FIELD OF INVENTION
The present invention relates to a process for Plasma texturing of multicrystalline silicon (mc-Si) wafers for development of solar cells. More particularly the invention relates to a process where higher reflection losses of the incident light are significantly reduced.
BACKGROUND OF INVENTION
The solar cells industry is growing at a very fast pace and solar cells based on the crystalline Si technology have the largest share (>85%) of the total global solar cell production. The major challenge is to bring the cost down by either enhancing the solar cell efficiency or using cheaper input material. Solar cells based on crystalline Si technology can be produced either by using mono or multi crystalline Si wafers. Multicrystalline Si wafers offer a cost advantage over mono

crystalline but give slightly lower efficiency (~1% absolute). One of the factors for low efficiency is the non-textured surface resulting in loss of incident light. Mono crystalline Si wafers can be textured easily using alkaline solution and the diffused reflectance can be reduced to 4-5% while in multicrystalline Si wafers, good quality texturing is not possible due to different orientation of the grains using alkali solution. Texturing with alkali or acid solutions result in diffused reflectance in the range of 14-15 %. Plasma texturing technique is being developed globally where texturing of the mc-Si wafer is possible and can be used to reduce diffused reflection to 4-5% without damaging the active device surface. In this technique the wafers are placed on a ground electrode and the process chamber is evacuated to less than ~50 mili Torr pressure. A mixture of SF6+O2 gas is introduced into the process chamber and radio frequency (RF) plasma is struck between the powered and ground electrode exposing the top surface of the wafers to plasma at around 245-250 mili Torr pressure. Plasma is the ionized state of gases where gases break into charged species. In the present invention, RF (radio frequency) power provides this

energy and ionizes the gases. These charged species are directed under the influence of electric field towards electrodes and react. While developing this technique, a number of trials were performed by altering a number of process parameters. It was observed that the surface of the wafer was not darkening just by placing the wafers on a ground electrode and striking the plasma. An additional process step was introduced where the wafers were exposed to atmosphere intermittently. This process step showed remarkable improvement in texturing the multicrystalline silicon (mc-Si) wafers.
OBJECTS OF THE INVENTION
Therefore, an object of the invention is to propose a process for plasma texturing of multi-crystalline silicon wafers for development of Solar Cells which is capable of producing good quality texturing of mc-Si wafers.

Another object of the invention is to propose a process for plasma texturing of multi-crystalline silicon wafers for development of Solar Cells which is able to reduce significantly the reflection losses of the incident light.
A further object of the invention is to propose a process for plasma texturing of multi-crystalline silicon wafers for development of Solar Cells which is enable to produce higher output power of plasma textured wafers in comparison to existing textured wafers.
SUMMARY OF THE INVENTION
Multicrystalline Si (mc-Si) wafers offer a cost advantage over mono crystalline but give slightly lower efficiency (~1% absolute). One of the factors for low efficiency is the non-textured surface which results in higher reflection losses of the incident light. Mono crystalline Si wafers can be textured easily using alkaline solution while in multicrystalline Si wafer, the chemical texturing does not produce good quality texturing. Plasma texturing technique has the potential to

texture mc-Si wafers and is being developed globally for large area reactors to match the high throughput requirement of the photovoltaics (PV) industry. In this technique the wafers are exposed to plasma of SF6+02. While developing this process at BHEL-ASSCP it was observed that an intermittent air exposure to wafers during its plasma texturing cycle produced amazing results. All the fifteen mc-Si wafers of area 6" x 6" each turned uniformly dark. Later on, the plasma time and exposure time were optimized as 5 min plasma + 5 minutes exposure to air at 10 Torr + 5 minutes plasma. The diffused reflectance of the surface could be reduced to 4-5 % in a controlled manner. Hundreds of mc-Si wafers were textured and converted to solar cells in this manner. The results showed that the output power of plasma textured is relatively higher in comparison to alkali textured wafers.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The drawing shows the arrangement of placing Si wafers (1) on the ground electrode (3). The vacuum chamber (2) is maintained at a low pressure in the

range 245-250 mili Torr with gas mixture of SF6+02 and RF power is applied. The plasma is ignited and Si wafer upper surface is exposed to it for reaction.
Fig.l - Shows arrangement for texturing silicon wafers in SF6+02 plasma.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Solar cells based on crystalline Si technology can be produced either by using mono or multi crystalline Si wafers. Multicrystalline Si wafers offer a cost advantage over mono crystalline but give slightly lower efficiency (~1% absolute). One of the factors for low efficiency is the non-textured surface which results in higher reflection losses of the incident light. Mono crystalline Si wafers can be textured easily using alkaline solution and the diffused reflectance can be reduced to 4-5% while in multicrystalline Si wafers, good quality texturing is not possible due to the presence or grains of different orientations using alkaline solutions. Plasma texturing technique can be used to texture the mc-Si

wafers. Using this technique cost advantage, over chemical method, can be realized in two ways. One by improving the conversion efficiency of solar cells as good quality texturing results in better photogenerated current and other by avoiding the use of costly chemicals and problem of their disposal, which is being practiced in industries. Plasma texturing technique is being developed globally for large area reactors to match the high through put requirement of PV industry. In this technique the wafers are placed on a ground electrode and the process chamber is evacuated to less than ~50 m Torr pressure (about 40-50 mili Torr). A mixture of SF6+02 gases is introduced into the process chamber and radio frequency (RF) plasma is struck between the powered and ground electrode exposing the top surface of the wafers to plasma. While developing this technique a number of trials were performed by changing process parameters such as process pressure, RF power, process time and ratios of gas mixture. It was observed that at gas flows of 335 seem (standard cubic centi meter) of SF6 and 205 seem of 02 and process pressure of ~ 250 mili Torr the surface of the wafer started darkening after 5-7 min of exposure to plasma but

not uniformly. This could be observed with naked eye inspection. In order to increase the exposure to plasma, the wafers were reinserted into the process chamber for 5-7 min. It was observed that another exposure to plasma darkened the wafers. In the next run the wafers were exposed to plasma continuously for twelve minutes but wafers came out without any texturing. After repeating number of experiments, it was concluded that exposure to atmosphere intermittently was doing the trick for which scientific explanations still need to be found out. The exposure to atmosphere was tried by venting the process chamber to 10 Torr after initial exposure of 5-7 minutes to plasma followed by another exposure to plasma at the optimized process conditions. All the fifteen wafers became jet black. Later on the plasma time and exposure time was optimized as 5 min plasma + 5 minutes exposure to air at 10 Torr + 5 minutes plasma. This process step showed remarkable improvement in making the wafer surface dark uniformly over the entire area of the 15 mc-Si wafers of size 6" x 6"

each. The diffused reflectance of the surface could be reduced in the range of 4-5 % in a controlled manner. Hundreds of mc-Si wafers have been textured and subjected to damage removal etch before converting them into solar cells in a manufacturing unit using conventional diffusion and printing process. The final results revealed that the output power of cells using the above technique was better when compared with alkali textured cells.
Thus the process for plasma texturing of multicrystalline silicon wafers has the following advantages over the prior art.
1. A unique process sequence for uniform texturing of mc-Si wafers for solar cell process using SF6+O2 plasma with intermittent exposure to air leading to a diffused reflectance of 4-5%.
2. An industrial process with high throughput for texturing 15-20 large size (6" x 6") mc-Si wafers for solar cells development.

3. A technique for uniform and good quality texturing of mc-Si wafers in a controlled manner.
4. An industrial dry process for texturing 15-20 large size (6" x 6") mc-Si wafers for producing higher efficiency solar cells in comparison to wet chemical texturing techniques.
5. The process is able to reduce significantly the reflection losses of the incident light.
6. The process is capable to produce higher output power of plasma textured solar cells in comparison to existing prior art.

WE CLAIM
1. A process for plasma texturing of multicrystalline silicon wafers for development of solar cells comprising; placing the Silicon wafers (1) on a ground electrode (3); evacuating the process chamber (2) to a reduced pressure of 40-50 mili Torr;
introducing (6) into the process chamber a mixture of SF6+O2 gas to expose the wafers to plasma of SF6+O2 (4);
striking; radio frequency (RF) plasma between the powered (5) and ground electrode exposing the top surface of the wafers to Plasma at around 245-250 mili Torr pressure; Characterized in that,
the wafers are exposed to SF6+O2 plasma (4) for 5 minutes and then exposed to atmosphere at ~10 Torr for about 5 minutes and again exposed to SF6+O2 plasma for 5 minutes to obtain improved texturing of wafers wherein the diffused reflectance of the surfaces of the wafers is reduced by 4-5%.

2. A process as claimed in claim 1, wherein the wafers are exposed to atmosphere at a pressure of 10 Torr to make the wafer surface uniformly darkened.

ABSTRACT

The invention relates to a process of placing the silicon wafers on a ground electrode when the process chamber is evacuated to a reduced pressure of 40-50 mili Torr. The wafers are then exposed to plasma of SF6+O2 in the process chamber for 5 minutes and then exposed to atmosphere for about 5 minutes and again exposed to SF6+O2 plasma for 5 minutes to obtain improved texturing of wafers wherein the diffused reflectance of the surfaces of the wafers is reduced by 4-5%.