may be fired on demamd. All the plurality of ~vav~ OEe supplied along a single beOEm line into the IllI~lU:>I,U~U~i optics, so that they OEe CUII ,; ,~11.ly positioned in the field of view of the llfi~lU:.~.UIJ-, on the subject of the probe station. In addition, the laser system includes a vOEiable XY shutter, so that the output beam has a controlled size rectangulOE footprint on the subject DUT. The laser head uses an invOE stabilized resonator cavity on which the optics of the laser OEe mounted. This insures trouble free operation under normal operating conditions. The flash lamp and power ~ w095/22429 2~ ~2j3~8 PCT/US95/~)1731 supply are passively air-cooled without the use of farls, or other active cooling ,,.. . l, .,:~.,.~ This prevents vibration at the probe station wAhich is critical for probing ,~.";...r,.l.l.-tnr devices arld the like which may have submicron tiimPneinne A layout of the optical design of the laser system in the preferred - emh~tlinA~rt is described in Figs. 2A and 2B. The laser system includes a flash lamp pumped electro-optically Q-switched Nd:YAG laser 100, such as the ~UIIIIll~,ll..;all~ available ACL-1 air-cooled laser available from the Assignee of the present application, New Wave ~esearch, Inc., of Sunnyvale, California. This system includes an invar stabilized, electro-optically Q-switched and passively air cooled laser resonator. The laser 100 includes a high reflector 101, electro-optic Q-switch 102, a flash lamp pumped Nd:YAG gain medium 103, and an output coupler 104. The output of the laser 100 is supplied along beam path 105 througha first non-linear crystal 106 for generating the second harmonic of the r~ output wavelength of the laser l oo In the preferred system, this non-linear crystal is KTP aligned for frequency doubling the 1064 nanometer line ofthe YAG laser. Next in the beam path 105 is a high reflecting mirror 107 for the r, ....lA.... l ~1 amd secorld harmonic ~ ,. The mirror 107 directs the beam path at a 90O angle through a polarizer 108 to repolarize the r,. I--II...AI
~ L,.. ~ ofthelaser100. The~ a ~ .l.yLIlisrepolarizedafterthe doublmg crystal 106 for more efficient The repolarized f~
frequency and frequency doubled component are then passed along the beam path 105 through a second non-linear crystal 109. The second non-linear crystal 109, in the preferred system, is used for generatmg the third harmonic and the fourth harmonicofther~ a~ . Inthis l.o~l;.. ' itconsistsofbeta bariurn borate BBO aligned for either the third or fourth harmonic generation.
The r"- I~ I 11~1, the second harmonic, and the third or fourth harmonics are then passed along the beam path 11 0 to a high reflecting mirror 111, which is high reflecting at the r~- ~tlA~ a~ the second harmonic, the third wo ss/2242s ~ 3 ~ 8.
harmonic, and the four~fh harmonic. The mirror 111 turns the beam 90O th~ough a variable attenuator 112.
The variable attenuator 112 consists of a multiple wavelength wave plate 113 and a calcite polarizer 114. The relative angular position ofthese two devices S is controlled usmg a mf-rh",liem I I S known in the art so as to control attenuation of the laser beam on path 105.
The multiple vvav.,l~ t;al wave plate operable at each of the four ~v~ ala identified must have an optical tbickness which is near an odd number of one-half ~av~ ala of all of the ~4~1V~ lla of interest. It has been discovered that an optical grade crystalline quartz plate having a physical thickness of near 0.77901 millim~t~rc provides about 180 degrees relative phase }etardation of the e- and o- waves for each of the ~ ' ~\, the second harmonic' the third harmonic and the fourth harmonic (1064, 532, 355, 266 nm). This ~ c with the 63rd order _alf-wave at the 266 1.- .. 1.. ~ of the fourth harmonic.
Although the relative phase retardation is not precisely half-wave for all four v~ ,h"l~ala, it is close enough that when combmed with a polarizer, an attenuator is formed which is effective at all four. In the ~., h.,.':..1. ~ . describe, the attenuator ., . when open is about 100% for the fourth harmonic, about 99.4% for the third harmonic, about 98.6% for the second harmonic and about 89.3% for the r.,.,.l .,.. :-' Other afi~,hl~,~a~,~ for the half-wave plate can be used to achieve similar results, but this is preferred because of the higher ~ .., in lower power ~av~ ;ala of the second, third and fourth harmonics. For instance, a thickness of about .0865 millim~f~rg is about 100% ll~la~ a:l;v~ at 266, 89% at 355, 100% at 532 and 62% at 1064. A thickness of about .3091 millirn~-tf-rg is about 100% tl~lallli~alv~ at 266, 98% at 355, 77% at 532 and 99% at 1064. A
thickness of about .5564 mjllirn~t~rg is about 100% tlollalll;i~ ~;v~ at 266, 85% at 355, 87% at 532 and 96% at 1064. A thickness of about .9274 millimeters is about100% l~ al~ a;Vt: at the fifth harmonic (213 " ...,. ~ ), about 100%
;a ,;v~ at 266, 85% at 355 ~nd 8~ at 1064, although it is not ll~ulal~ ;v~

WO 95/22429 2 1 ~ 2 3 5 8 PCT/IJS95/01731 at the second harmonic. For single plate b If-wave plates, it is desirable to keep the thickness below about I millimeter to avoid incurring thermal problems associated with thicker plates.
The attenuated beam is supplied on path 105 out of the variable attenuator 112 through a switchable filter mechanism 116. The switchable filter mrrhS~ni~m mounts a plurality of filters used for selecting the output wavelength of the system.
By moving one of the plurality of ~a~ selective filters into the beam path, the output ~av.,l.,llgtll is selected.
Then non-linear crystals 109 for generating the third or fourth harmonic cause walkoff of tbe harmonic ~va ~ SO that they are separated from the beam path I OS by an amount of about one-half of a millimeter. This walkoff is critical for the U~UU~ mounted laser system where the output beam must proceed along the same beam line for all the selected ~ ,.,gth., into the field of view of the IIIII..IU:I~.U~
By tilting filter 117 which is used to select the third or fourth harmonic , this walkoff is corrected. Thus, the third or fourth harmonic ~a~ , and the other wa~,L,Il~;llls are supplied along the beam path 105 in alignment, i"1~ of which wc~ lll is selected.
The KTP crystal 106 aligned for second harmonic generation causes negligible walkoff. Thus, the BBO crystal 109 is primarily ~ r for the walkoffwhich must be corrected by the switchable optics 116, using tilted colored glass filters which select for the desired output.
Next in the beam path 105 is telescope 118. This telescope is used to expand the beam about three times from about a 3 millimeter cross-section to about a 9 millimeter cross-section. This allows for matching the cross-section of the beam with the UU~ " - X-Y aperture 120 described below. After the telescope 118, the beam is supplied along the path 105 to a high reflector 119, which is reflective of the four selectable output wavelengths. The beam is turned at reflector 119 by 90 to reflector 150. ~,llec~or 150 is refl~ctive at the harmonic .

woss/22429 2 1 8~:3 58 v~ al~, and at the sccond, third, and fourth harmonics of the harmonic w~ als. Also, it is Ll~ ;Vt~ at 600, ~ and above in the ~;111~ ' described so that Yisible light is transmitted from a white light source IS 1 into the beam line such as a 150 watt white lamp, to act as an aimmg beam or a spot marker.
The reflector 150 turns the beam path through an X-Y aperture 120 which is used to form a squ~re or ICi-,Llll~UL cross-section for the beam being delivered to the Illi, lva.,u,u~.
The beam passes from aperture 120 to beam splitter 121. The beam splitter 121 is 50% or more ~ ;Vt~ at all of the four ~ ,al, selectable by the output system. The output of the laser system is then supplied on the beam line 122 irlto the luil,lU:l~,V~J~i optics amd on a line ~ ' ' to the drawing in Fig.2A to a camera adapter 123, as shvwrl irl Fig. 2B. An image frvm the field of view of the ,..i~,.u~,v~ is reflected at beam splitter 121 to mirror 124 m the cameraadapter 123. The camera adapter includes a fitting 125 to which a video camera or other imaging system may be coupled to the assembly.
The laser layout illustrated in Figs. 2A and 2B is capable of proYiding three selectable output V~ with the flip of a switch for a probe station or laser cutter, accordmg to the present mvention. By tuning the non-linear crystal 109 to select for either the third or fourth harmonics, the laser system cam be adapted to select for 1~1....l,..,.~ ,: 1 output ~a~ in the irlfrared, the second harmonic in the visible, or the tbird harmonic in the ultraviolet, or to select for the r~
output wavelength in the infrared, the second harmonic in the visible, or the fourth harmonic m the ultraviolet.
The variable attenuator 112 and the switchable optics 116 are especially designed to overcome the prvblems associated with multi-v~ ' laser systems which must supply controlled attenuation outputs, on a single beam line with theexacting standards of probe stations or laser cutters.

wo 95/22429 ~ 1 8 2 3 ~ 8 PCT/US95/01731 Because the optics, including attenuator 112, _nd high reflectors 111, 119 and 150, work for all four possible ~ V~ 6~ the laser system of Fig. 2A cam be extended to a four ~_v.,l~ h system by inserting an additional nonlinear crystdl in series with crystdl 109. Any changes in the walkoff of the beam are ~ .1 by adjusting the tilts of the filters as before.
A simplified diagram of the switchable optics, according to the present invention, is provided in Figs. 3A and 3B. Fig. 3B illustrates a wheel 200 on which a pluralit,v of filters 201, 202, 203 and 204 are mounted. Each filter consists of a colored glass filter designed to select for a particular output ~_v~,1.,.l6Lll.
AlL-,ludti~.,l.y, a plurality of the filters on the wheel might select for the same h~ll6lll but provide for different amounts of attenuation of that ~ ,;h.
Thus, filters 201 and 202 might select for the third and second harmonics of theharmonic ~.v.,l~,l.~,lLa ~ ly. Filters 203 dnd 204 might select for the r-~ output ~a~ ..6l.1l with a 70% attenudtion and a 50% sttf-mlstiA,n ~ .,ly. The wheel carl be expdnded to hold a larger number of filters, as needed. Also, more than one wheel in series may be used to A~ a variety of effects.
In order to correct for walkoff, the filter 201 which selects for the third or fourth harmonic ~ 6Lha is mounted with a tilt. Fig. 3B illustrdtes a side view of the selectable optic . .. ~ which includes a wheel 200. The wheel 200 is mounted with a motor 210 for selecting the position of the filter. Filters 202, 203 and 20~ are mounted flat to pass the ~ ' ' and second harmonic ~_v~ ;LlL~ straight through when the .L~ v~ , filter is in the beam path 105.
However, the filter 201 is mounted with a tilt to cvl~l for the walkoff205 of the third or fourth h. rmonic ~ on path 206. In the preferred system, a colored glass filter having a thickness of 2.5 millim~f~r.~, tilted at ~r ~/
180 is used to - -r ' for the walkoff caused by a BBO crystal aligned for generation of the third harmonic. When the BBO crystal is aligned for the fourth harmonic, the angle of tilt of the filter is aL)~ 20 .

wo gs/22429 Pcr/usss/0l73l 3~:8 For a system w1 1ich is designed to supply an ultraviolet output ~
(355 or 266 " ~ . t -~, the optics of most cu.l...~ ,lu:~-,U~J~,.7 must be replaced with W tlal~ll~l~a;~. optics. Thus, the Mitutoyo FS-60 microscope must be fitted with a beam splitter prism and zoom lens attachment made of fused silica which transmits all of the ~av~ of interest. An objective lens which is tl~l~ ;ve of the W is available cull~ ,;ally from the llloll~ of the Mitutoyo llfi~lU~U~
The KTP and BBO crystals may be replaced with a wide variety of non-linear crystals, as known in the art. However, KTP is quite efficient for frequency doubling the 1064 nanometer line ofthe Nd:YAG laser. Also, the doubling occurs with an alignment of almost 90O so that the walkûffis negligible. The BBO crystal is used for tbird or fourth harmonic generation.
Fig. 4 illustrates an alternative layout for â laser system generating tbe second and third halmonics of the harmonic ~ for selection. In this system, ~ which are similar to thûse used in the I ., .1~1;........... ,; of Fig. 2A
are given like reference numbers and are not .c;.lc,,~,l;l,.,d here.
As can be seen, the layout is similar to that shown in Fig. 2A, except that the repolarizer 108 for the harmonic ~a~ ls is eliminated. Also, the attenuator 112 need only be operable at the two selectable vv~ ,k,ll~llls. Also, the crystal 109 can be tuned for either tbird or fourth harmonic generation, as before.
The spot marker 151 in this ~ ...1,-~.1; ....,1 is also a white light source.
However, as known in the art, this spot marker may be replaced vvith laser diodeor other aim beam l. . 1 " ..~
Fig. S illustrates the ' . . _..,I.ll~s~l Iaser system according to the present system moumted on a simple laser cutter m~ ;cm which includes a ~ u~-,ulJ~
300, amd a stage 301 mounted on a microscope base 302. The microscope 300 has afieldofview3030nasubject3040fthelasercuttingoperation. Thestage301 includes fine X and Y III;UIUIII~ I stages 305, 306 for controlling the position of the subject 304 ofthe laser cutter operation.

wo g5n2429 2 3 5 ~
The multi-wavelength laser 307 supplies a selectable output wavelength on the beam line 308 into the field of view 303 of the laser. The beam line 308 does not change when the output V~a~ ,Lh of the laser 307 chamges under operation of the system as described above. The ~ ,lUa~.OIJC; 300 includes a plurality of S objective lenses 310, 311, and 312 all of which are adapted to pass the plurality of ~,vav~ 15 which are generated by the multi ..a~ Lll laser 307. Further, the ua~vlJc optics, including prisms and the like, arc llallall~laa;vt; at all the plurality of v~a~.,l.,ll~;LllD selected by the laser system.
The compact size of tbe laser system 307 is critical for the probe station or laser cutter application as described above. The ' .. _v.,I.,l.~,lh Iaser 307, as shown in Fig. 5, is small enough to mount on a IlP~,lua~,u~)~i, stable enough so that the output ~a~,lcll~la are Culla;~ ly generated along a single beam line 308, and light enough that it does not upset the focusing mechanism ûf the ll~lua-~U~Further, the laser system does not use valuable laboratory space to produce a plurality of ~a~ llla.
In addition, the laser multi ..~ LII Iaser 307 is applicable in a variety of other situations wbich require compact, stable, multi ~. .v~ ll systems.
Unique ability to provide a plurality of controlled attenur4tion outputs along asingle beam line using a single laser and specially designed optics, allows ~rrlir~ti~n of multi-~a. ~ 11l laserS m a variety of ~lv ;lulllll.,llla not available before because of the expense and bulk of prior art ..a~ ,LII systems.
Accordingly, the present invention provides an air-cooled, pulsed, Nd:YAG
laSer system designed to be mounted on a stand alone llf.~,lua~,u,u~, or a probestation. Moumted on an analytical probe station, the system provides unique 25 flexibility for ,.. .~.. ,.. l.. ~.. , design ~l;rll~dtiull and failure analysis ~
In the stand alone laser cutter system, the laser is mounted on a ~ ,lua~,ou~ with its own stand and X-Y stage, making it available for a larger number of engineers than would have access to the standard probe station.

wo ssl22~2s ~ _ _ r~
`3~8 The system provides mûny advanced features, including electro-optic Q-switch, multiple ~u~ operation, fanless operûtion, and compact size.
Output is stable and repeatable, which insures precise cutting and allows for large uniform cuts. The invar stabilized resonator structure improves t~ Ul~
stability, which makes the laser system more tolerant of t~lllU~.a~ul~ changes and allows it to have consistent energy over a broader t~ UlC range.
The electro-optic Q-switch in the preferred, ~ - ' uses a KDP crystal in the Pockel cell and polarizer, which allows for precise control of the energyburst from the gain medium.
The unique half-wave plate and dielectric polarizer of the variable attenuator are not subject to heat buildup, .1. ~,.",~1;."" and energy variûtions, which might be ~ 1 by lower cost Att.-ml~t .re Also, they provide attenuation over the entire spectrum of output ~U~ lE1h.. generated by the laser System.
The present invention provides, for the first time, the ability to mount a single laser on a probe station or laser cutter, which provides outputs selectable from the infrared, the visible, and the ultraviolet ranges. The outputs are available with precise~y controlled ~ttl-ml~ n and proceed along a single beam line through the III;~,IU~,V~J~ for consistent operation. The compact size and air-cooled nature make it ideally suited to the probe station ~IIY 11~ t, where laboratory space is ût a premium and vibration calmot be tolerated.
The foregoing description of a preferred ( ., .l .o~ l; . . . I of the invention has been presented for purposes of illustration and ~ errirtil~n It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many ."~ and variatiorls will be apparent to l~ skilled in this art. It is intended that the scope of the invention be defmed by the following claims and their equivalents.
What is claimed is:

Claims

1. A probe station system, comprising:
a base;
a chuck mounted on the base to hold a device to be analyzed or tested;
a probe platen mounted on the base on which to mount probes for the device;
a microscope mounted on the base, having a field of view on the device on the chuck; and a laser, mounted with the microscope, supplying am output beam through the microscope optics on a beam line to the field of view of the microscope, thelaser including optics to selectively generate the output beam on the beam line in a plurality of wavelengths, and wherein the microscope includes optics transmissive in the plurality of wavelengths and wherein the plurality of wavelengths includes more than two selectable wavelengths for the output beam.

2. The system of claim 1 or 11, wherein the laser comprises a solid state laser, a harmonic generator coupled with the solid state laser, and switchable optics for selecting the wavelengths of the output beam.

3. The system of claim 1 or 11, further including a variable attenuator for the plurality of wavelengths.

4. The system of claim 1 or 11, wherein the laser comprises:
a solid state laser generating a beam at a fundamental wavelength along a beam path;
one or more non-linear optics in the beam path which generate at a plurality of harmonics of the fundamental wavelength;
a variable attenuator in the beam path for the plurality of wavelengths; and switchable optics to select the wavelength of the output beam from among the plurality of harmonics and the fundamental wavelength.

5. The system of claim 5, wherein the non-linear optics cause walkoff of at least one of the plurality of wavelengths, and further including an optic in the beam path to compensate for the walkoff so that each of the plurality of wavelengths is supplied when selected on the beam lime.

6. The system of claim 1 or 11, wherein the laser comprises:
a solid state laser generating a beam at a fundamental wavelength along a beam path;
one or more non-linear optics in the beam path which generate a plurality of harmonics of the fundamental wavelength;
a variable attenuator in the beam path for the plurality of wavelengths; and switchable optics to select the wavelength of the output beam from among the plurality of harmonics.

7. The system of claim 6, wherein the non-linear optics cause walkoff of at least one of the plurality of harmonics, and further including an optic in the beam path to compensate for the walkoff so that when selected, each of the plurality of harmonics is supplied on the beam line.

8. The system of claim 1, 11, or 12, wherein the laser comprises a passively air cooled solid state laser.

9. The system of claim 1, 11, or 12, wherein the laser comprises a passively air cooled, electro-optically Q-switched, Nd:YAG laser.

10. The system of claim 1, 11, or 12, wherein the plurality of wavelengths includes at least one wavelength in the infrared range, at least onewavelength in the visible range, and at least one wavelength in the ultraviolet range.

11. A laser cutter system comprising:
a base;
a stage mounted on the base to hold a subject;
a microscope mounted on the base, having a field of view on the subject on the stage; and a laser, mounted with the microscope, supplying an output beam through the microscope optics on a beam line to the field of view of the microscope, thelaser including optics to selectively generate the output beam on the beam line in a plurality of wavelengths, and wherein the microscope includes optics transmissive in the plurality of wavelengths and wherein the plurality of wavelengths includes more than two selectable wavelengths for the output beam.

12. A laser system for supplying a plurality of wavelengths with controllable attenuation along a single beam line, comprising:
a solid state laser generating a beam at a fundamental wavelength along a beam path;
one or more non-linear optics in the beam path which generate a plurality of harmonic of the fundamental wavelength;

a variable attenuator in the beam line for the plurality of wavelengths; and switchable optics to select the wavelength of the output beam.

13. The laser system of claim 12, wherein switchable optics select the output wavelength from among the plurality of harmonics and the fundamental wavelength, and the one or more non-linear optics cause walkoff of one of the plurality of wavelengths, and further including means in the beam line for compensating for the walkoff so that when selected, each of the plurality of wavelengths is supplied on the beam line.

14. The laser system of claim 13, wherein the switchable optics select the wavelength of the output beam from among a plurality of harmonics.

15. The laser system of claim 12, wherein the switchable optics select the wavelength of the output beam from among a plurality of harmonics and the fundamental wavelength, and wherein the non-linear optics cause walkoff of at least one of the plurality of wavelengths, and further including means in the beam line for compensating for the walkoff so that when selected each of the plurality of wavelengths is supplied on the beam line.

16. The laser system of claim 12 or 25, wherein the switchable optics include a plurality of filters, and a mechanism for switching a selected one of the plurality of filters into the beam path.

17. The laser system of claim 12, wherein the switchable optics include a plurality of filters, and a mechanism for switching a selected one of the plurality of filters into the beam path to select the output wavelength from among the plurality of harmonics and the fundamental wavelength, and in which the one or more non-linear optics cause walkoff of at least a particular one of the plurality of wavelengths, and wherein one of the plurality of filters selects the particular one of the plurality of wavelengths, the one filter mounted at a predetermined angle to the beam path to compensate for the walkoff.

18. The laser system of claim 12, wherein the variable attenuator comprises:
a half-wave plate in the beam line tuned to the at least one harmonic and the fundamental wavelength;
a polarizer; and a mechanism which controls relative angular position of the wave plate and the polarizer to attenuate the plurality of wavelengths.

19. The laser system of claim 18 or 25, wherein the plurality of wavelengths includes the fundamental wavelength, a second harmonic of the fundamental wavelength and a third harmonic of the fundamental wavelength.

20. The laser system of claim 18, wherein the plurality of wavelengths includes the fundamental wavelength, a second harmonic of the fundamental wavelength and a fourth harmonic of the fundamental wavelength.

21. The laser system of claim 12, wherein the variable attenuator comprises:
a half-wave plate in the beam line tuned to a plurality of harmonics;
a polarizer; and a mechanism which controls relative angular position of the wave plate and the polarizer to attenuate the plurality of harmonics

22. The laser system of claim 21, wherein the plurality of wavelengths includes a second harmonic of the fundamental wavelength and a third harmonic of the fundamental wavelength.

23. The laser system of claim 21, wherein the plurality of wavelengths includes a second harmonic of the fundamental wavelength and a fourth harmonic of the fundamental wavelength.

24. The laser system of claim 12 or 25, wherein the plurality of wavelengths include an infrared, fundamental wavelength, a visible second harmonic, and at least one of an ultraviolet third harmonic and an ultraviolet fourth harmonic.

25. A laser system for supplying a plurality of wavelengths with controllable attenuation along a single beam line, comprising:
a solid state laser generating a beam at a fundamental wavelength along a beam path;
one or more non-linear optics in the beam path which generate at least one harmonic of the fundamental wavelength, in which the one or more non-linear optics cause walkoff of at least a particular wavelength of the plurality of wavelengths;
a component in the beam line which compensates for the walkoff so that when selected each of the plurality of wavelengths is supplied on the beam line;and switchable optics to select the wavelength of the output beam.

26. A laser system for supplying a plurality of wavelengths with controllable attenuation along a single beam line, comprising;
a passively air cooled, electro-optically Q-switched, Nd:YAG laser, generating a beam at a fundamental wavelength along a beam path;
a first non-linear crystal in the beam path to generate a second harmonic of the fundamental wavelength;
a second non-linear crystal in the beam path to generate at least one of a third or a fourth harmonic of the fundamental wavelength;
a variable attenuator in the beam path for the fundamental wavelength, the second harmonic and at least one of the third and fourth harmonics; and switchable optics to select the wavelength of the output beam from among the fundamental wavelength, the second harmonic and at least one of the third and fourth harmonics.

27. The laser system of claim 26, wherein the variable attenuator comprises:
a half-wave plate in the beam line tuned to the fundamental wavelength, the second harmonic and at least one of the third and fourth harmonics;
a polarizer; and a mechanism to control relative angular position of the wave plate and the polarizer to attenuate the fundamental wavelength, the second harmonic and at least one of the third and fourth harmonics.

28 . The laser system of claim 26, wherein the switchable optics include a plurality of filters, and a mechanism for switching a selected one of the plurality of filters into the beam path.

29. The laser system of claim 26, wherein the switchable optics include a plurality of filters, and a mechanism for switching a selected one of the plurality of filters into the beam path to select the output wavelength, and in which at least one of the first and second non-linear crystals causes walkoff of at least a particular wavelength of the plurality of wavelengths, and wherein a particular one of the plurality of filters selects the particular wavelength, the particular one filter mounted at a predetermined angle to the beam path to compensate for the walkoff.

30. The laser system of claim 26, in which at least one of the first and second non-linear crystals causes walkoff of at least a particular wavelength of the plurality of wavelengths, and further including an optic in the beam line which compensate for the walkoff.

31. The laser system of claim 26, wherein the switchable optics include a plurality of filters, and a mechanism for switching a selected one of the plurality of filters into the beam path, and wherein the optic which compensates for the walkoff is a particular one of the plurality of filters which selects for the particular wavelength.