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The XRG-800 generator produces radicals, i.e. excited, uncharged molecule
parts. The radical generator offers extremely high yield (>95%) of all gas
molecules are converted into radicals), extremely high plasma density
(>60W/cm3) as well as simple and reliable operation.
The radicals have many applications, some of the most important for Solar
wafer are: -
Damage Etching
Solar wafers made of polysilicon, are processed very thin, they are
cut to 150 - 200 microns thickness then etched for a stock removal of 10 -
25 microns for damage removal. In dry chemical etching, the removal of a
10 - 25 micron thick layer is sufficient, the challenge for the equipment
manufacturer is to meet the required throughput of 200 - 200 substrates
per hour.
Plasma process damage etch of whole surface of "as cut" wafers with
10 - 20 µm removal, both sides without ion damage. High rate etch for removal
of grinder induced damage and for wafer thinning to 130 µm or even 70 µm. A
two steps process: the wafers are ground to 200 µm from their original
thickness and then damage etched and thinned by high rate etching to the
required thickness of 70 µm. Healing of cracks in the edges of EFG (Edge-defined
Film-fed-Growth)-wafers for improvement of breaking
strength. The laser cut of EFG substrates can cause cracks throughout the entire
substrate thickness of approx. 310 µm. The improvement in breaking strength
comes from rounding the inner edges of the cracks by radicals thus eliminating
the danger of breakage. After that, the EFG wafer will be clean. The residues of
molten silicon, 10 - 100 µm in size, which come from laser cutting, are removed
from the surface by radicals and without attacking the surface itself.
Secon solution is the XCD - 7XX system. For processing of 125 x 125 mm2
substrates, they are placed in boats with a separation distance of 1 cm between
the substrates and on a square base of 140 x 140mm. Each boat holds 10
substrates and are made of anodized aluminum and are not subject to wear. The
process chamber of the XCD - 7XX holds 200 substrates in four rows at 5 boats
each. The required etching takes 3.3 minutes at an etch rate of 3.3 micron/min
with the total cycle taking 12 minutes (etching, evacuation, handling), the
hourly throughput is therefore 1000 substrates.
When etching with microwave generated radicals, fluorine containing gases
such as SF6 are almost completely dissociated in the microwave
generator, so that the resulting F-radicals from volatile SiF4
together with the Si of the substrate. This SiF4 is then pumped out
by means of a vacuum pump and neutralized in the gas scrubber. The substrates
are not damaged by ion bombardment, because the anisotropic etching functions
without application of high bias voltages.
The advantages of the dry chemical etch process in comparison to wet
processes are:
- Environmentally friendly (the reaction products are incomparably lighter
and therefore cheaper to dispose of than those resulting from wet etch
process)
- Cleanliness - dry etch processes are cleaner than wet etch processes
- Easier process control
- Better reproducibility and reliability
- A Silicon removal of only 30 - 40% of the wet etch figure is required to
achieve the same qualities damage removal and tensile strength improvement
- Reduced wafer breakage by reducing the handling of the brittle solar
substrates
- Lower cost of ownership
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Si3N4 Antireflective Coating
Monocrystalline and Polycrystalline solar substrates can be coated with
Si3N4 on the active side, this antireflective layer
increases the efficiency of the cell. The refractive index of the nitride
is almost an ideal figure for optimizing the absorption of light passing
from air to silicon.
Secon has developed the machine XED - 7XX microwave system for deposition of
Si3N4 layers on solar wafers, 104 (125 mm x 125 mm)
substrates per process, 6 processes per hour, 624 substrates per hour. During
the deposition process in the XED - 7XX, Si3N4 layers from
N-radicals are produced, which are gained in the high density microwave plasma
of the microwave generator XRG - 800 by dissociation of N2. The
radical generator is able to concentrate the necessary energy in the plasma for
the dissolution of the N2 triple-compound. This creates highly active
nitrogen without having to use ammonia with it's disadvantages (i.e. increased
H-bonds to Si within the Nitride layer).
The N-nitride react at the substrate surface with SiF4 to Si3N4.
At substrate temperatures of ~ 260 ºC and with bias voltage applied, the
deposition rate is ~ 300 nm/min. The Si3N4 layers show
excellent values for density, hydrogen bonds and stoichiometry, especially
considering the low generation temperature.
Si3N4 can be deposited with correct stoichiometry and a
refraction index of 2 or with higher Si content and higher refraction index
respectively. The 3 low temperature process steps are:
- SF6 / N2O microwave etch step for surface cleaning
and activation
- H2 - RIE - plasma for the adsorption of a sufficient number of
H-radicals by the Si surface for saturation of free Si valences
- N2 / SiN3 deposition step, where the N-radicals come
from the dissociated N2 directly. The high efficiency Secon
radical generator can break the triple bond of N2. This makes NH3
avoidable and results in pinhole free layers.
Texturing of polysilicon substrates for utmost equal reflectivity by
increasing the surface roughness. This is done by etching small cones and pyramids
into each crystal of the surface. Moreover, Secon Dry etcher system
capable to process the edge etch of stapled substrate. This is for removal of an
n-layer on p-material in order to cut conductive links between front and
backside of solar wafers. The wafers are stapled, the edge are etched in common.
Solar wafer stripping for removal of thick (for instance 4 µm) photoresist
layers, one side or both sides, in a no-damage process can be done with Secon
Dry Etcher.
The advantages of the Si3N4 plasma deposition process
are:
- Low temperature process (~ 260 ºC) compared with a conventional process,
which runs in an expensive LPCVD system at 600 - 700 ºC enabling the
application to be made late in the fabrication process without affecting
device performance
- Easy process control and therefore higher reproducibility and reliability
- Better quality of the Si3N4 layer with respect to density, hydrogen bonds
and stoichiometry
- Uncritical handling of brittle substrates
- Lower cost of ownership
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Etching of moats in silicon wafers
Some applications in solar technology manufacturing require deep moats in
silicon wafers, This is usually achieved using photoresist masks and wet
etching. For a dry etch solution for this application, Secon has developed the
series model XCD-7XX etcher, which together with the XRG-900 high rate source
and an HF bias voltage (100 - 400 Hz), produces ions, radicals and UV radiation,
for optimum etch rate. In the large process chamber batches with several wafers
can be processed, etch rate up to 50 µm/min can be achieved with hard aluminum
masks replacing photoresist.
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