U.S. patent application number 09/335113 was filed with the patent office on 2002-12-19 for sc-2 based pre-thermal treatment wafer cleaning process.
Invention is credited to HACKENBERG, DIANA LYNN, LINN, JACK H., NOLAN-LOBMEYER, ROBERTA R., RAFIE, SANA, ROUSE, GEORGE V., SLASOR, STEVEN T., VALADE, TIMOTHY A..
Application Number | 20020189640 09/335113 |
Document ID | / |
Family ID | 22053109 |
Filed Date | 2002-12-19 |
United States Patent
Application |
20020189640 |
Kind Code |
A1 |
LINN, JACK H. ; et
al. |
December 19, 2002 |
SC-2 BASED PRE-THERMAL TREATMENT WAFER CLEANING PROCESS
Abstract
Pre heat-treatment processing of a silicon wafer to grow a
hydrophilic oxide layer includes an initial step of contacting the
wafer with a pre-clean SC-1 bath, thereby producing a silicon wafer
surface that is highly particle free. After a deionized water
rinse, the wafer is scoured with an aqueous solution containing
hydrofluoric acid and hydrochloric acid to remove
metallic-containing oxide from the wafer surface. In order to grow
a hydrophilic oxide layer, an SC-2 bath (containing hydrogen
peroxide and a dilute concentration of metal-scouring HCl) is used.
The resulting hydrophilic silicon oxide layer grown on the surface
of the silicon wafer using the combined
SC-1.fwdarw.AF/HCL.fwdarw.SC-2 wafer cleaning process has a metal
concentration no greater than 1.times.10.sup.9. The diffusion
length of minority carriers is increased from a range on the order
of 500-600 microns to a range on the order of 800-900 microns.
Inventors: |
LINN, JACK H.; (MELBOURNE,
FL) ; ROUSE, GEORGE V.; (INDIALANTIC, FL) ;
RAFIE, SANA; (MELBOURNE, FL) ; NOLAN-LOBMEYER,
ROBERTA R.; (WEST MELBOURNE, FL) ; HACKENBERG, DIANA
LYNN; (WEST MELBOURNE, FL) ; SLASOR, STEVEN T.;
(PALM BAY, FL) ; VALADE, TIMOTHY A.; (MELBOURNE,
FL) |
Correspondence
Address: |
ALLEN, DYER, DOPPELT, MILBRATH & GILCHRIST P.A.
1401 CITRUS CENTER 255 SOUTH ORANGE AVENUE
P.O. BOX 3791
ORLANDO
FL
32802-3791
US
|
Family ID: |
22053109 |
Appl. No.: |
09/335113 |
Filed: |
June 17, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09335113 |
Jun 17, 1999 |
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09064029 |
Apr 21, 1998 |
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5932022 |
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Current U.S.
Class: |
134/3 ;
257/E21.228; 257/E21.285 |
Current CPC
Class: |
H01L 21/02307 20130101;
H01L 21/31662 20130101; H01L 21/02052 20130101; H01L 21/02238
20130101 |
Class at
Publication: |
134/3 |
International
Class: |
C23G 001/02 |
Claims
What is claimed
1. A method for processing a silicon wafer subjected to thermal
processing that may cause contaminant metals present on the surface
of the wafer to be driven into the silicon wafer and degrade bulk
silicon minority carrier recombination lifetime, said method
comprising the steps of: (a) contacting said wafer with a pre-clean
SC-1 aqueous cleaning solution, so as to remove organic
contaminants and particulates, and form soluble complexes of said
contaminant metals; (b) contacting the surface of the silicon wafer
processed in step (a) with an aqueous solution containing
hydrofluoric acid and hydrochloric acid to remove contaminant
metallic-containing oxides from the wafer surface; and (c)
contacting the hydrofluoric and hydrochloric acid treated wafer
processed in step (b) with an ozone-free aqueous SC-2 solution
containing peroxide and hydrochloric acid to grow a hydrophilic
oxide layer on the surface of the silicon wafer.
2. A method according to claim 1, further including the step of:
(d) heating the wafer processed in step (c) to a temperature of at
least about 300.degree. C. for a duration of at least about one
second.
3. A method for heat-treating a silicon wafer comprising the steps
of: (a) contacting the surface of the silicon wafer with an aqueous
solution containing hydrofluoric acid and hydrochloric acid to
remove metals from the wafer surf aced (b) contacting the
hydrofluoric and hydrochloric acid treated wafer processed in step
(a) with an ozone-free aqueous solution containing peroxide and
hydrochloric acid to grow a hydrophilic oxide layer on the surface
of the silicon wafer, with the concentration of each of iron,
chromium, calcium, titanium, cobalt, manganese, zinc, and vanadium
on the surface of the silicon wafer being less than
1.times.10.sup.9 atoms/cm.sup.2; and (c) heating the wafer
processed in step (b) to a temperature of at least about
300.degree. C. for a duration or at least about one second.
Description
FIELD OF THE INVENTION
[0001] The present invention relates in general to the manufacture
of semiconductors, and is particularly directed to a new and
improved process for cleaning silicon wafers prior to a heat
treatment, so as to minimize the presence of particulates, organics
and metallic contaminants that impact minority carrier
recombination lifetimes. BACKGROUND
BACKGROUND OF THE INVENTION
[0002] The production of single crystal silicon wafers customarily
involves growing a single crystal ingot, slicing the ingot into
wafers, and then lapping, etching and polishing the wafers. Based
upon the required specifications of a circuit device manufacturer,
the silicon wafers may also be subjected to thermal processing,
such as, but not limited to, oxygen donor annihilation annealing,
thermal processing to control oxygen precipitation, low temperature
chemical vapor deposition (CVD) oxidation, epitaxial deposition,
and polysilicon deposition.
[0003] In the course of such thermal processing, a silicon wafer
will typically be exposed to a temperature of at least about
300.degree. C. for a duration of at least about one second. Under
these conditions, (contaminant) metals that may be present on the
surface of the wafer, such as but not limited to nickel, copper,
iron, chromium, calcium, titanium, cobalt, manganese, zinc and
vanadium, can be driven into the silicon crystal material, where
they can degrade bulk silicon minority carrier recombination
life-time. Ideally, the silicon wafer should be metal-free when
subjected to thermal processing.
[0004] In many applications it is also preferred that silicon
wafers to be subjected to thermal processing be passivated by a
hydrophilic silicon oxide layer. Unfortunately, a number of
limitations associated with conventional processes for growing
hydrophilic surface layers of silicon oxide have made it
impractical to grow such a silicon oxide layer at a surface
concentration of less than 1.times.10.sup.11 atoms/cm.sup.2 of
contaminant metals (minority carrier recombination lifetime
killers). As a result, silicon wafers have been routinely stripped
of their surface oxide layers prior to thermal processing.
Unfortunately, stripping such oxide layers prior to thermal
processing is not without its disadvantages, as silicon wafers that
have hydrophobic surface layer can be prone to localized (metal)
contamination.
[0005] A proposal to solve this problem, described in the U.S. Pat.
to Pirooz et al. U.S. Pat. No. 5,516,730, is to initially immerse
the wafer in a conventional pre-clean SC-1 aqueous cleaning
solution containing H.sub.2O.sub.2 and NH.sub.4OH, in order to
remove organic contaminants and particulates, and to form soluble
complexes of contaminant metals such as iron, copper, gold, nickel,
cobalt, zinc and calcium. To remove the complexed metals, the
surface of the pre-cleaned silicon wafer is subjected to the flow
of an aqueous solution hydrofluoric (HF) acid (which contains
hydrochloric (HCl) acid to enhance metal removal), followed by
rinsing the HF and HCl acid-treated wafer in deionized water. In
order to grow a hydrophilic oxide layer on its surface, the rinsed
wafer is then contacted with ozonated water. The ozonated
water-treated wafer upon which the oxide layer has been grown is
then heated to a temperature of at least about 300.degree. C. for a
duration of at least about one second.
[0006] Now although the patentee states that the concentration of
contaminant metals on the surface of the oxide-grown wafer at the
beginning of the anneal/heating step is less than 1.times.10.sup.9
atoms/cm.sup.2, his use of ozonated water creates several problems
that are antithetical to the objective of forming a hydrophilic
oxide on the metal free surface of a silicon wafer. First of all,
during the production of ozone in present day commercially
available ozone generation equipment, contaminant metals may be
leached from the hardware into the ozonated water. Thus,
irrespective of how well cleaned the surface of the silicon wafer
is following the post HF and HCl acid-treatment (deionized water)
rinse, the silicon wafer surface may be exposed to minority carrier
recombination lifetime killer contaminants in the ozonated
water.
[0007] Secondly, ozone concentration in the produced solution is
extremely difficult to control. As a result, the degree to which
the ozone (which tends to be an extremely aggressive solution with
respect to the surface of the silicon wafer) may modify the surface
of the wafer can vary. Consequently, it may be expected that the
silicon wafer surface will have a texture that may readily trap
contaminant metals that are likely to be present in the ozonated
water. When the oxide layer begins to grow, these minority carrier
recombination lifetime killer metals become fixed or trapped on the
wafer surface, and are eventually driven into the bulk silicon
material during subsequent thermal processing.
SUMMARY OF THE INVENTION
[0008] In accordance with the present invention, the
above-described metal decontamination problem is successfully
remedied by avoiding the use of ozone and instead, growing the
hydrophilic oxide layer in an SC-2 bath (a hydrogen peroxide
solution that also contains a dilute concentration of
metal-scouring HCl). An Hf/HCl bath immersion is used subsequent to
the initial SC-1 clean, in order to etch away or purge chemical
oxide metallics that may have grown on the wafer surface during the
SC-1 cleaning step, thereby producing a silicon wafer surface that
is highly hydrophobic and metal-free.
[0009] The HCl in the SC-2 bath serves to mitigate metallic
contamination during the chemical oxide growth caused by the
H.sub.2O.sub.2 in the SC-2 bath. The SC-2 medium in which the oxide
is grown is not itself a source of metallic contaminants. The
resulting hydrophilic silicon oxide layer grown on the surface of
the silicon wafer using the combined SC-1.fwdarw.HF/HCL.fwdarw.SC-2
wafer cleaning process of the invention has a metal concentration
no greater than 1.times.10.sup.9 atoms cm.sup.-2. Measurements have
revealed that subsequent to the heat treatment, the diffusion
length of minority carriers is increased from a range on the order
of 500-600 microns to a range on the order of 800-900 microns, or
about a fifty percent improvement in P-type silicon.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The single FIGURE shows successive steps of a pre-heat
treatment and thermal processing flow sequence in accordance with
an embodiment of the present invention.
DETAILED DESCRIPTION
[0011] As described above preliminary steps of the pre-heat
treatment process according to the present invention are
essentially the same as those described in the above-referenced
Pirooz patent, and are included within the process flow sequence
diagrammatically illustrated in FIG. 1. In accordance with a
preferred, but non-limiting example, the steps of the pre-heat
treatment cleaning sequence (shown in steps 101-113) may be carried
out in a conventional wet bench cleaning apparatus having a series
of tanks containing approximately one to one hundred liters of
pre-heat treatment solution. In addition, the process is preferably
controlled such that a wafer cassette or cassettes holding a
plurality of wafers, e.g., up to one hundred wafers, is
automatically transported to and immersed in the pre-cleaning bath,
oxide growth solution, etc. All wetted parts may be constructed of
quartz, polyvinylchloride (PVC), polyvinylidinedfluoride (PVDF),
polypropylene, or teflon.
[0012] Referring now to the process flow of FIG. 1, at step 101,
the silicon wafers are immersed in any of a number of
conventionally employed cleaning solutions, including piranha
mixtures (mixtures of sulfuric acid and hydrogen peroxide), HF
mixtures and SC-1 mixtures. The SC-1 cleaning solution contain may
contain about 1000:1:1 to 1:1:1 parts by volume
H.sub.2O:H.sub.2O.sub.2:NH.sub.4OH, preferably about 100:1:1 to
about 5:1:1 parts by volume H.sub.2O:H.sub.2O.sub.2:NH.sub.4OH,
(which may be supplied as 30-35 wt % H.sup.2O.sub.2 in water and
28-30 wt % NH.sub.4OH in water), and having a temperature on the
order of about 0.degree. C. to about 100.degree. C., preferably
about 25.degree. C., to 90.degree. C. As described above, the SC-1
solution is effective to remove particulates and organic
contaminants by the solvating action of ammonium hydroxide and the
vigorous action of hydrogen peroxide. Following the SC-1 step 101,
the wafers are rinsed with ultra pure water in step 103.
[0013] Metal removal is effected in step 105 by immersing the
silicon wafers in an HF/HCl aqueous solution, which effectively
removes metallic-containing oxids. The HF/HCl serves to tie up such
metals--forming soluble metal complexes--so as to inhibit their
redeposition from the solution and leaving the wafer surface
hydrophobic. For this purpose, the HF/HCl aqueous solution may
contain from about 1:1 to about 1:10,000 parts by volume of
HF:H.sub.2O, (which may be supplied as 49 wt % HF in water). To
enhance metals removal, the solution additionally contains HCl
(1:1:0 to 1:1:10,000 parts by volume HF:HCl:H.sub.2O, (which may be
supplied as 36.5-38 wt % HCl in water).
[0014] A preferred volumetric range of this aqueous HF/HCl metals
removal solution is about 5:2:200 parts by volume of
HF:HCl:H.sub.2O, (supplied as 49 wt % HF in water), and 36.5-38 wt
% HCl in water). The temperature of the solution may be in a range
of from about 10.degree. C. to about 90.degree. C., preferably in a
range of from about 25.degree. C. to about 60.degree. C. The
duration of immersion may be at least about six seconds, and
preferably in a range on the order of thirty seconds to ten
minutes.
[0015] Following metals removal, the wafers are rinsed in step 107
by a deionized water flow for a period of at least about 0.1
minutes and typically about two to ten minutes. The deionized rinse
water may have a resistivity from about 3 to about 18 megohms, and
preferably greater than about 17 megohms.
[0016] After rinsing away the metal-scouring HF/HCl solution in
step 107, an oxide layer is grown on the silicon wafers in step 109
by contacting the silicon wafers with an SC-2 bath that contains a
hydrogen peroxide solution and a dilute concentration of
metal-scouring HCl. The HCl/H.sub.2O.sub.2 oxide growth SC-2
solution may comprise HCl (1:0 to 1:10,000 parts by volume
HCl:H.sub.2O, (which may be supplied as 36.5-38 wt % HCl in water),
H.sub.2O.sub.2 (1:1:0 to 1:1:10,000 parts by volume
HCl:H.sub.2O.sub.2:H.sub.2O, supplied as 30-35 wt % H.sub.2O.sub.2
in water). A preferred volumetric range of this oxide growth
solution is about 1:1:5 parts by volume of
HCl:H.sub.2O.sub.2:H.sub.2O, (supplied as 30-35 wt % H.sub.2O.sub.2
in water), and 36.5-38 wt % HCl in water). The temperature of the
oxide growth solution may be in a range of from about 10.degree. C.
to about 90.degree. C., preferably in a range of from about
20.degree. C. to about 60.degree. C. The duration of immersion may
be at least about six seconds, and preferably in a range on the
order of two to ten minutes. The resulting silicon oxide layer may
have a thickness on the order of about 0.6 to about 2.5
nanometers.
[0017] As described briefly above, the HF/HCl solution serves to
remove metallic-containing oxides that may have grown on the wafer
surface during the initial pre-clean SC-1 bath of step 101, thereby
producing a silicon wafer surface that is highly hydrophobic and
metal-free. The SC-2 solution is not a source of metallic
contaminants. The silicon oxide layer grown within the aqueous
HCl:H.sub.2O.sub.2 bath employed in step 109 has a metal
concentration that is considerably reduced compared to that of
conventional processes, particularly the ozone-based process
described in the Pirooz patent. Measurements have shown that the
metal concentration in an oxide formed by the process of the
present invention is no greater than 1.times.10.sup.9 atoms
cm.sup.-2.
[0018] Upon completion of oxide growth step 109, the wafers are
rinsed in step 111 for a period of at least about 0.1 minutes, and
typically about two to ten minutes in deionized water having a
resistivity of about 3 to about 18 megohms, preferably greater than
about 17 megohms. After this post oxide growth rinse, the oxided
wafers are dried in step 113, by using any drying method which does
not recontaminate the wafers with metals or other contaminants. As
non-limiting examples, such a non-contaminating drying method may
include conventional spin-drying and isopropyl alcohol vapor drying
techniques.
[0019] Upon completion of the pre-heat treatment sequence of steps
101-113, described above, the dried wafers are transferred to a
furnace, rapid thermal annealer or other apparatus, in which
thermal processing is to be performed, as shown in step 115. To
minimize unnecessary handling (and potential recontamination) and
facilitate processing, the drying station of the pre-heat treatment
cleaning process is preferably integrated with the thermal
processing apparatus.
[0020] As described earlier, it is customary practice to subject
silicon wafers to oxygen donor annihilation annealing, thermal
processes to control oxygen precipitation, low temperature chemical
vapor deposition (CVD) oxidation and nitridation, polysilicon
deposition and other thermal process steps prior to being polished.
As a result of the pre-heat treatment steps of the invention, the
wafers upon which such thermal processing operations are carried
out have a relatively metal-free, hydrophilic surface, so that the
finally processed wafers have a minority carrier diffusion length
that is increased considerably compared to wafers obtained using
conventional processing, as described above.
[0021] While silicon is hydrophobic, silicon oxide is hydrophilic
and easily wetted by water. The degree of hydrophilicity or
hydrophobicity can be readily determined by reference to the
contact angle of a droplet of water placed on the surface. A
surface is considered to be hydrophilic if the contact angle is
less than 30 degrees; the surface is considered to be hydrophobic
if the contact angle is greater than 30 degrees. Preferably, the
contact angle for the hydrophilic surfaces described herein is less
than 10 degrees and preferably, about three to about five
degrees.
[0022] Due to the analytical approach used to determine the
concentration of metals on the surface of a hydrophilic wafer, the
concentrations given here include quantities of metal located on
the surface of the silicon dioxide layer, metal incorporated in the
silicon dioxide layer, and-metal located at the silicon
dioxide/silicon interface. Such methods for determining the surface
contamination of silicon are conventional. For example, the surface
metal content of silicon may be determined as described in an
article by K. Ruth et al, Proceedings of the ECS Fall Meeting,
Electrochemical Society 1993 (Vol. II) p. 488, the disclosure of
which is incorporated herein.
[0023] Various techniques may be employed to measure the minority
carrier recombination lifetime (or minority carrier diffusion
length) of a silicon wafer and typically involve injecting carriers
into a wafer sample by means of a flash of light or voltage pulses
and observing their decay. A non-limiting example of a process for
measuring minority carrier recombination lifetime is the surface
photovoltage (SPV) technique described in an article by Zoth et al,
in the Journal of Applied Physics, Vol. 67, 6764 (1990). Another
scheme for measuring diffusion length is to use an electrolytic
metal analysis tool (ELYMAT) instrument, such as that manufactured
by GeMeTec, Munich, Germany. This tool measures (to a resolution of
about 1 mm) photocurrents generated by a scanning laser beam.
Minority carrier diffusion lengths are calculated from these data
and diffusion length images can be generated. For a description of
this process attention may be directed to an article by H. Foell,
Proc. ESSDERC Conference, p. 44, Berlin, Germany, 1989. The
calculated diffusion length values are readily converted to
minority carrier recombination lifetime values using known
formulas.
[0024] The following example illustrates the invention.
EXAMPLE
[0025] A cleaning process was carried out on smooth P-type silicon
wafers having a resistivity of 5-20 ohm-cm, using a conventional
wet bench cleaning apparatus. The cleaning sequence for one set of
the wafers (set "A") was as follows:
[0026] STEP 1: 10 minutes in an SC-1 bath (1:1:5
NH.sub.4O:H.sub.2O.sub.2:- H.sub.2O) with megasonics at 50.degree.
C.;
[0027] STEP 2: Ultra pure water rinse for 5 minutes;
[0028] STEP 3: 1 minute in a metallic-containing oxide removal
solution (5:2:200 HF:HCl:H.sub.2O);
[0029] STEP 4: Overflow deionized water rinse for 5 minutes;
[0030] STEP 5: Immersion in an SC-2 bath (1:1:5
HCl:H.sub.2O.sub.2:H.sub.2- O) for 5 minutes;
[0031] STEP 6: Ultra pure water ines for 5 minutes;
[0032] STEP 7: Spin dried for 10 minutes; and
[0033] STEP 8: Processed through a diffusion furnace (600.degree.
C. for 60 minutes) in a nitrogen atmosphere.
[0034] A second set of wafers (set "B") was subjected to steps 1-2
and 5-8.
[0035] The wafers of sets A and B were analyzed for minority
carrier diffusion length by Elymat. The results were 900 microns
for set A and 450 microns for set B.
[0036] As will be appreciated from the foregoing description, the
above-described metal decontamination problem of using an ozonated
ambient to grow a hydrophilic oxide layer on a silicon wafer is
obviated in accordance with the present invention by using an SC-2
bath containing hydrogen peroxide and a dilute concentration of
metal-scouring HCl. The presence of the HCl in the SC-2 bath serves
to mitigate metallic contamination during the chemical oxide growth
caused by the H.sub.2O.sub.2 in the SC-2 bath. The SC-2 medium in
which the oxide is grown is not itself a source of metallic
contaminants. As a consequence the hydrophilic silicon oxide layer
grown on the surface of the silicon wafer using the combined
SC-1.fwdarw.HF/HCL.fwdarw.SC-2 wafer cleaning process of the
invention enjoys a substantially improvement in diffusion length of
minority carriers (from a range on the order of 500-600 microns to
a range on the order of 800-900 microns).
[0037] While we have shown and described an embodiment in
accordance with the present invention, it is to be understood that
the same is not limited thereto but is susceptible to numerous
changes and modifications as known to a person skilled in the art,
and we therefore do not wish to be limited to the details shown and
described herein, but intend to cover all such changes and
modifications as are obvious to one of ordinary skill in the
art.
* * * * *