U.S. patent application number 13/820537 was filed with the patent office on 2013-09-05 for method for the wet-chemical etching of a highly doped semiconductor layer.
This patent application is currently assigned to SCHOTT SOLAR AG. The applicant listed for this patent is Norman Hermert, Agata Lachowicz, Berthold Schum, Knut Vaas. Invention is credited to Norman Hermert, Agata Lachowicz, Berthold Schum, Knut Vaas.
Application Number | 20130228220 13/820537 |
Document ID | / |
Family ID | 44583021 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130228220 |
Kind Code |
A1 |
Schum; Berthold ; et
al. |
September 5, 2013 |
METHOD FOR THE WET-CHEMICAL ETCHING OF A HIGHLY DOPED SEMICONDUCTOR
LAYER
Abstract
A method for the wet-chemical etching of a silicon layer in an
alkaline etching solution is provided, where the silicon layer is
the surface region of a solar cell emitter. The method ensures that
the surface region of the emitter is etched-back homogeneously
using an oxidant-free alkaline etching solution comprising at least
one organic moderator is used for the isotropic etching back of the
surface region of the emitter, where the moderator has a dopant
concentration of at least 10.sup.18 atoms/cm.sup.3.
Inventors: |
Schum; Berthold;
(Biebergemuend, DE) ; Vaas; Knut; (Alzenau,
DE) ; Lachowicz; Agata; (Kahl, DE) ; Hermert;
Norman; (Rodgau, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Schum; Berthold
Vaas; Knut
Lachowicz; Agata
Hermert; Norman |
Biebergemuend
Alzenau
Kahl
Rodgau |
|
DE
DE
DE
DE |
|
|
Assignee: |
SCHOTT SOLAR AG
Mainz
DE
|
Family ID: |
44583021 |
Appl. No.: |
13/820537 |
Filed: |
September 2, 2011 |
PCT Filed: |
September 2, 2011 |
PCT NO: |
PCT/EP11/65222 |
371 Date: |
May 15, 2013 |
Current U.S.
Class: |
136/256 ;
136/252; 438/57; 438/98 |
Current CPC
Class: |
H01L 31/02363 20130101;
C09K 13/08 20130101; Y02P 70/50 20151101; H01L 31/1804 20130101;
H01L 31/186 20130101; Y02E 10/547 20130101; H01L 31/022425
20130101; H01L 21/30604 20130101 |
Class at
Publication: |
136/256 ; 438/57;
438/98; 136/252 |
International
Class: |
H01L 31/18 20060101
H01L031/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2010 |
DE |
10 2010 037 311.7 |
Jun 7, 2011 |
DE |
10 2011 050 903.8 |
Claims
1-19. (canceled)
20. A method for the wet-chemical etching of a surface region of a
solar cell emitter in an alkaline etching solution, comprising:
isotropic etching back the surface region using an oxidant-free
alkaline etching solution comprising at least one organic
moderator, the surface region has a dopant concentration of at
least 10.sup.18 atoms/cm.sup.3.
21. The method according to claim 20, wherein the oxidant-free
alkaline etching solution has an alkaline component in a
concentration range between 1 g/L and 50 g/L.
22. The method according to claim 20, wherein the oxidant-free
alkaline etching solution has a concentration range of the at least
one organic moderator between 0.1 g/L and 5 g/L.
23. The method according to claim 20, wherein the step of isotropic
etching back comprises contacting the surface region with the
oxidant-free alkaline etching solution for a time between greater
than or equal to 10 second and less than or equal to 180
seconds.
24. The method according to claim 23, wherein the time is between
greater than or equal to 15 seconds and less than or equal to 16
seconds.
25. The method according to claim 20, further comprising adjusting
the oxidant-free alkaline etching solution to a temperature of
between less than or equal to 35.degree. C. and greater than or
equal to 65.degree. C. during the etching back.
26. The method according to claim 21, wherein the alkaline
component has at least one component selected from the group
consisting of LiOH, NaOH, KOH, ammonium hydroxide, quaternary
ammonium hydroxides, tetraalkyl ammonium hydroxide, organic bases,
and organic amines.
27. The method according to claim 20, wherein the organic moderator
comprises at least one component selected from the group consisting
of sulfuric acid alkyl esters and the salts thereof, alkyl
carboxylic acids and the salts thereof, alkyl and alkyl benzene
sulfonic acids and the salts thereof, mono- and diesters of
orthophosphates, fluorinated carboxylic acids, fluorinated sulfonic
acids, polyalkylene glycol ethers, such as fatty alcohol
ethoxylates or fatty alcohol propoxylates, alkyl polyglucosides,
saccharose esters, sorbitan fatty acid esters, N-methylglucamides,
alkyl phenol ethoxylates, alkyl phenol propoxylates, alkanol
amides, alkyne diols, substituted alkynols, ethoxylated alkyne
diols, fluorinated alkyl alkoxylates, alkyl betaines, amidoalkyl
betaines, alkyl sulfobetaines, amidoalkyl sulfobetaines, alkyl
amino oxides, alkyl amidoalkyl amino oxides, fluorinated amphoteric
alkyl compounds, alkyl amine ethoxylates, dialkyl dimethyl ammonium
salts, and alkyl benzyldimethyl ammonium salts.
28. The method according to claim 20, wherein the oxidant-free
alkaline etching solution further comprises at least one component
selected from the group consisting of complexing agents, complexing
agents for silicic acid, complexing agents for metal cations,
chelating agents for metal cations, and buffering substances.
29. The method according to claim 20, wherein the at least one
organic moderator is a surface-active substance or surfactant.
30. The method according to claim 20, wherein the at least one
organic moderator comprises at least one component selected from
the group consisting of sodium dodecylbenzenesulfonate, saccharose
stearic acid ester, dodecyldimethylamino oxide, and
dioctyldimethylammonium chloride.
31. The method according to claim 20, wherein the step of isotropic
etching back the surface region comprises etching back the surface
region to a thickness between 0 and less than or equal to 10
nm.
32. The method according to claim 28, wherein the complexing agent
is selected from the group consisting of hydroxyphenols, amines,
hydroxycarboxylic acids, polyalcohols, phosphonic acids, and
polyphosphates.
33. The method according to claim 20, wherein the step of isotropic
etching back the surface region comprises employing the
oxidant-free alkaline etching solution in batch processing unit or
continuous process unit.
34. The method according claim 20, wherein the surface region
comprises, as a dopant, a material selected from the group
consisting of phosphorus, arsenic, boron, gallium, and
aluminum.
35. A crystalline solar cell with a solar cell emitter that is
etched-back according to the method of claim 20.
36. A method for the wet-chemical etching of a surface region of a
solar cell emitter in an alkaline etching solution, comprising:
isotropic etching back the surface region using an oxidant-free
alkaline etching solution containing at least one organic
moderator, the surface region having a dopant concentration of at
least 10.sup.18 atoms/cm.sup.3, wherein the oxidant-free alkaline
etching has an alkaline component with a concentration range
between 1 g/L and 50 g/L and the organic moderator has a
concentration range that lies between 0.1 g/L and 5 g/L, wherein
the step of isotropic etching back comprises contacting the surface
region with the oxidant-free alkaline etching solution for a time
between greater than or equal to 10 second and less than or equal
to 180 seconds; and adjusting the oxidant-free alkaline etching
solution to a temperature of between less than or equal to
35.degree. C. and greater than or equal to 65.degree. C. during the
etching back.
37. The method according claim 36, further comprising, after the
step of isotropic etching back, applying a metal layer at least
selectively onto the surface region by a method selected from the
group consisting of chemical deposition, electrodeposition, and
physical vapor deposition.
38. The method according claim 37, wherein the metal layer
comprises a layer selected from the group consisting of
nickel/silver layer, a nickel/copper layer, and a
titanium/palladium/silver layer.
39. A crystalline solar cell with a solar cell emitter that is
etched-back according to the method of claim 36.
Description
[0001] The invention relates to a method for the wet-chemical
etching of a silicon layer in an alkaline etching solution, the
silicon layer being the surface region of a solar cell emitter.
[0002] The quality factor/structure of an emitter is one of the
crucial factors for the efficiency of a solar cell. The emitter
depth, the dopant profile, the surface concentration of the dopant,
and the emitter layer resistance must be precisely adjusted to
obtain optimal cell properties.
[0003] The emitter properties are influenced by the diffusion
parameters of temperature and time as well as the type of doping
agent. In the framework of economically applicable industrial
processes, however, it is not possible to adjust all properties at
will and independently of one another.
[0004] In the currently widespread methods for producing
crystalline solar cells, homogeneously doped mono- and
multicrystalline p-type silicon wafers, usually with boron base
doping, are used as starting materials. The concentration of the
dopant is on the order of 10.sup.16 atoms/cm.sup.3. The emitter is
produced by in-diffusion of phosphorus. The most important target
parameter, which is readily accessible by measurement technology,
is the emitter layer resistance as a measure of the amount of
electrically active phosphorus atoms that have diffused in.
[0005] The phosphorus surface concentration can lie in the range of
10.sup.19 to more than 10.sup.21 atoms/cm.sup.3, depending on the
diffusion conditions and the type of doping agent, with the maximum
concentration of electrically active phosphorus atoms as well as
the solubility of phosphorus in silicon (approximately
5.times.10.sup.20 atoms/cm.sup.3) being exceeded. The phosphorus is
then present in inactive form and forms recombination centers for
the charge carrier pairs generated (so-called "dead layer").
[0006] The metal contacts on the front side are produced
predominantly by means of thick-film silver pastes in the
silk-screen printing process with subsequent sintering. On the one
hand, a high phosphorus surface concentration is advantageous for
the creation of a low-ohmic contact between the silver paste and
the emitter; on the other hand, such a high surface concentration
of the doping agent causes, as mentioned, a more enhanced
recombination of the charge carrier pairs generated by light
absorption.
[0007] An improvement of the emitter surface can be achieved by
etching back of the highly doped layers near the surface. The
surface recombination of the charge carriers generated is hereby
reduced.
[0008] This is evident through improved spectral sensitivity and
internal quantum efficiency in the short-wavelength region of the
spectrum and positively influences above all the short-circuit
current density as well as the open-circuit voltage of the solar
cell.
[0009] According to the prior art, in a first chemical treatment
step, the phosphosilicate glass formed from silicon oxide and from
dopant oxides is removed in dilute hydrofluoric acid after
diffusion during the production of solar cells with p-silicon-based
wafers.
[0010] In a second treatment step, alkaline cleaning solutions,
based on ammonia or alkyl ammonium hydroxides and hydrogen
peroxide, are used. Solutions of this type were originally
developed for semiconductor cleaning, known as "SC-1," a part of
the RCA cleaning sequence.
[0011] Standard RCA cleaning comprises treatment in the SC-1
solution, consisting of ammonia or ammonia derivatives and hydrogen
peroxide, rinsing, and treatment with hydrogen peroxide in a dilute
hydrochloric acid solution (known as "SC-2"). Ammonia and
hydrochloric acid are generally employed in the concentration range
of 3 to 10 weight percent; the hydrogen peroxide concentration
generally is approximately 1 weight percent.
[0012] The temperatures employed are in the range of 60.degree. C.
to 85.degree. C.; the contact times are approximately 10 minutes.
Such conditions can be realized only in batch process units.
[0013] In widely employed continuous process units, solutions based
on alkyl ammonium hydroxides and hydrogen peroxide are employed at
temperatures of approximately 60.degree. C. and contact times of
approximately 60 s. For reasons of economic efficiency, markedly
longer contact times cannot be implemented.
[0014] The change of the emitter layer resistance is a readily
accessible measured parameter, by means of which the depth of the
emitter etching back and the reduction in the surface concentration
of the dopant can be measured approximately. Mass erosion due to
the etching solution is very small (on the order of 1 mg for wafers
that are 156.times.156 mm in size) and corresponds to the erosion
of only a very thin emitter layer of less than 10 nm.
[0015] Emitters produced by standard methods have a layer
resistance in the range of 50 to 70 ohm/sq after diffusion. The
layer resistance is raised by 5 to 10 ohm/sq by treatment in dilute
hydrofluoric acid and subsequent treatment in the RCA sequence,
that is, 10 minutes in SC-1 solution, rinsing, and 10 minutes in
SC-2 solution. The emitter etching back takes place predominantly
in the alkaline SC-1 solution.
[0016] The layer resistance is increased by only approximately 1-2
ohm/sq by treatment in continuous process units at approximately
60.degree. C. and a contact time of 60 s in a solution based on
alkyl ammonium hydroxides and hydrogen peroxide.
[0017] Offered as an alternative cleaning and emitter etching
solution is an acidic aqueous mixture made up of
tetramethylammonium hydroxide (TMAH), acetic acid, surfactants,
complex-forming agents, fluoride, and peroxide. This solution makes
possible stronger etching back of the emitters even at lower
temperatures, that is, at approximately 40.degree. C.
[0018] The emitter can be strongly etched-back by using an acidic
etching mixture consisting of nitric acid and hydrofluoric acid;
that is, the increase of the emitter layer resistance is, for
example, 20 ohm/sq and more at shorter contact time and lower
temperature, such as, for example, 60 s at up to 15.degree. C. The
etching rate of this solution is too high for a controlled moderate
etching back of a flat emitter. The remaining surface doping would
be too small to form a low-ohmic contact with silk-screened silver
paste. This solution can be used to produce a selective emitter. In
this type of emitter, subregions beneath the metal contacts have a
low layer resistance and a high surface concentration of the dopant
as well as a high layer resistance between the metal contacts.
[0019] Described as another possibility for emitter batch etching
is a repeated sequence consisting of chemical oxidation--for
example, with nitric acid--and removal of the formed silicon oxide
in dilute hydrofluoric acid. The etching back of the emitter in
this way is very slow and requires several process solutions.
[0020] The drawback of the available alkaline solutions for emitter
etching back is that the etching rate is too low. Long contact
times and high temperatures are necessary for an expedient,
uniform, and moderate etching back of the emitter and an increase
in the layer resistance in the range between 5 and 15 ohm/sq.
[0021] Typical alkaline emitter etching solutions are based on
ammonia or ammonia derivatives and hydrogen peroxide. By way of
example, reference is made to the "SC1 solution" of the RCA
cleaning developed for semiconductor manufacture (W. Kern, "The
Evolution of Silicon Wafer Cleaning Technology" in J. Electrochem.
Soc., Vol. 137, No. 6, June 1990, 1887-1891). Alkyl and
hydroxyalkyl derivatives of ammonia offer the advantage of a lower
vapor pressure and hence less of a problem with emissions in
comparison to ammonia. Further components, such as complexing
agents, surfactants, and stabilizers, can also be employed (see,
for example, WO A 2006/039090).
[0022] The drawback of these solutions is the low etching back of
the emitter surface layer within the contact time available in
standard processes for solar cell manufacture, which usually is
less than 1 min in a continuous process unit.
[0023] Described in EP A 1 843 389 is a sequence consisting of
repeated chemical oxidation with subsequent dilute HF to remove the
silicon oxide, so as to erode the uppermost highly doped emitter
layers. Specified for chemical oxidation are: ozone,
ozone/H.sub.2O, O.sub.3/H.sub.2O/HF, H.sub.2O.sub.2, HNO.sub.3,
H.sub.2SO.sub.4, and NH.sub.4OH at a temperature of between
20.degree. C. and 90.degree. C. This method is supposed to offer
the advantage of a better degree of control of the emitter
profile/phosphorus surface concentration created during diffusion
with respect to oxidation. However, owing to chemical oxidation
under the given conditions, an oxide layer with a thickness of only
approximately 1 nm is produced. Several repetitions of the
oxidation/HF sequence would be needed to erode the highly doped
layer.
[0024] Described in EP A EP 0 731 495 as cleaning solutions for
semiconductors in the framework of a modified RCA cleaning sequence
are aqueous HF solutions containing ozone (and surfactant for
improvement of the ozone solubility) or hydrogen peroxide.
[0025] An alternative possibility of avoiding the drawback of a
high surface concentration of dopant is offered by the development
of the selective emitter. Thus, the production of a selective
emitter via etching back of an emitter, diffused by conventional
processes, in the regions between the metal contacts may be
inferred from WO A 2009/013307. The regions beneath the metal
contacts are protected by an etching barrier introduced beforehand.
In the first step, a mixture made up of nitric acid and
hydrofluoric acid is used for etching back for controlled
production of a porous silicon layer. The etching progress is
readily evident, because the porous silicon appears in various
colors depending on the layer thickness. In the second step, the
porous silicon is subjected to wet-chemical oxidation. Specified as
oxidants are HNO.sub.3 and H.sub.2SO.sub.4. The removal of
SiO.sub.2 in dilute HF occurs subsequently.
[0026] A drawback of the mixed acid used is that it is difficult to
control the formation of a homogenously porous Si layer by process
engineering, so that--and as a consequence of inhomogeneous etching
back--a strong scatter of the emitter layer resistance values over
the wafer surface results.
[0027] The effect of alkaline emitter etching solutions based on
TMAH and hydrogen peroxide may be inferred from the literature
reference J. J. Tool et al. "Almost 1% Absolute Efficiency Increase
in mc--Solar Cell Manufacturing with Simple Adjustment to the
Processing Sequence," Proceedings 21.sup.st European Photovoltaic
Solar Conference, Dresden (2006).
[0028] A cleaning sequence consisting of repeated oxidation with
HNO.sub.3 and oxide dissolution in HF may be taken from S.
Keipert-Colberg et al., "Investigation and Development of
Industrial Feasible Cleaning Sequences Prior to Silicon Nitride
Deposition Enhancing Multicrystalline Silicon Solar Cell
Efficiency," Proceedings 24.sup.th European Photovoltaic Solar
Energy Conference, Hamburg (2009).
[0029] The present invention is based on the problem of further
developing a method of the type mentioned in the beginning such
that the drawbacks of prior art can be avoided. At the same time,
homogenous etching back of the surface region of the emitter should
be possible. Furthermore, it should be possible to employ process
times and parameters that do not lead to a negative influence on
the solar cell manufacturing process in a process line. It should
also be possible to adjust precisely dopants, such as phosphorus
surface concentrations. The blue sensitivity in solar cells should
be improved.
[0030] In order to solve this problem, the invention provides
essentially that an oxidant-free alkaline etching solution,
containing at least one organic moderator, is used for isotropic
etching back of the emitter in its surface region, which has a
dopant concentration of at least 10.sup.18 atoms/cm.sup.3,
particularly at least 10.sup.19 atoms/cm.sup.3. It is provided, in
particular, that the etching solution used is one in which the
alkaline component lies in the concentration range between 1 g/L
(0.1 wt %) and 50 g/L (5 wt %). In particular, it is provided that
the concentration range of the alkaline component lies between 2
g/L (0.2 wt %) and 15 g/L (1.5 wt %). Furthermore, the etching
solution used should be one whose organic moderator lies in the
concentration range between 0.1 g/L (0.01 wt %) and 5 g/L (0.5 wt
%), in particular in the range between 0.2 g/L (0.02 wt %) and 1
g/L (0.1 wt %). Furthermore, it is provided, in particular, that
the contact time between etching solution and the surface region is
between 10 sec and 80 sec, preferably between 15 sec and 60 sec,
with the temperature of the etching solution preferably being in
the range between 35.degree. C. and 65.degree. C., particularly
between 45.degree. C. and 60.degree. C.
[0031] On the basis of the teaching of the invention, the surface
region of the emitter, which exhibits planar homogeneous doping,
that is, lateral homogeneous doping, with at least a dopant
concentration of greater than 10.sup.18 atoms/cm.sup.3, is
isotropically etched-back, with the topography of the previously
textured surface being maintained. An emitter surface layer of
constant thickness is consequently eroded. It is provided, in
particular, that, through the choice of contact time and
temperature as well as the concentration ranges of the alkaline
component and of the organic moderator, less than 10 nm is eroded,
in particular between 3 nm and 7 nm, preferably in the range of
about 5 nm.
[0032] The teaching of the invention avoids anisotropic etching
without alteration of the topography of the surface, so that even
fragile structures projecting from the surface are retained.
[0033] Proposed in accordance with the invention is a method for a
microscopically homogeneous, wet-chemical, isotropic depth etching
of a highly doped emitter zone near the surface, with the greatest
degree of retention of the topographic details of the surface
having a dopant concentration of more than 10.sup.18 atoms/cm.sup.3
and a preferred back-etching depth of between 3 nm and 7 nm, in
particular <5 nm, by using an oxidant-free alkaline etching
solution containing at least one organic moderator as etching
solution.
[0034] Although the use of alkaline etching solutions containing
organic moderators is known, they are not known for isotropic
etching back of a highly doped surface region of an emitter of a
silicon-based crystalline solar cell.
[0035] Thus, a texturing and cleaning medium for surface treatment
of wafers may be inferred from DE A 10 2007 058 829. Proposed is an
anisotropic etching for creation of surface roughness, so as to
minimize light reflections. Because of the anisotropy, the alkaline
solutions used are suitable only for texturing of monocrystalline
silicon, with pyramids being formed. Coming into consideration as
etching medium are KOH, NaOH, THAH, or other inorganic etching
media.
[0036] Provided as additives are aliphatic and aromatic carboxylic
acids, amino carboxylic acids, polyalcohols, EDTA, polyethylene
sorbitan monolaurate, and alkly-substituted pyrocatechol. The
concentration of the etching medium lies between 4 and 15 wt % and
that of the additives between 1 and 20 wt %. Provided as contact
times are 10-30 min at a temperature of greater than 80.degree. C.
The silicon layer that is etched off lies in the range of between 5
and 10 .mu.m. Use for etching back a highly doped emitter surface
region is not possible with a respective etching solution, because
the emitter layer would be totally destroyed in the case of
conventional emitter thicknesses in the range of 350 nm.
[0037] The subject of DE A 10 2008 052 660 is a method for
manufacturing a solar cell with two-stage doping. A selective
emitter is supposed to be produced by means of an inorganic mask.
The mask is produced by application of a paste, which contains
SiO.sub.2 glass, and subsequent fusion. HF--HNO.sub.3 is used for
etching back. In this process, the mask is partially or completely
co-etched off. Employed for removal of the porous silicon resulting
from the etching is an alkaline etching solution. The function of
the alkaline etching solution involves selectively etching off the
porous silicon layer that was created beforehand in an upstream
acidic etching step. An isotropic etching back in the range of 5 nm
cannot be controlled using the respective measures.
[0038] Carried out in accordance with the invention, by contrast,
is a one-step, chemically simple process, which, in comparison to
the technically difficult process that, owing to the toxicity of
hydrofluoric acid, is also risky, is quite suitable
economically.
[0039] An anisotropic etching by means of KOH with isopropanol, for
example, is described in GB A 2 209 245 for the creation of a
three-dimensional structure. In this process, more highly doped
regions are selectively removed. Described is a multi-stage method
for producing lateral etched steps in the surface, with the
creation of a highly boron-doped surface that is resistant to the
anisotropically acting etching medium.
[0040] A substrate that is 40 nm thick is thinned to 10 nm
according to US A 2004/0259324. Used as etching medium is an
alkaline etching medium, which contains quaternary ammonium
hydroxides, substituted amines, particularly TMAH in a
concentration of between 10 wt % and 45 wt %. The etching rate at
25.degree. C. amounts to 7.5 nm/min. Provided as possible further
additives in the etching solution are anionic, cationic, and
neutral surfactants, oxidants in the form of peroxide or
persulfate, or, alternatively, reductants. Acids such as silicic
acid can also be employed to change the pH. The described method is
aimed at substantial thinning of the substrate made of silicon,
with the attainment back-etching depths that, solely on account of
the etching rate, do not lead to reproducibly adjustable
back-etching depths for highly doped emitters.
[0041] Etching solutions for undoped silicon, highly doped silicon,
and silicon nitride are also known from US A 2005/0065050. The
etching solution is supposed to enable the highest possible etching
selectivity, that is, different etching rates for the respective
substrates, to be achieved. Coming into consideration as etching
medium are, for example, KOH or TMAH. Possible additives have the
properties of being water soluble, non-volatile, non-flammable,
with preferably ethylene glycol being specified. The etching method
aims at the manifestation of a high selectivity of the etching rate
of various dopant concentrations in the silicon or at the boundary
surfaces to adjacent layers, such as SiH or SiOx.
[0042] A selective dissolution of any unevenness in a thin-layer
film is the subject of US A 2010/0126961. The etching solution
consists of a strong base, such as TMAH, NaOH, or KOH, an etching
moderator, an oxidant, such as a persulfates, and a wetting agent.
The contact time between the etching solution and the polysilicon
thin-layer film that is being planarized lies between 0.5 min and
10 min at a temperature between 40.degree. C. and 80.degree. C.
Projections on a size order of between 80 nm and 100 nm are
supposed to be etched. Corresponding etching solutions are not
suitable for controlled used on highly doped surface regions of an
emitter, particularly for a back-etching depth of <5 nm.
[0043] A selective etching of non-doped regions in silicon is
described in U.S. Pat. No. 5,431,777. Coming into consideration as
etching medium are KOH, NaOH, and TMAH in a concentration range of
approximately 8 wt %. As additives, it is possible also to use
aromatic alcohols and ethers, including substituted ones, Novolacs,
and polyvinyl alcohol. The etching rate ranges from a few
nanometers to a few micrometers per minute, with a temperature
range of between 45.degree. C. and 104.degree. C. be chosen.
[0044] EP A 2 302 701 relates to the texturing of a semiconductor
substrate. Coming into consideration as etching medium are
hydroxides and alkanol amines. The concentration range for use of
KOH is given as 6 wt % by way of example. Coming into consideration
as additives are alkoxylated glycols or glycol alkyl ethers, such
as triethylene glycol and diethylene glycol monomethyl ether.
Additionally provided are chlorides and silicates. The etching
erosion approaches 10 .mu.m at temperatures of between 80.degree.
C. and 100.degree. C. The described mixtures are utilized for
anisotropic texturing of a Si surface.
[0045] The subject of the invention is an alkaline etching solution
containing at least one organic etching moderator. This solution
makes possible an optimal etching back of the emitter within short
contact times, because the etching rate for silicon is higher than
that of alkaline solutions that contain hydrogen peroxide as
etching-moderating component.
[0046] Another advantage of the solution according to the invention
is that porous silicon, which can form in preceding process steps,
is removed.
[0047] Surface-active substances (surfactants) can be employed as
organic moderators that inhibit the etching attack of the alkaline
solutions. Because surfactant molecules contain a hydrophobic group
and a hydrophilic group, both H-terminated (hydrophobic) and
oxidized (hydrophilic) substrate surfaces are protected.
[0048] The advantage of organic moderators over hydrogen peroxide
is that the silicon surface is not completely "blocked." Moderate
and uniform silicon dissolution is still not yet possible. Hydrogen
peroxide results in oxidation of the silicon surface; the etching
rate of silicon oxide in an alkaline solution is extremely low.
Even at very low concentrations, hydrogen peroxide is effective
above 0.1 wt %. Smaller concentrations do not interfere and can
also be present in the etching solutions according to the
invention. Hence, it is insofar possible to speak of an
oxidant-free aqueous etching solution.
[0049] It has been found that many surface-active substances
produce the desired moderating effect on the etching operation.
Differences in the adsorption strength of surfactants on the
silicon surface and hence in their ability to moderate the etching
attack of the alkaline solution can be compensated for by adjusting
the surfactant and alkaline concentration as well as by adjusting
the etching parameters temperature and time to the desired etching
rate or else to the desired increase in the emitter layer
resistance.
[0050] The solution according to the invention contains an alkaline
component and at least one organic etching moderator. Complexing
agents and buffering substances can be employed as further
constituents.
[0051] Used as alkaline component is at least one substance from
the group of LiOH, NaOH, KOH, ammonium hydroxide, quaternary
ammonium hydroxides, organic bases, and organic amines.
[0052] Quaternary ammonium hydroxides comprise tetraalkyl ammonium
hydroxides and substituted tetraalkyl ammonium hydroxides
containing hydroxyl- and alkoxy-substituted alkyl groups, such as,
for example, tetraalkyl ammonium hydroxide,
trimethyl-2-hydroxyethylammonium hydroxide (choline), and
triethyl(ethoxypropyl)ammonium hydroxide.
[0053] Examples of organic bases are pyrimidine and guanidine.
[0054] Organic amines comprise alkyl amines, polyalkylene amines,
alkanolamines, cyclic N-substituted amines, and derivatives
containing substituted alkyl groups.
[0055] Examples of alkyl amines are mono-, di-, and triethylamine
and dodecyldimethylamine. An example of polyalkylene amines is
diethylenetriamine. Examples of alkanolamines are mono-, di-, and
triethanolamine and 2-(2-aminoethoxy)ethanol.
[0056] Examples of cyclic N-substituted amines are
N-methylpyrrolidine, N-methylpiperidine, and
N-ethylpyrrolidone.
[0057] The following substances can be employed as etching
moderators.
[0058] anionic surfactants, such as sulfuric acid alkyl esters and
the salts thereof, alkyl carboxylic acids and the salts thereof,
alkyl and alkyl benzene sulfonic acids and the salts thereof, mono-
and diesters of orthophosphates, fluorinated carboxylic acids, and
fluorinated sulfonic acids,
[0059] nonionic surfactants, such as polyalkylene glycol ethers
(for example, fatty alcohol ethoxylates, fatty alcohol
propoxylates), alkyl polyglucosides, saccharose esters, sorbitan
fatty acid esters, N-methylglucamides, alkyl phenol ethoxylates and
alkyl phenol propoxylates, alkanol amides, alkyne diols,
substituted alkynols, ethoxylated alkyne diols, and fluorinated
alkyl alkoxylates,
[0060] amphoteric surfactants, such as alkyl betaines, amidoalkyl
betaines, alkyl sulfobetaines, amidoalkyl sulfobetaines, alkyl
amino oxides, alkyl amidoalkyl amino oxides, and fluorinated
amphoteric alkyl compounds,
[0061] cationic surfactants, such as amine ethoxylates, dialkyl
dimethyl ammonium salts, and alkyl benzyldimethyl ammonium
salts.
[0062] Examples of anionic surfactants are: sodium dodecyl hydrogen
sulfate, sulfosuccinic acid dihexyl ester, sodium
dodecylbenenesulfonate, ammonium lauryl sulfate, 2-ethylhexanol
phosphoric acid ester, and perfluorooctanesulfonate.
[0063] Examples of nonionic surfactants are: tetraethylene glycol
octyl ether, lauryl myristyl polyglycol ether, octylphenol
ethoxylate, saccharose stearic acid ester, and
3,5-dimethylhexyn-3-ol.
[0064] Examples of amphoteric surfactants are: cocoamidopropyl
betaine, dodecyldimethyl-amino oxide, octylimino dipropionate, and
N-dodecyl-N,N-dimethylammonium propanesulfonate.
[0065] Examples of cationic surfactants are: oleyl
bis(2-hydroxyethyl)methylammonium chloride, dioctyldimethylammonium
chloride, and cocobenzyldimethylammonium chloride.
[0066] The emitter solution can additionally contain:
[0067] complexing agents for silicic acid (reaction product), such
as, for example, o-dihydroxybenzene and other hydroxyphenols as
well as aromatic ethers,
[0068] complex-forming agents or chelating agents for metal
cations: amines such as EDTA, DTPA, di- and tricarboxylic acids,
hydroxycarboxylic acids (for example, citric acid, tartaric acid),
polyalcohols, such as, for example, glycerin, sorbitol, and other
sugars and sugar alcohols, phosphonic acids, and
polyphosphates,
[0069] buffering substances, such as ammonium acetate and potassium
hydrogen phthalate,
[0070] peroxides, such as hydrogen peroxide in very low
concentrations (<0.1 wt %).
[0071] In addition, the invention is characterized by the use of
one of the previously described etching solutions for etching back
of the emitter, with a metal layer being deposited at least
selectively onto the surface of the crystalline solar cell by
chemical deposition or electrodeposition of a nickel/silver or
nickel/copper layer or by physical vapor deposition methods after
etching back of the emitter. When a vapor deposition method is
used, a titanium/palladium/silver layer, in particular, is
deposited.
[0072] The field of application of the invention is the manufacture
of solar cells made of silicon. Therefore, the invention is
characterized by a solar cell, the emitter of which is etched-back
by using measures that have been previously described.
[0073] The invention will be discussed below on the basis of
examples, from which ensue further features, in themselves and/or
in combination.
EXAMPLE 1
[0074] The etching effect of a hydrogen peroxide-containing
alkaline solution on the emitter of a solar cell was compared with
the etching effect of a surfactant-containing solution. The
hydrogen peroxide-containing solution contained 1 to 6 weight
percent caustic soda and approximately 2 weight percent hydrogen
peroxide. The temperature was in the range of 49.degree. C. to
55.degree. C. The surfactant-containing solution contained 1.2
weight percent NaOH and 0.1 wt % sodium dodecylbenzenesulfonate.
The temperature of the solution was constant at 53.degree. C.
[0075] Multicrystalline wafers, after diffusion and after chemical
edge isolation, including dissolution of phosphosilicate glass in
dilute hydrofluoric acid, were etched in the alkaline solutions.
The contact times and the resulting increase in the emitter layer
resistance are compiled in Table 1 and 2.
Emitter Layer Resistance
TABLE-US-00001 [0076] TABLE 1 Increase in the emitter layer
resistance by treatment in hydrogen peroxide-containing solution.
Contact Before After time etching etching Delta Solution seconds
ohm/sq ohm/sq ohm/sq NaOH 1 wt %, H.sub.2O.sub.2 2 wt % 60 48.1
48.7 0.6 300 49.3 50.5 1.2 NaOH 3 wt %, H.sub.2O.sub.2 2 wt % 30
49.6 50.2 0.6 300 48.8 51.4 2.6
Emitter Layer Resistance
TABLE-US-00002 [0077] TABLE 2 Increase in the emitter layer
resistance by treatment in surfactant-containing solution. Contact
Before After time etching etching Delta Solution seconds ohm/sq
ohm/sq ohm/sq NaOH 1.2 wt %, sodium 25 54.0 58.9 4.9
dodecylbenzenesulfonate 50 55.3 68.1 12.8 0.1 wt % 75 55.9 78.6
22.7
[0078] It can be seen from the values that the etching rate of the
surfactant-containing solution is markedly higher than that of the
hydrogen peroxide-containing solution.
EXAMPLE 2
[0079] Multicrystalline wafers, after diffusion, were etched in a
hydrogen peroxide-containing or in a surfactant-containing solution
after chemical edge isolation, including dissolution of
phosphosilicate glass in hydrofluoric acid. The wafers were then
coated with a silicon nitride antireflection layer in standard
production processes and metallicized in the silk-screen printing
process. The electrical values of the cells as well as the emitter
layer resistance directly before a nitride coating are presented in
Table 3.
TABLE-US-00003 TABLE 3 Electrical values after emitter etching back
in a hydrogen peroxide-containing or surfactant-containing
solution. Jsc Voc [mA/ Eta R.sub.sheet Emitter etching solution
[mV] cm.sup.2] FF [%] .OMEGA./.quadrature. NaOH, 1.2 wt %, hydrogen
peroxide 0.609 34.07 0.772 16.02 64.0 0.3 wt % (20.degree. C.,
contact time 40 s) NaOH, 1.2 wt %, sodium 0.611 34.27 0.771 16.14
67.3 dodecylbenzenesulfonate 0.1 wt % (50.degree. C., contact time
25 s)
[0080] After a stronger etching back of the emitter, that is, at
higher layer resistance, higher short-circuit current densities
(Jsc) and higher open-circuit voltages (Voc) are observed,
resulting in a higher efficiency of the solar cells.
EXAMPLE 3
[0081] The etching effect of the solution according to the
invention from Example 1 and 2 was compared with the etching effect
of a cleaning solution consisting of 1 weight percent
tetramethylammonium hydroxide (TMAH) and 1 weight percent hydrogen
peroxide.
[0082] Used were multicrystalline wafers after diffusion and
chemical edge isolation, including dissolution of phosphosilicate
glass in dilute hydrofluoric acid. After treatment in the emitter
etching solutions was finished, further standard production
processes were carried out.
[0083] The electrical values and the increase in the emitter layer
resistance are compiled in Table 4.
TABLE-US-00004 TABLE 4 Electrical values after emitter etching back
in a solution according to the invention and in an aqueous solution
containing TMAH and hydrogen peroxide. Jsc Delta Voc [mA/ Eta
R.sub.sheet Emitter etching solution [mV] cm.sup.2] FF [%]
.OMEGA./.quadrature. NaOH, 1.2 wt %, sodium 0.603 34.24 0.768 15.84
5.0 dodecylbenzenesulfonate 0.1 wt % (50.degree. C., contact time
25 s) Tetramethylammonium hydroxide 0.601 34.03 0.769 15.77 1.5 1.0
wt %, hydrogen peroxide 1.0 wt % (60.degree. C., contact time 60
s)
EXAMPLE 4
[0084] Multicrystalline wafers, after diffusion, with an emitter
layer resistance of 48 ohm/sq, were treated in a vertical
laboratory unit initially in dilute hydrofluoric acid for 2
minutes, rinsed, and subsequently etched in a solution of the
following composition for 2 minutes at 50.degree. C.
[0085] KOH, 1.5 wt %
[0086] ammonium lauryl sulfate, 0.5 wt %
[0087] 1,2-dihydroxybenzene (pyrocatechol), 0.1 wt %
[0088] The measured layer resistance after treatment in the etching
solution was on average 53 ohm/sq.
EXAMPLE 5
[0089] In the same arrangement as in Example 2, a solution of the
following composition was used.
[0090] tetramethylammonium hydroxide, 1 wt %
[0091] ammonium lauryl sulfate, 0.1 wt %
[0092] diethlyenetriaminepentaacetic acid, 0.05 wt %
[0093] Monocrystalline wafers, after diffusion, with an emitter
layer thickness of 52 ohm/sq, were treated in dilute HF for 2
minutes, rinsed, and etched at 60.degree. C. for 1 minute in the
solution according to the invention. The measured layer resistance
was on average 58 ohm/sq. The increase in layer resistance was thus
6 ohm/sq.
EXAMPLE 6
[0094] An aqueous emitter etching solution with the following
composition was employed:
[0095] diethylenetriamine, 3.5 wt %
[0096] dodecyldimethlyamine oxide, 0.1 wt %
[0097] ammonium chloride, 0.5 wt %
[0098] hydrogen peroxide, 0.06 wt %
[0099] Used were multicrystalline wafers after inline diffusion.
After removal of phosphosilicate glass and after chemical edge
isolation in a horizontal unit, the emitter layer resistance was 53
ohm/sq.
[0100] After 1 minute of treatment time in a vertical laboratory
unit at 50.degree. C. in the solution according to the invention,
the layer resistance was on average 56 ohm/sq.
* * * * *