WO2009098109A1

WO2009098109A1 - Method of manufacturing a thin silicon slice

Info

Publication number: WO2009098109A1
Application number: PCT/EP2009/050338
Authority: WO
Inventors: Rainer Klaus Krause; Gerd Pfeiffer; Xiaoyan Shao
Original assignee: International Business Machines Corporation
Priority date: 2008-02-07
Filing date: 2009-01-14
Publication date: 2009-08-13
Also published as: TW200949930A

Abstract

The invention relates to a method of manufacturing a thin silicon slice (114), the method comprising: - providing a silicon block (100), - applying a protection pattern (102) to a surface of the silicon block (100), the protection pattern (102) defining the lateral slice boundaries, - applying an etchant to the surface comprising the protection pattern (102), the etchant being adapted to induce silicon trenches (104) in the silicon block (100), the silicon trenches (104) resulting from a - removing of the silicon located below the protection pattern (102), the removing of the silicon resulting in said thin silicon slices (114).

Description

D E S C R I P T I O N

Method of manufacturing a thin silicon slice

Field of the invention

The invention relates to a method of manufacturing a thin silicon slice, a method of manufacturing a solar cell and a method of manufacturing a semiconductor device.

Background

Solar cells are devices which convert light energy into electrical energy by the photovoltaic effect. Today, there is a high demand on solar cells since solar cells have many applications. For example, solar cells are used for powering small devices like calculators. Further, an increasing demand in solar cells is due to their usage in vehicles and satellites. Solar cells even have the potential of substituting state of the art power plants since solar cell technology is a technology branch favored by the society nowadays. The reason for this favoring can be found in the fact that electricity produced by solar cells is renewable ^λclean' electricity.

Solar cells comprise a semi conducting material which is used to absorb photons and generate electrons via the photovoltaic effect. A semi conducting material typically used for manufacturing solar cells is silicon. In solar cells, silicon can be either used as mono or polycrystalline silicon.

Production of solar cells using silicon technology is nowadays driven by the high material cost of silicon wafers, which contributes roughly up to 40% of the cost of the cell itself. The reason for that is that currently the silicon wafer thickness used in the solar industry is in the range from 180 μm to 330 μm. Individual wafers are typically obtained from a silicon ^λingot' . Such silicon ingots are cut into individual wafers using a wire saw which typically consumes an additional >150 μm per cut. Therefore, the total amount of silicon consumed per wafer is between 330 μm and 480 μm, such that the material loss due to the cutting is in the unacceptable large range of 30% up to 45%.

Therefore, there is a need to provide a method of manufacturing silicon slices, wherein the silicon loss is reduced to a minimum.

Summary of the invention

In accordance with the present invention there is provided a method of manufacturing a thin silicon slice, the method comprising providing a silicon block, applying a protection pattern to a surface of the silicon block, the protection pattern defining the lateral slice boundaries and applying an etchant to the surface comprising the protection pattern, the etchant being adapted to induce silicon trenches in the silicon block, the silicon trenches resulting from a removing of the silicon located below the protecting pattern, the removing of the silicon resulting in said thin silicon slices.

The method according to the invention has the advantage, that ultra thin silicon slices can be manufactured which reduces the silicon loss during the manufacturing process to a minimum. The thickness of the silicon slices can be easily predefined by simply adjusting the lateral slice boundaries defined by the protection pattern.

In accordance with an embodiment of the invention, during the etching process silicon bars are interconnecting neighboring silicon slices, wherein the bars are located on the bottom of the silicon trenches, wherein the application of the etchant is stopped in case said bars are reaching a predetermined thickness. The application of the etchant results in a comb- like silicon structure. Preferably in an industrial process the etching process is stopped after a predetermined etching time period, which can be determined depending on the etch rate .

The resulting comb-like silicon structure has the feature, that after termination of the etching process the thin silicon slices are still attached to a base. This allows for an easy handling of the silicon slices, for example for the purpose of cleaning or annealing the silicon slices.

In accordance with an embodiment of the invention, the predetermined thickness of the interconnecting silicon bars is less than the thickness of the silicon slices. This minimizes the silicon loss during the slice production process.

In accordance with an embodiment of the invention, the method further comprises removing of the remaining silicon bars, the removing being performed by mechanical ablation and/or by thermal ablation. For example, ion bombardment can be used to remove the silicon bars or a laser scribing process can be used.

In accordance with an embodiment of the invention, the laser scribing is for example applied parallel to the trenches or perpendicularly to the trenches at the level of the remaining silicon bars. Using laser scribing has the advantage, that the remaining silicon bars can be removed from the comb-like silicon structure in a highly accurate manner. Also, by laser scribing clean and smooth cutting edges are obtained.

Optionally or additionally before removing of the remaining silicon bars it may be required to also remove in a separate treatment step the remaining material originating from the protection pattern which was previously applied to the surface of the silicon block.

In accordance with an embodiment of the invention, the method further comprises cleaving the silicon slices of the comb-like structure which results in said thin silicon slices. Cleaving in combination with mechanical ablation or thermal ablation has the advantage that with minimal effort a separation of the individual silicon slices from the comb-like silicon structure is possible. For example the laser scribing process does not have to be performed in such a highly accurate manner that the remaining silicon bars are completely removed - a weakening of the silicon bars is sufficient for a subsequent cleaving step.

Due to a preferred usage of monocrystalline silicon, clean lines of breakage can be obtained resulting in almost perfect silicon slices. Optionally, in addition further etching processes can be applied to the thin silicon slices which will further smooth a possible roughness of the thin silicon slices, the roughness resulting from the previous etching process or the cleaving process.

In accordance with an embodiment of the invention, the etchant comprises an oxidizer. The oxidizer has preferably a more positive redox potential than Si⁴VSi. For example, the oxidizer is selected from the group of H2O2, Ag⁺, Cu²⁺, Pt⁴⁺, Fe³⁺. The usage of an additional oxidizer has the advantage, that the etching process is accelerated which is an important feature for industrial mass production processes.

In accordance with an embodiment of the invention, the etchant comprises diluted hydrofluoric acid. In accordance with a further embodiment of the invention, the protection pattern comprises a noble metal. Preferably, the protection pattern comprises silver (Ag) . Ag in combination with diluted hydrofluoric acid has the advantage that during the etching process silicon located below the silver protection pattern is dissolved and transported through the protection pattern into the etching bath. This requires that in accordance with a further embodiment of the invention the thickness of the protecting pattern is selected to permit a diffusion of Siθ2 and/or further reaction products through the protecting pattern, the Siθ2 and the further reaction products emerging from the etching process below the protecting pattern. In order to facilitate a removing of Siθ2 and the reaction products, a certain flow in the etching bath is preferred. This enhances the efficiency of the etching process which significantly determines the manufacturing process time.

In accordance with an embodiment of the invention, the aspect ratio of the trench depth versus the trench width is more than 1000:1.

In accordance with an embodiment of the invention, the thickness of the thin silicon slices is in between 5 μm to 70 μm, preferably 20 μm. In accordance with a further embodiment of the invention, the protection pattern is applied to the surface of the silicon block by sputtering, for example using hard mask technologies, and/or lithographic techniques and/or printing techniques.

In another aspect, the invention relates to a method of manufacturing a solar cell, the method comprising manufacturing a set of thin silicon slices, the manufacturing being performed in accordance with the method of manufacturing thin silicon slices according to the invention. The method of manufacturing a solar cell further comprises doping the set of thin silicon slices, providing a support and attaching the set of doped thin silicon slices to the support. Finally, the thin silicon slices are electrically connected.

In accordance with an embodiment of the invention, the doping of the set of silicon slices further comprises providing a carrier plate, the carrier plate comprising an array of indentations, each indentation of the set of indentations being adapted for receiving a silicon slice. The method further comprises insertion of the silicon slices into said indentations and doping of the silicon slices.

By using a carrier plate which comprises an array of indentations, for example by pick and place technologies the thin silicon slices can be easily positioned on the carrier plate and also transported within a production line to a doping facility without the risk of moving individual silicon slices on top of each other due to vibrations and shocks resulting from the transport of the carrier plate.

In another aspect, the invention relates to a method of manufacturing a semiconductor device, the method comprising manufacturing a set of thin silicon slices, the manufacturing being performed in accordance with the method of manufacturing a thin silicon slice according to the invention. The method further comprises bonding the silicon slices on top of each other .

In general, the thin silicon slices can be used to replace silicon material used in all kinds of state of the art silicon technologies. For example sensor applications using defined silicon slices can be manufactured using the thin silicon slices, which reduces the production costs significantly and further enhances the sensitivity of such sensors. Also nano structures can be manufactured based on an assembly of said thin silicon slices.

Brief description of the drawings

In the following preferred embodiments of the invention will be described in greater detail by way of example only making reference to the drawings in which:

Fig 1: illustrates the method steps according to the invention of manufacturing a thin silicon slice,

Fig. 2: illustrates the scribing process used for manufacturing of a thin silicon slice,

Fig. 3: illustrates a doping process usable for manufacturing for example a solar cell,

Fig. 4: is a flow chart illustrating the method steps according to the invention of manufacturing a thin silicon slice.

Detailed description

In the following similar elements are depicted by the same reference numerals.

Fig. 1 illustrates the method steps according to the invention of manufacturing a thin silicon slice. In step a), a silicon block 100 is provided. Preferably, the silicon block 100 is a monocrystalline silicon block. In step b) , a protection pattern is applied to a surface of the silicon block, the protection pattern defining lateral slice boundaries. In the embodiment of fig. 1, the protection pattern consists of a set of silver stripes 102 which are arranged in parallel on top of the silicon block 100.

In step c) , the application of the protection pattern to the surface of the silicon block is shown in a side view. Preferably, the direction 118 perpendicularly to the surface of the silicon block 100 and perpendicularly to the silver stripes 102 is the (1-1-0) or (1-0-0) lattice direction of the monocrystalline silicon block 100. This has the advantage that a subsequent etching process illustrated in step d) yields a set of trenches 104 with smooth side wall surfaces. The trenches are originating from a cut of the silicon block 100 in said lattice directions. Hence, a minimum surface roughness within the trenches 104 is guaranteed.

The thickness of the silver stripes 102 is selected to permit a diffusion of Siθ2 and Siθ2 reaction products 106 through the silver stripes 102. For example, in case the etching process illustrated in step d) is performed in an etching bath, the flow conditions within the etching bath are selected in such a way that the reaction products 106 are transported effectively away from the trenches 104, such that permanently fresh etching fluid like diluted hydrofluoric acid can penetrate into the trenches 104. This ensures that in a homogeneous manner silicon material originally located below the silver stripes 102 is dissolved and transported away such that the silver stripes 102 are moving from the surface of the silicon block 100 down to the bottom of the silicon block, leaving behind the trenches 104.

Step e) shows the final etched silicon block 100, wherein the silicon block has a comb-like silicon structure. The reason for the comb-like silicon structure is that the etching process was previously stopped such that silicon bars 116 which interconnect neighboring silicon slices are remaining. The bars 116 are located on the bottom of the silicon trenches 104. The control when to stop the etching process in step d) is for example performed by timing the application of the etchant to the silicon block 100, wherein the timing depends on the etching rate. An optimal timing can be determined previously once by experimental analysis.

The stopping of the etching process leaves the silicon bars 116 behind, which have typically a thickness 110 which is less than the thickness 112 of the silicon slices. The silicon slices are made up by the remaining silicon located in between adjacent trenches 104.

By applying in step e) for example a laser beam 108 through the trenches 104 to the silicon bars 116, the silicon bars are evaporated, which results in a set of individual thin silicon slices 114 shown in step f) . By using of for example vacuum tweezers, the individual thin silicon slices 114 can then be picked and placed onto a respective carrier for further processing.

Fig. 2 illustrates the scribing process used for the manufacturing of a thin silicon slice in more detail. The laser beam for cutting the silicon comb into individual silicon slices can be either applied through the trenches from top to bottom of the silicon block 100, which corresponds in fig. 2 to an application of the laser beam to the bars 116 into the direction 200. Or alternatively it is possible to at least partially remove the bars 116 by applying a laser beam perpendicularly to the trenches 104 at the level of the remaining silicon bars. In fig. 2 the application of a laser beam perpendicularly to the trenches is indicated by the laser beam directions 202 and 204.

Typical dimensions of the comb-like silicon structure illustrated in fig. 2 are a height of 1 cm, a depth of 2 cm and a total length in between 5 to 10 cm. However, these dimensions can be varied in a broad range by simply modifying the geometrical dimensions of the silicon block 100 and also by adjusting the geometrical structure of the protection pattern .

Regarding production and cost effectiveness, an increase of the silicon block length and a decrease of the silicon block height and depth are favored. The reason is, that a reduced silicon block height decreases the time required for etching the trenches 104 due to a reduced trench depth. Further, an increased length of the silicon block 100 allows for a manufacturing of more individual silicon slices 114 in one etching step, which in the present manufacturing method is the most time consuming process.

Fig. 3 illustrates a doping process usable for manufacturing a solar cell. The doping process illustrated in fig. 3 already assumes the availability of the thin silicon slices produced by means of the method according to the invention. By pick and place technology, the thin silicon slices 114 are placed on a carrier plate 300. Such a process can be performed automatically, which also includes sorting of the individual silicon slices 114 by their size, quality etc. The carrier 300 is a quartz glass carrier since it typically needs to resist high temperatures used for carrying out the doping process.

In order to facilitate a placing of the individual silicon slices 114 onto the carrier plate 300, the carrier plate 300 comprises a multitude of indentations, each indentation having the size for receiving exactly one silicon slice 114.

After having positioned the individual silicon slices 114 into the indentations of the carrier plate 300, a state of the art doping process is carried out. For example, a dopant 304 like for example phosphor is applied to the silicon slice surfaces through spray techniques.

After this application in step 3a) , in step 3b) a high temperature treatment is performed which leads to a diffusion of the dopant from the surface of the individual slices to the bulk of the slices into the direction 302.

Fig. 4 is a flowchart illustrating the method steps according to the invention of manufacturing a thin silicon slice. In step 400 a silicon block is provided. Step 400 is followed by step 402 in which a protection pattern is applied to a surface of the silicon block. Preferably, the protection pattern consists of silver lines which are spaced apart from each other, the spacing defining the final slice thickness. In an alternative embodiment, instead of using straight silver lines, it is also possible to use a rectangular mesh of silicon lines. In this case the silver lines encase rectangles, each rectangle defining the final thickness and width of a silicon slice. This means, that large sized silicon blocks can be used for manufacturing of a multitude of small, thin silicon slices.

After carrying out step 402, in step 404 an etchant, for example diluted hydrofluoric acid comprising an oxidizer, is applied to the silicon block. Since the etchant is only etching the silicon of the silicon block in the presence of the protection pattern, the complete silicon block comprising the protection pattern can be immersed into an etching bath: an etching and removing of silicon will only occur below the protection pattern itself. The application of the etchant in step 404 can be further optimized by using ultrasonic or megasonic excitations and etch bath recirculation, which optimizes the etching conditions and minimizes the process time .

In case in step 406 a stop condition is met, the method continues with step 408, which is a laser scribing step. In case in step 406, the stop condition is not met, the method returns to step 404 which is a continuation of the application of the etchant to the silicon block. The stop condition is for example that the silicon bars which are located on the bottom of the silicon trenches and interconnecting neighboring silicon slices are reaching a predetermined thickness. As already mention above, in case in step 406 the stop condition is met, the method is continued with step 408, the laser scribing step. By means of the laser scribing step the individual silicon slices interconnected by the silicon bars can be separated from each other. By means of pick and place techniques, the individual silicon slices can be handled for further manufacturing steps.

Not shown in fig. 4 is an additional annealing step which further can be used to smoothen the surface of the thin silicon slices. However, as already mentioned above, the etching direction of the silicon block is selected in such a way that the etching is being performed in a lattice direction of the monocrystalline silicon block which permits a smooth edged silicon slice surface.

L i s t o f R e f e r e n c e N u m e r a l s

100 silicon block

102 Ag stripe

104 trench

106 Siθ2 reaction products

108 laser beam

110 thickness

112 width

114 silicon slice

116 bar

118 direction

200 direction

202 direction

204 direction

300 carrier plate 302 direction 304 dopant

Claims

C L A I M S

1. A method of manufacturing a thin silicon slice (114), the method comprising:

- providing a silicon block (100),

- applying a protection pattern (102) to a surface of the silicon block (100), the protection pattern (102) defining the lateral slice boundaries,

- applying an etchant to the surface comprising the protection pattern (102), the etchant being adapted to induce silicon trenches (104) in the silicon block (100), the silicon trenches (104) resulting from a - removing of the silicon located below the protection pattern (102), the removing of the silicon resulting in said thin silicon slices (114) .

2. The method of claim 1, wherein during the etching process silicon bars (116) are interconnecting neighbouring silicon slices (114), the bars (116) being located on the bottom of the silicon trenches (104), wherein the application of the etchant is stopped in case said bars (116) are reaching a predetermined thickness (110) with the application of the etchant resulting in a comb-like silicon structure.

3. The method of claim 2, wherein the predetermined thickness (110) of the interconnecting silicon bars (116) is less than the thickness (112) of the silicon slices (114) .

4. The method of claim 2 or 3, further comprising removing of the remaining silicon bars (116), the removing being performed by mechanical ablation and/or by thermal ablation .

5. The method of claim 4, wherein the thermal ablation is performed by laser scribing.

6. The method of claim 5, wherein the laser scribing is applied parallel (200) to the trenches (104) or perpendicularly (202; 204) to the trenches (104) at the level of the remaining silicon bars (116) .

7. The method according to any of the previous claims 2 to 6, wherein the method further comprises to cleave the silicon slices (114) of the comb like structure resulting in said thin silicon slices (114) .

8. The method according to any of the previous claims, wherein the etchant comprises an oxidizer.

9. The method of claim 9, wherein the etchant comprises diluted hydrofluoric acid.

10. The method of claim 9, wherein the oxidizer has a more positive redox potential than Si⁴VSi.

11. The method of claim 10, wherein the oxidizer is selected from the group of H₂O₂, Ag⁺, Cu²⁺, Pt⁴⁺, Fe³⁺.

12. The method according to any of the previous claims, w- herein the aspect ratio of the trench depth versus the trench width is more than 1000:1.

13. The method according to any of the previous claims, wherein the thickness (112) of the thin silicon slices

(114) is in between 5 μm to 70 μm, preferably 20 μm.

14. The method according to any of the previous claims, wherein the protection pattern (102) comprises a noble metal .

15. The method according to any of the previous claims, wherein the protection pattern (102) is applied to the surface of the silicon block (100) by sputtering and/or lithographic techniques and/or printing techniques.

16. The method according to any of the previous claims, wherein the thickness of the protection pattern (102) is selected to permit a diffusion of Siθ2 and/or further reaction products through the protection pattern (102), the Siθ2 and the further reaction products emerging from the etching process below the protection pattern (102) .

17. A method of manufacturing a solar cell, the method comprising :

- manufacturing a set of thin silicon slices (114), the manufacturing being performed in accordance with any of the previous claims,

- doping the set of thin silicon slices (114),

- providing a support,

- attaching the set of doped thin silicon slices (114) to the support,

- electrically contacting the thin silicon slices (114) .

18. The method of claim 18, wherein doping of the set of silicon slices (114) further comprises:

- providing a carrier plate (300), the carrier plate

(300) comprising an array of indentations, each indentation of the set of indentations being adapted for receiving a silicon slice (114), - insertion of the silicon slices (114) into said indentations,

- doping of the silicon slices (114) .

19. A method of manufacturing a semiconductor device, the method comprising:

- manufacturing a set of thin silicon slices (114), the manufacturing being performed in accordance with any of the previous claims 1 to 16,

- bonding the thin silicon slices (114) on top of each other .