Method for patterning and etching film layers of semiconductor devices

In a laser assisted semiconductor etching process, a krypton fluoride excimer laser operating at 248 nm excites a carbonyl dichloride COCl.sub.2 radical precursor gas which decomposes into carbon monoxide and also atomic chlorine that bonds to laser illuminated surface layer materials of semiconductor devices to create gaseous chlorides which desorb to perfect selective etching, the surface layer material being Cu, Al, amphorous silicon, Ga.sub.(x) Al.sub.(1-x) As, CuIn.sub.(x) Ga.sub.(1-x) Se.sub.2, CdZnS, ZnO and other materials useful in the manufacture of semiconductor devices and solar cells.

FIELD OF THE INVENTION 
This invention relates generally to photon beam assisted etching processes 
which are particular useful during the manufacture of semiconductor or 
solar cell devices. 
BACKGROUND OF THE INVENTION 
Solar cells are presently fabricated commercially by photolithographic 
processes. These processes often are complex and time consuming. The use 
of these processes has lead to high cell manufacturing costs which has 
limited the use of solar cells for terrestrial and space power 
applications. 
During solar cell manufacture, solar cell device structures are built up by 
successively depositing and then patterning and selectively etching 
various combinations of metal, semiconductor, and insulator film layers on 
a suitable substrate. Solar cells typically employ many such film layers. 
Currently, conventional photolithographic processing methods are employed 
to pattern and etch the different film layer combinations during the 
manufacture of solar cells of all types. 
During the patterning and etching of a single film layer combination, 
several steps are required to pattern and develop the photoresist used to 
mask the topmost film layer for subsequent etching in desired regions. 
Several other acid, reactive solution, or plasma etching steps are 
required to selectively remove this film layer from an underlying layer in 
unmasked regions. 
For example, time consuming and costly steps in the fabrication of copper 
indium diselenide (CuInSe.sub.2), copper indium gallium diselenide 
(CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, where x can vary between 0 and 1) and 
amorphous silicon (a-Si) solar cells currently include the 
photolithographic processes that are separately used to pattern and 
selectively remove semiconductor film layers used in the cells from 
underlying metal electrode layers, and to pattern and selectively remove 
the metal electrode layers from underlying glass, polymeric, or other 
supporting substrates. For CuInSe.sub.2 and CuIn.sub.(1-x) Ga.sub.(x) 
Se.sub.2 solar cells, this most often involves the removal of combination 
semiconductor layers of CdZnS/CuInSe.sub.2 and ZnO/CdZnS/CuIn.sub.(1-x) 
Ga.sub.(x) Se.sub.2, respectively, from molybdenum (Mo) electrode layers 
and these Mo layers from glass. For a-Si cells, this often involves 
removal of an a-Si layer from a metal electrode layer [typically copper 
(Cu), aluminum (Al), molybdenum (Mo), or titanium (Ti)] and this metal 
layer from glass. 
Complex photolithographic processing steps currently are also employed to 
produce etch mesas around the perimeters of gallium arsenide (GaAs) and 
gallium aluminum arsenide (Ga.sub.(1-x) Al.sub.(x) As, where x can vary 
between 0 and 1) solar cells during their manufacture as a means of 
reducing cell leakage currents. In addition, GaAs or Ga.sub.(1-x) 
Al.sub.(x) As solar cells used in space applications are generally 
shielded by a protective coverglass to minimize proton-induced damage that 
can degrade cell performance. Because cells are electrically connected to 
each other at their busbars, it is often cumbersome to extend a protective 
coverglass of the cell beyond the busbar region. The removal of all 
junction material in the region along the edge of the cell adjacent to, 
and outside of the busbar of a cell would eliminate the necessity to 
shield the cell in this region. Thus, methods capable of reliably 
producing etch mesas in order to remove junction material in this region 
are desired. The photolithographic processes available for producing these 
etch mesas in the GaAs and Ga.sub.(1-x) Al.sub.(x) As semiconductor layers 
for minimizing cell leakage currents and for minimizing cell radiation 
damage are sufficiently expensive that they can only be used in the 
fabrication of custom cells. 
The fabrication of multiple layer solar cells requires the use of a number 
of processing steps. Many different production stations may be required to 
carry out these photoresist patterning and film layer selective etching 
steps. The need for these steps and associated production stations limits 
the production throughput of solar cells and adds to their fabrication 
costs. One method of patterning and removing a surface layer on a 
semiconductor substrate is the laser dry etching method wherein a laser 
excites a gaseous radical precursor compound to decompose it into at least 
a radical species which combines with and acts to volatilize constituent 
elements of the surface layer. For example, a krypton fluoride laser 
operating at 248 nm can excite gaseous carbon tetrachloride (CCl.sub.4) by 
photon absorption to create atomic chlorine which reacts with a copper 
surface forming cupric chloride and cuprous chloride which are desorbed 
from the surface by the incident laser light. This method has been 
disadvantageously only applied to a single constituent metal and is prone 
to create carbon containing deposits on the surface which limits the 
efficiency of the dry etching process. This method has limited use during 
the manufacture of the solar cells which contain complex surface layers, 
such as CuInSe.sub.2, CuIn.sub.(x) Ga.sub.(1-x) Se.sub.2, CdZnS, ZnO, 
GaAs, and Ga.sub.(x) Al.sub.(1-x) As. These and other disadvantages are 
solved or reduced using the present invention. 
SUMMARY OF THE INVENTION 
An object of the present invention is to apply photon beam assisted etching 
techniques to lower the cost and increase the throughput of fabrication 
processes. 
Another object of the present invention is to apply photon beam assisted 
etching techniques to lower the cost and increase the throughput of 
fabrication processes for solar cells. 
Yet another object of the present invention is the use of laser assisted 
etching processes for patterning and selectively removing CuInSe.sub.2 and 
CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 and other compound semiconductor layers 
from molybdenum or other electrode materials. 
Yet another object of the present invention is the use of laser assisted 
etching processes for patterning and selectively removing metals, such as 
copper or aluminum, from underlying film layers by reaction with atomic 
chlorine. 
A further object of the present invention is the use of a single laser 
assisted etching process for removing combination semiconductor compound 
layers, such as, CdZnS/CuInSe.sub.2 or ZnO/CdZnS/CuInSe.sub.2 layers from 
molybdenum, or other metallic or nonmetallic electrode materials, as well 
as for removing the combination semiconductor compound layers 
CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 or ZnO/CdZnS/CuIn.sub.(1-x) 
Ga.sub.(x) Se.sub.2 layers from molybdenum or other electrode materials. 
Yet a further object of the present invention is the use of laser assisted 
etching processes for patterning and selectively removing an amorphous 
silicon (a-Si) semiconductor layer from a metal layer, such as molybdenum, 
or titanium. 
Still a further object of the present invention is the use of laser 
assisted processes for etching the GaAs and Ga.sub.(1-x) Al.sub.(x) As 
semiconductor layers during the production of etch mesas at the edges of 
GaAs and Ga.sub.(1-x) Al.sub.(x) As devices. 
The present invention provides a maskless and resistless, photon beam 
assisted, dry etching technique that is suitable for the direct, single 
step patterning and etching of film layers having constituent elements 
that react with atomic chlorine, especially those film layers used in 
solar cells. 
A krypton fluoride laser beam assisted etching technique is carried out by 
placing the part containing the film layer to be patterned and selectively 
etched in the presence of a carbonyl dichloride (COCl.sub.2) radical 
precursor compound. The topmost film layer of the part is then exposed to 
a photon beam. Radical chlorine etchant species, each having an unpaired 
electron, are produced on or near the surface of the film layer to be 
etched as the result of the photon beam inducing the decomposition of the 
radical precursor compound. The radical species produced induce etching of 
the film layer in the regions exposed to the photons from the laser 
source. The photons striking the surface can facilitate removal of 
material from the surface by photon assisted ejection. Compounds 
containing the surface elements may form that are either volatile at the 
surface temperatures produced during the beam-induced processing, or are 
desorbed by photostimulated processes at the photon fluences and energies 
employed. The topmost film layer of a part can be efficiently and 
selectively removed with respect to its underlying layer of a different 
material by production of the proper radical species. 
The laser assisted etching processes of the present invention will replace 
the time consuming, yield reducing, and expensive photolithographic 
processes currently used in the fabrication of solar cells. These laser 
etching processes will significantly lower the fabrication costs of 
various types of solar cells. These and other advantages will become more 
apparent from the following detailed description of the preferred 
embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an ultraviolet krypton fluoride (KrF) laser is used as 
a photon energy beam source. Examples of other photon sources include 
lamps and synchrotron radiation sources, having wavelength outputs 
absorbed by the precursor gas. In the embodiment shown, the part to be 
patterned and etched is placed in a chamber containing a carbonyl 
dichloride (COCl.sub.2) radical precursor compound in the gas phase and 
the part is exposed through a window, not shown, on the chamber. By the 
photon beam assisted etching process, a part is thus patterned and etched 
by placing it in the presence of an appropriate radical precursor compound 
and exposing it to the output of an appropriate photon source. 
The output of a photon source is transmitted to the surface of the part to 
be patterned and etched through a suitable window on the chamber holding 
the part. The window may be made of solid materials or fluid (e.g. flowing 
gas or liquid) that are transmissive or partially transmissive at the 
wavelengths and fluences of light produced by the photon source. In some 
cases it may be possible to eliminate the window on the container holding 
the part. In such cases the surface of the part to be patterned and etched 
would be directly illuminated with photons from the photon source while 
also being immersed in a gaseous, liquid, solution, solid, or absorbed 
layer of the radical precursor compound. 
Using a photon beam source, the desired exposure regions on the film's 
surface can be defined by projecting the photon beam onto the surface 
through a contact, proximity, or remote transmission photomask containing 
the desired etch pattern. An optical projection system can be employed to 
image the desired photon beam pattern onto the surface when a remote 
transmission mask is used. The desired exposure regions on the surface of 
the film can also be defined by writing with the photon beam. In this 
case, the photon beam is rastered across the surface and the intensity of 
the beam or dwell time, or both, are modified to provide the desired 
exposure dose in a given area. 
Solar cell manufacturing processes involve many steps, such as the removal 
of compound semiconductor layers or a combination of compound 
semiconductor layers from the molybdenum (Mo). In a preferred embodiment 
for patterning and etching semiconductor films during the fabrication of 
CuInSe.sub.2 solar cells, a chlorine atom etching process is used based on 
the KrF laser induced decomposition of carbonyl dichloride precursor gas 
to selectively and very rapidly remove either the CuInSe.sub.2 layer from 
Mo, the CdZnS/CuInSe.sub.2 combination layer from Mo, or the 
ZnO/CdZnS/CuInSe.sub.2 combination layer from Mo. 
The KrF laser 248 nm wavelength output overlaps the absorption band of the 
carbonyl dichloride resulting in decomposition of the carbonyl dichloride 
into carbon monoxide (CO) and atomic chlorine. The chlorine atoms bond to 
surface sites of one of the surface atoms, for example, a selenium (Se) 
atom of a CuInSe.sub.2 layer. As the chlorine atoms create bonds with one 
constituent atom, e.g. SeCl.sub.2, other molecular bonds at the surface 
break thereby atomizing all of the constituent surface atoms, for example 
indium, selenium and copper, of the molecular lattice. The atomized 
species at the surface form chlorine based molecular gases, SeCl.sub.2, 
InCl.sub.4, CuCl and CuCl.sub.2, all of which become volatile and desorb 
from the surface sites. The copper chlorides formed are the slowest to 
desorb of all of the constituent material of CuInSe.sub.2 and limits the 
etching rate, but nonetheless is efficient. The laser photon energy may 
further lie within absorption bands of surface located copper chlorides 
enhancing the photoejection of etched products to further increase the 
etching rate. In this manner, surface material may be etched using a 
photon beam which decomposes the precursor gas into the radical 
constituents which bond to the surface material. The new molecules 
produced as the result of reactions of radical etchant species with 
surface material species are volatile and desorb from the surface sites. 
In another embodiment for producing CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 
devices, this same process is used to remove the CuIn.sub.(1-x) Ga.sub.(x) 
Se.sub.2, CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, or 
ZnO/CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 layers from Mo. The 
efficiency is enhanced using a KrF laser induced, chlorine atomic etching 
process for removing CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, 
CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.(2), or ZnO/CdZnS/CuIn.sub.(1-x) 
Ga.sub.(x) Se.sub.2 films from Mo. The semiconductor/Mo film layer 
combinations are placed in a gas cell or chamber and exposed at normal 
incidence in the presence of a COCl.sub.2 and helium gas mixture with the 
248 nm, pulsed output of a KrF excimer laser. Regions of the 
CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, CdZnS/CuIn.sub.(1-x) Ga.sub.(x) 
Se.sub.2, or ZnO/CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 film surfaces 
where etching is desired are exposed to the necessary number of KrF laser 
pulses of the necessary fluence in the presence of a COCl.sub.2 and helium 
gas mixture containing the appropriate partial pressure of COCl.sub.2 gas 
to accomplish the etching process. The KrF laser is particularly preferred 
because it is an efficient laser at the desired wavelength for decomposing 
the COCl.sub.2. Further it may tend to aid in the photoejection of the 
copper chlorides. The laser illuminates the surface so that radical 
precursor decomposition occurs in close proximity thereto. 
FIG. 2 shows the laser etching results obtained at different KrF laser 
fluences on a sample containing the CdZnS/CuInSe.sub.2 combination layer 
deposited on Mo. Etching in this case was carried out at a laser 
repetition rate of 10 Hz in the presence of a gas mixture containing 65 
Torr of COCl.sub.2 and 695 Torr of helium. The depth of etching produced 
in the exposure regions is plotted versus the total number of incident 
laser pulses. At fluences of 138 mJ/cm.sup.2 -pulse and higher, removal of 
both the CdZnS and CuInSe.sub.2 layers is efficient. At fluences of 188 
mJ/cm.sup.2 -pulse or higher, the entire 7-7.5-micron thick combination 
layer is removed under the conditions employed within 1500 laser pulses. 
The laser etch rates are given by the slopes of such plots. These etch 
rates are plotted in FIG. 3 versus laser fluence. The removal rates 
increase linearly with laser fluence once a threshold fluence of slightly 
more than 100 mJ/cm.sup.2 -pulse is exceeded. Removal of the combination 
layer would be best carried out in the 230-330 mJ/cm.sup.2 -pulse fluence 
regime since ablation damage to the underlying Mo layer may occur once 
laser fluences of 330-380 mJ/cm.sup.2 -pulse or greater are used. Highly 
selective removal of the CdZnS/CuInSe.sub.2 layer from Mo is possible 
since Mo is etched with very poor efficiencies by this chlorine-atom 
etching process at KrF laser fluences of 330 mJ/cm.sup.2 -pulse and lower. 
Studies were performed to determine the dependencies of the CuInSe.sub.2, 
CdZnS/CuInSe.sub.2, and ZnO/CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 etch 
rates on laser fluence, COCl.sub.2 pressure, and laser pulse repetition 
rate. These results, together with data on expected laser performance and 
operating costs, have been used to calculate expected processing costs and 
part fabrication rates for these laser processes. These processing cost 
and part fabrication rate calculations indicate that the KrF 
laser-assisted, chlorine-atom etching process will allow the lower-cost 
and higher-throughput fabrication of CuInSe.sub.2 and CuIn.sub.(1-x) 
Ga.sub.(x) Se.sub.2 cells, and tandem-cell structures in which these cells 
are used, than is presently possible using conventional photolithographic 
processing methods. 
The dry etching process is also applicable to the removal of silicon from 
metal electrode layers. The fabrication of a-Si solar cells requires the 
ability to pattern and selectively remove the a-Si semiconductor layer 
from an underlying metal electrode layer. The atomic chlorine etching 
process developed, based on the KrF laser-induced decomposition of 
COCl.sub.2, will be useful for this patterning and selective removal 
process, for example, when the a-Si layer has been deposited on Mo. This 
is possible since Si can form volatile chlorides and can be efficiently 
etched by this process, while Mo is etched with very poor efficiency by 
this process. 
The dry etching process is also applicable to the removal of metal 
electrode layers from glass. The removal of Cu and A1 metal electrode 
layers from glass is typically required during the fabrication of 
CuInSe.sub.2, CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, or a-Si solar cells. The 
atomic chlorine generation process, based on the KrF laser induced 
decomposition of COCl.sub.2, can etch Cu and Al with very high 
efficiencies. Because this process etches glass (SiO.sub.2) with poor 
efficiency, it is well-suited for rapidly and selectively removing Cu or 
Al from glass. These laser-assisted atomic chlorine etching processes can 
thus be used for patterning and selectively removing typical metal film 
layers from glass substrates during the fabrication of CuInSe.sub.2, 
CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, a-Si, GaAs or GaAlAs solar cells. The 
process is also well-suited for patterning and selectively removing Cu, 
Al, or Cu/Al alloy layers from SiO.sub.2 (or other dielectric materials 
not efficiently etched by chlorine atoms) during the fabrication of 
integrated circuit devices or for forming reliable, 
electromigration-resistant electrical interconnects between various types 
of electronic devices and energy storage devices (e.g. solar cell and 
microbattery arrays). 
The atomic chlorine generation process, based upon the KrF laser induced 
decomposition of COCl.sub.2, can also efficiently etch GaAs and 
Ga.sub.(1-x) Al.sub.(x) As. The process thus could be readily applied to 
the patterning and etching of these semiconductor materials during the 
production of etch mesas around the perimeters of GaAs and Ga.sub.(1-x) 
Al.sub.(x) As solar cells, or during the manufacture of electronic devices 
comprised of GaAs or Ga.sub.(1-x) Al.sub.(x) As. 
The previously described atomic chlorine etching process would not be 
useful for selectively removing a-Si from Cu or Al electrode layers, since 
this process etches both of these metals with high efficiency. 
A single commercial, 100-W KrF laser can be used, for example, with the 
atomic chlorine etching process for the low-cost and high-throughput 
patterning and etching of CdZnS/CuInSe.sub.2 over Mo parts used in 
CuInSe.sub.2 solar cells. The laser process would allow, for example, the 
cost-effective production of four 16 inch long etch lines of 175 microns 
width through a 7 micron thick CdZnS/CuInSe.sub.2 combination film layer 
in each part. This process is also capable of efficiently etching 
CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2. Thus, similar desirable fabrication 
rates and costs will be possible by the laser process for producing the 
CdZnS/CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 over Mo parts used in 
CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2 solar cells. Laser-generated, atomic 
chlorine etching process will have a highly favorable impact on lowering 
the fabrication costs of CuInSe.sub.2 and CuIn.sub.(1-x) Ga.sub.(x) 
Se.sub.2 solar cells. 
The maskless, laser etching technique can pattern film layers directly 
without the resist patterning steps and the acid, reactive solution, or 
plasma etching steps required in the conventional photolithographic 
methods now used to fabricate solar cells. The disclosed laser etching 
processes will thus replace the most time consuming, yield reducing, and 
costly steps used in the fabrication of solar cells. The laser etching 
method will allow the lower-cost, higher-throughput fabrication of various 
types of solar cells than is currently possible using conventional 
photolithography. These include, but are not limited to, solar cells 
having CuInSe.sub.2, CuIn.sub.(1-x) Ga.sub.(x) Se.sub.2, a-Si, GaAs, and 
Ga.sub.(1-x) Al.sub.x As materials and various tandem-cell structures 
involving these cells. 
Although the invention has been described in terms of a preferred 
embodiment, it will be obvious to those skilled in the art that 
alterations and modifications may be made without departing from the 
invention. For example, it is obvious that conditions can be readily 
modified to suit the circumstances. Laser modifications include but are 
not limited to wavelength(s), fluence(s), pulse length(s), and repetition 
rate(s). Radical precursor modifications include but are not limited to 
precursor(s), buffer gases, such as, N.sub.2 or Ar and accompanying 
pressures. For most solar cell semiconductor materials, the radical 
species are preferably either chlorine, bromine, iodine, or 
trifluoromethyl radical that is, a heavy halogen or a halogen-substituted 
polyatomic radical. Fluorine efficiently etches Si or Ge but it does not 
efficiently etch most other solar cell semiconductor materials. Reaction 
chamber modifications include substrate temperature and total chamber 
pressure. The remainder portion of the decomposed radical precursor, for 
example, CO from COCl.sub.2, must be substantially inert to the surface 
layer reacting with the radical species. Accordingly, it is intended that 
all such alterations and modifications be included within the spirit and 
scope of the invention as defined by the appended claims. Also, the 
invention can be practiced to produce a wide variety of solar cells 
(miniature, micro- or nano-scale) fabricated separately or in conjunction 
with electrical storage devices (e.g. microbatteries) or optical or 
mechanical or electronic devices.