DE4033658A1 - Processing trench edges in semiconductor substrates - using stream of particles, for simple, accurate silicon@ solar cell mfr. - Google Patents

Abstract

Processing an upper edge section of trenches etched in semiconductor substrates with a stream of particles comprises directing an anisotropic stream of particles at a selected angle of incidence W on to the substrate surface so that the lower edge of the trench, which is not to be processed, is shielded by the opposite edge of the substrate surface. USE/ADVANTAGE - The method is simple, accurate, and can be used instead of high-output solar cells made from crystalline Si. The method is simple, accurate and can be used instead of costly lithographic techniques. It can be used for any shape of trench.

Description

Trenches and holes in the surface of semiconductor substrates can be used to hold electrically conductive material Inclusion of insulation material to create a ditch iso or with doped trench walls and with dielectric serve as trench cells.

With increasing integration density of semiconductor components wins in addition to the structuring of horizontal surfaces a vertical structure in importance. The concept of so-called 3-D integration includes the arrangement one above the other NEN of individual components and is for example for top integrated memory components, for example for a 16 M DRAM used.

Another category of devices, the trenches and others Have depressions on their surface represent certain Types of high-performance solar cells made of, for example, crystalline nem silicon. The surface of which is regular Trench or hole pattern "roughened" to the percentage of through Reflection lost for photovoltaic energy conversion To minimize solar radiation. Not a higher density of Generate structures, but to avoid shading losses wrestle and thus increase the efficiency of these solar cells, is also an exact structuring here during production with precise machining steps necessary.

Such a solar cell structure is described, for example, in an article by A. Cuevas et al in IEEE Electron Device Let ters, Vol. 11, No. 1, 1990, pages 6 to 9. The active areas of the surface of this cell have a hole  pattern of inverted pyramids. On continuous walkways with a V-shaped cross-section are punctiform highly doped zones attached, the metal running along these webs strips can be contacted. The endowed areas are by opening punctiform windows in one the whole Surface of the solar cell covering oxide layer and on closing diffusion of dopants (for example from phosphorus-containing glass). These doping windows are only in the upper area of the webs mentioned opened.

The high efficiency of achieved with this type of solar cell However, 26 percent under concentrated light is with one extremely complex manufacturing process connected. It requires on the light incidence side at least four photolithographic Steps and at least one on the back, one Structural fineness down to 2 µm is necessary. Aggravating In addition, some of these lithography steps are over strongly inclined surfaces and some only in the upper ones Flank section of the trenches and holes are to be made.

The object of the present invention is therefore to unite to specify a well-known procedure, with the targeted in the upper Flank sections of trenches are processed can be, and which on a complex photoresist technique waived.

According to the invention, this object is achieved by a method for machining an upper flank section of in half etched trenches with a particle stream, wherein the particle flow is anisotropic with a selected angle of incidence kel W is directed onto the substrate surface such that the lower flank section not to be machined by the in Trench opposite edge of the substrate surface is shaded.

Further embodiments of the invention are the subclaims refer to.  

The method according to the invention is characterized by its on expertise and can be sufficiently anisotropic when used Steps to replace a complex lithography process. The Shading is at a suitable angle of incidence W by the because flanks / substrate edge lying in the direction of incidence acts, which in a sense represents a mask. So that's it possible to process only in an upper flange ken section and the lower flank section to be left out of processing.

The accuracy of the procedure or the amount of contrast the border between unprocessed and machined edges cut depends on the anisotropy of the machining used particle stream, on the nature of the sub stratkanten, from the structure of the flank surfaces and except that of the angle of incidence W and the flank angle FW that the Flank wall forms against the substrate surface.

The method can be used for trenches of any shape will. The setting of the angle of incidence is easy to do take, for example, by tilting the substrate towards the Particle current source or the preferred direction of the particle current. Should individual trenches be used for different trenches? Processing can be excluded, so this can be done by a fold photoresist technique with the help of a compared to the processing suitable resists are covered. The procedure is particularly useful for different processing of Flank sections, or for exclusive processing an upper flank section in corresponding trenches is suitable.

Trenches suitable for carrying out the method can be lowered have right or inclined flanks, the through narrow or extend the knife of a trench can, so that overhanging trench flange ken can be edited. The flank angle FW, the one Trench flank forms against the surface, can over the Gra Bending depths vary, both convex and concave  curved flanks are suitable. Also in the horizontal cross section the trench shape is arbitrary, the term trench in white test meaning is to be interpreted. Round to oval are suitable Trenches that can also be called a hole. Before however, trenches with a rectangular cross section and one are added high ratio of trench length to trench width. The flanks areas on the narrow sides of such trenches do not fall weight, so that a corresponding processing one or includes both of the flanks, which cover the long sides of the trench form. To avoid further and unwanted shadowing when machining the flanks, it is still an advantage if these have no further corners and edges.

The aspect ratio of trenches suitable for processing can be of any height, but must not be too low be chosen so that there are still manageable angles of incidence Shadowing through the opposite substrate edges have a father.

The trench width parallel to the machining direction must not be too large to avoid blurring in the "illustration". In a way, it corresponds to the mask spacing for one Photolithography, which increases with increasing size Un sharpen the image because it creates the negative Influence of scattering and diffraction effects as well as of the isotropic Part of the particle stream is amplified.

All anisotropic particle streams are suitable for use in the process, the processing resulting in an etching or a material deposition, or the processed surface changing chemically or physically. Machining with a particle beam can be: ion beam or plasma etching, plasma deposition, CVD processes, vapor deposition, sputter etching, sputter deposition, implantation, compaction or oxidation. Unless these processes work with a highly anisotropic particle beam, anisotropy can occur be maintained or strengthened through further measures. By applying or improving the vacuum during processing, collisions of the particles in the particle stream are reduced. The best possible vacuum is therefore sought, preferably below 10 ^-5 Torr. If the particle stream consists of charged particles, these are preferably generated with low kinetic energy and then strongly accelerated. In the case of a particle beam composed of uncharged particles, the divergence of the particles striking the substrate decreases with increasing distance of the substrate from the particle current source. Additional devices for bundling the particle stream can be electromagnetic lenses or diaphragms for uncharged particles.

To generate shading, the angle of incidence W klei ner than 90 ° and with sloping flanks additionally smaller than the flank angle FW can be selected. A sharp picture due to the shadowing effect towards particle flow source / trench edge located with steeper and preferably 90 ° impact angle of the particle stream preserved on the trench flank. This happens when the angle of incidence W and the slope of the trench flank FW addie at least in the area of impact of the particle stream to 90 ° or do not deviate too much from this value.

For different applications it may be necessary to to process the smallest upper flank section. In the In this case, the angle of incidence W is as small as possible chooses.

More complicated structures can be created with the invention Procedure by running several anisotropic ones in succession Processing steps are generated, being for different Processing steps selected different angles of incidence W. can be.

An inversion of the working principle is also possible in order to machine a lower flank section. For this purpose, with the aid of the method according to the invention, a first machining step is carried out at a first angle of incidence W 1 in such a way that, compared to a second machining step, the upper flank section behaves differently from the lower flank section shaded in the first machining step. The second processing step can also be carried out according to the invention using a shading effect with a larger angle of incidence W 2 . It is also possible to carry out the second processing step in such a way that the entire substrate surface, with the exception of the upper flank sections, is covered thereby. For example, in the first processing step, a mask can be generated in the upper flank section that is resistant to a second processing step, for example an implantation or an etching step. Also, in the first processing step, a layer containing dopant produced over the entire surface can be removed in the upper flank section, the doping agent being driven into the substrate by increasing the temperature in the second processing step.

With the method according to the invention initially only the in Machining direction of the trench flank machined. In others Trench flanks pointing in the right direction can be repeated of the processing step are detected, the substrate is rotated beforehand by the angle that the new one to be machined Flank with the one edited first. At essentially Lichen extending in the length of these trenches Example 180 ° C.

If a trench has no straight lines in horizontal cross-section lines of demarcation so that no individual flanks differ can be, so the upper flank section in ge entire circumference of the trench evenly processed by who that the substrate during the processing by a vertical to its surface or parallel to the particle stream Axis is rotated.

The following is an example of the method game and the associated six schematic figures explained. Show  

Fig. 1 is a silicon wafer having a V-shape etched grooves in a perspective view;

Fig. 2 shows this wafer in a schematic cross-section during a first anisotropic processing step

FIGS. 3 and 4 steps of the wafer during further processing and

Fig. 5 and 6 show two embodiments of a finished So larzelle in schematic cross section.

Production of a solar cell with a V-shaped trench send surface.

Using conventional photolithography, a strip-like mask is formed on an undoped or slightly n- or p-doped crystalline silicon wafer. This consists, for example, of approx. 10 to 30 µm wide oxide strips that run parallel to each other at a distance of approx. 1 to 2 µm. In the next step, a crystal-oriented anisotropic etching with alkali is carried out, or, as suggested in the Cuevas document mentioned, using ammonium hydroxide. This special type of etching creates trenches with a V-shaped cross-section. The etching is carried out until the trench edges of two adjacent trenches coincide. The entire original substrate surface is thus etched away, the oxide mask being lifted off at the same time. Fig. 1 shows the surface of the thus-treated silicon substrate S with the parallel V-shaped grooves in the substrate surface.

In the upper flank section OFA, p⁺-doping is now generated by an anisotropic oblique implantation of boron atoms or ions containing boron. The oblique implantation, shown in FIG. 2 by the arrows TS _i , it follows at an angle of incidence W 1 , the substrate edge SK shading the lower flank section UFA from the particle stream, as well as the opposite flank GF.

In an oxygen-containing, moist atmosphere, the entire substrate surface is now oxidized at elevated temperature. At the same time, the p⁺ doping generated by angled implantation is driven deeper into the substrate by diffusion and produces doped regions DB with p⁺ doping there. Fig. 3 shows the arrangement after this step. The oxide layer OS is designed so that it can simultaneously serve as an anti-reflective layer for the future solar cell. For this purpose, the thickness and density of the oxide layer is checked and, for example, a layer thickness of 110 nm is set.

By means of an ion beam etching process, a contact window KF is now opened in the oxide layer OS in the region of the upper flank section OFA. For this purpose, an incident angle is selected W 2 for the ion beam etching is preferably W 2 is less than W1. The strip-shaped contact windows KF in the upper flank section, freed from the oxide layer, preferably have a width of at most 1 to 2 μm.

Fig. 4 shows the arrangement immediately at the beginning of the subsequent processing step in which a conductor strip is produced by anisotropic vapor deposition of aluminum over the contact window KF. The particle beam TS used for this purpose is directed onto the substrate S with an angle of incidence W 3 , W 3 being set at least greater than W 2 .

Fig. 5 shows the completed solar cell with the vapor-deposited aluminum conductor stripe LS. As can be seen from the figure, the conductor strips LS slightly overlap the oxide mask OM.

For structuring the opposite of the incidence of light There are several options for the back of the substrate. In the darge illustrated embodiment are the highly doped areas HB formed as point contacts, corresponding to that of Cueva's proposed structure. This is the back of the wafer Provide a thick oxide layer O over the entire surface. In these punctiform contact windows are then etched. The endowed  Areas can be created by implanting dopant and closing driving are generated, the driving Exercise process in the process of structuring the solar cell len front can be integrated. It is also possible Application of a dopant-containing layer, for example a layer of phosphor silicate glass, which acts as a diffusion source for can serve the collection process. After collecting the Thursday animal substance and removal of the glass layer becomes full surface generates a back electrode RE, which contains the highly doped areas contacted in the contact windows.

Another embodiment relates to a solar cell, the back of which is structured similarly to the front. FIG. 6 shows this arrangement, in which the trench etching, oblique implantation, oxidation with driving in of the dopants and opening of the strip-shaped contact windows are carried out analogously to the structuring of the front side. An n-doping material, for example phosphorus, is used as the dopant. Since the back of the solar cell is not intended for the incidence of light, the entire surface of the back electrode can also be deposited over the contact window and the oxide layer in this embodiment too.

This second embodiment, in addition to the front also the back of the solar cell through V-shaped trenches is structured, only needed for manufacture per surface a lithography step to produce the stripe-shaped Trench etching oxide mask. All other structuring Steps use the method according to the invention, in which in simply by setting an angle of incidence strip-like structuring of the here only from the trench flanks existing substrate surface succeed. By saving the Lithography steps will unite the overall manufacturing process folds and shortens. Through the process it is possible to Shading of active solar cell surface by narrow off keeping the conductor strips extremely low and the Conductor strips also to posi on the upper edges of the trenches tion, which has a high reflection even without conductor strips  have potential and therefore little for light trapping and Contribute to the efficiency of the solar cell.

For solar cells manufactured according to the invention are therefore similar expected high efficiencies, as with those in the article described by Cuevas point contact solar cells, which under normal lighting 21 percent, under concentrated light however, reach 26 percent.

Claims (10)

1. Method for machining an upper flank section of trenches etched in semiconductor substrates with one particle current, with an anisotropic part in one processing step chenstrom at a selected angle of incidence W such the substrate surface is directed that the not too bear finishing lower flank section by the in the trench overlying edge of the substrate surface is shadowed. 2. The method according to claim 1, characterized records that the particle stream is a plasma. 3. The method according to claim 1 or 2, characterized ge indicates that the particle stream is an ion beam is. 4. The method according to claim 1, characterized records that the particle stream is a metal vapor. 5. The method according to claim 1, characterized records that an Implanta tion for generating a doping is carried out. 6. The method according to any one of claims 1 to 5, characterized characterized that several anisotropic processing steps in succession and if necessary under different which angles of incidence are carried out. 7. The method according to at least one of claims 1 to 6, characterized in that at least a processing step from a different angle of incidence of 180 ° minus W is repeated. 8. The method according to any one of claims 1 to 7, characterized characterized in that the semiconductor substrate one or between two treatment steps by one Axis is rotated perpendicular to the surface of the substrate.   9. The method according to any one of claims 1 to 8, characterized characterized in that the upper flank section of the trench in a first processing step its properties is changed to a second subsequent processing step serves as a mask, so that in the second processing step only the lower flange ken section is edited. 10. The method according to at least one of claims 1 to 9, characterized in that for the manufacture of a solar cell

• the semiconductor substrate has a plurality of parallel rows of V-shaped etched trenches (V-grooves),
• in a first processing step in the upper flank section UFA of the trenches by means of an oblique implantation at an angle of incidence W 1 a doping of a first conductivity type is generated,
• an oxide layer OS is produced in a tempering step on the entire surface of the substrate S, doped regions DB being produced by driving in the dopant,
• over the doped regions DB by ion beam etching at a W 1 -like angle W 2 contact window KF in the oxide layer OS are opened,
• - Are generated by oblique vapor deposition at an angle of incidence W 3 of an electrically conductive metal conductor strip LS over the contact window KF, and that
• - Highly doped areas of a second conductivity type are provided on the back of the substrate, which are contacted by an overlying oxide layer contact window with an all-over back electrode.