Philips Res. Repts
R 701
24, 299-321,1969
INVESTIGATION OF THE CHEMICAL PROPERTIES OF
STAIN FILMS ON SILICON BY MEANS OF INFRARED
SPECTROSCOPY AND OMEGATRON MASS ANALYSIS
by Satoshi
YOSHIOKA
Abstract
Study of the chemical kinetics of the growth of stainfilms on silicon in
an HF-N02 gaseous mixture reveals that the film growth obeys a parabolic rate law. An equation for the rate constant is presented. Possible
chemical reactions for the preparation of the film material are proposed.
The chemical composition of the films was determined from the Lr.
absorption spectra and confirmed by omegatron mass analysis of the
evolved gas. The composition depends upon neither the conduction
type nor the boron surface concentration of the substrate silicon, while
it changes appreciably with the HFjNOrconcentration
ratio in the
gaseous mixture. The stain films were heat-treated in air and in vacuo,
and the chemical reactions of the film material were firmly established.
Rough estimates of the activation energies for the evolution of these
gases are presented. Irradiation of the film with u.v. light using a mercury lamp as the light source causes photochemical decomposition of
the film material. The absorption study in the i.r. visible and u.v. frequency regions and the omegatron mass analysis established the mechanism of the photochemical decomposition of the film.
1. Introduction
Stain films on silicon have been produced by various methods such as the
electrochemical technique 1), immersion in an HF-HN03
solution 2) and exposure to an HF-N02 gaseous mixture 3). Archer has investigated some physical
and chemical properties of the stain films formed in an HF-HN03 solution and
speculated that the films are silicon hydride and elemental silicon or a mixture
of both, depending on the conduction type and resistivity of the substrate
silicon 4). The stain films formed by the electrochemical method and chemical
corrosion in an HF-HN03 solution have been intensively studied by Beckmann
by means of infrared spectroscopy and the conclusion was drawn that they are
composed of silicon hydride 5). In a previous communication the present author
has reported a study of stain films by means of i.r. spectroscopy and omegatron
mass analysis, and has established that the films produced in an HF-N02
gaseous mixture consist of silicon hydride, SiOH, SiF, N03 - and N204, and
that their bonds with Si are ruptured by thermal or photochemical decomposition 6). A study of the photochemical
reaction of stain films was published
recently 7). In the present paper the kinetics of the growth of stain films on
silicon in an HF-N02
gaseous mixture and further information
about the
chemical properties of these films will be presented.
All reactions between silicon and HF-N02 took place at room temperature.
300
Satoshl YOSHIOKA
2. Growth of stain films on silicon
2.1. Experimental set-up
A diagram of the experimental set-up used to prepare stain films is shown
in fig. 1. Sources of the staining gaseous mixture were liquefied hydrogen
fluoride with a purity of about 99·9 % by weight and liquefied nitrogen peroxide
which was produced by pyrolytic decomposition of a reagent-grade lead nitrate
at 450 oe. The substrates used were mirror-polished silicon slices of both conduction types and resistivity ranging from 0·005 to 100 0 cm, with (Ill) faces.
9
Fig. 1. Schematic diagram of the apparatus used to prepare stain film;
I. Nrgas purifier,
6. N02 saturator,
2. flow meter,
7. reaction chamber,
3. HF cylinder,
8. Si slice,
4. HF saturator,
9. slice holder.
5. NOrgas generator,
Before preparations of the films the substrates were cleaned in hot benzene,
boiled in concentrated nitric acid, rinsed with deionized water and then dried
in hot air at 200-300 oe. After these cleaning .operations the substrate surface
showed the hydrophobic state which is required for the growth of a film of
homogeneous thickness.
The gaseous mixtures were prepared by bubbling two flows of N2 carrier gas
through the liquid HF and N02 *) respectively and mixing these saturated gases
with another flow of N2• The concentrations of HF and N02 in the mixtures
were varied independently by changing the flow rates of carrier gas and the
*) For the sake of brevity, N02 is used throughout this paper to designate, unless otherwise
specified, an equilibrium mixture of N02 and N204•
CHEMICAL
PROPERTIES
'OF STAIN FILMS ON SILICON
301
temperature of each saturator; HP was ranged between 0·5 and 8·0 mole %
and N02 between 0·25 and 5·0 mole %. The total gas flow rate was I I/min.
The film thickness was measured by an interference method after the stain
films were changed to a chemically stable oxide by heating them in ambient
O2 at 600-700 oe for 16 hours, because the stain film is not chemically stable
in light. The thickness of thermal silicon-oxide film grown between the substrate
and the stain film is negligible under these conditions.
2.2. Results
The stain films produced by this method display two kinds of appearance
depending on the conduction type and resistivity of the silicon substrate as has
been reported for the films prepared in an HP-HN03 solution 4). Those formed
on an n-type substrate in all resistivity ranges and on a p-type substrate above
0·1 Q cm give bright interference colours, while those formed on a p-type
silicon of resistivity lower than 0·1 Q cm show faint bluish interference colours.
The former corresponds to the type-I film as described by Archer and the latter
to the type-Ill film. The films corresponding to type I are changed to an oxide
which is soluble in an HP solution by a heat treatment above 150 oe in an
O2 atmosphere for 1 hour. Those corresponding to type-Ill film, on the other
hand, are not completely dissolved in an HP solution even after heat treatment
at 600 oe in O2 for 16 hours.
By the exposure of a wafer to the HP-N02 mixture the film thickness increases to a certain value, 3000-5000 A, and then decreases. The rate of change
in the film thickness before and after oxidation is probably not rigidly constant
for films produced under different conditions. Only a rough estimate of the
reaction kinetics is therefore possible in this work. The relationship between
the thickness of the stain film and staining time, as shown in fig. 2, suggests
that the growth of the film obeys a parabolic rate law during the early growth
period. The film thickness L with reaction time t is given as
L2
= kt,
where k is a rate constant. The rate constant changes both with the concentration of N02 and with that of HP in the gaseous mixture as shown in fig. 3.
When the HF concentration is rather low, k increases with both N02 and HP;
k begins to decrease with further increase of HP concentration. The straight
lines in fig. 3 give the following rate-constant equation:
k
=
N02
5.105 [N02]o.s [HP]-o.s
:
0·25 - 2·0 mole
%,
A2 s-t,
HP: 1 - 8 mole
(2)
%,
where brackets denote the concentration of the gas in mole %.
No effect ofthe conduction type and resistivity ofthe substrate on the growth
302
Satoshi YOSHIOKA
N-~~ 8r---;---P+----r---+----r--~
Q
'-.!.
N
~ 6r---,.r--+--~r---+---~--_4
(IJ
~
.\,!
~
14J----+---t-IH---H-!---I-----+--l
10
20
30
(s)
-Time
Fig. 2. Growth rate of stain films.
HF concentration (mole %)
N02 concentration (mole
1
2
1
1
1
0·25
8
1
4
·1
2
1
0·5
1
Symbol
o
b,.
o
.•..
•
X
V
1
0·2
-:
/
1
-N02
0·5
aJ
%)
V
_,..---.
/
2
(mole oio)
1
~
~
~
2
0·5
-HF
5
10
(mole%J
b}
Fig. 3. Dependence of rate constant on HF and N02 concentrations.
rate was detected. An addition 'of steam to the N2-HF-N02 gases giving a
humidity of 0·03-3 mole % brings about little change in the growth rate, but
increases the pin-hole density and the surface roughness. These pin holes are
probably caused by chemical etching of the grown film by the locally adsorbed
water molecules which absorb HF and N02 gases.
CHEMICAL
PROPERTIES
OF STAIN FILMS ON SILICON
303
3. Study of the chemical properties of the stain film by means of infrared
spectroscopy
3.1. Experimental set-up
The specimens for the i.r.-absorption measurements were zone-purified
silicon slices of which both surfaces were mirror-polished and covered with
stain films about 2500 A thick. The slice had an n-type resistivity of 100 Q cm
and (111) faces. The optical absorption due to Si-O bonds in the bulk of the
slices was negligible.
For absorption measurements on the films produced on a heavily doped
substrate, boron was diffused into the surface of a slice of silicon by a gasdiffusion technique before preparation of the films. The boron distribution
showed a complementary error-function diffusion profile. Employment of these
substrates enabled the large optical absorption by free carriers, which is
observed when a homogeneously doped crystal is used, to be avoided. The
specimens were pumped in vacuo at room temperature for 1 hour before each
measurement to eliminate any residual gases which had possibly been absorbed
in the as-grown films. The specimens were stored in the dark, since we found
that the stain films are chemically decomposed when they are subjected to light.
The absorption measurements were carried out in air at room temperature,
except for the study of thermal decomposition of the stain film, which took
place in a vacuum. A diagram of the vacuum chamber used for the thermaldecomposition study is shown in fig. 4. A mercury lamp was used as an ultraviolet-light source for the study of photochemical decomposition of the stain
film. An increase of the film temperature during irradiation with the lamp was
prevented by placing the specimens on a smooth-surfaced copper plate which
was cooled with a water jacket. The i.r. spectrum was recorded with the help
t
Pump
Fig. 4. Vacuum chamber for i.r.-absorption measurement.
1. Sample (Si slice),
4. furnace,
2. C.C. thermocouple,
5. NaCI window,
3. sample holder,
6. water jacket.
304
Satoshi YOSHIOKA
.
of a Leitz double-beam spectrophotometer with an NaCI prism and a grating.
The frequencies were corrected for each run by recording a spectrum of polystyrene.
3.2. Results
3.2.1. Chemical
composition
of the stain
film
Figure 5 shows typical i.r. absorption spectra of the stain films produced on
the substrate in three sets of gaseous mixtures for three sets of boron surface
concentrations. These results reveal that the qualitative chemical composition
of the films is independent of the boron concentrations in the substrate surfaces; type-I and type-Ill films, grouped by Archer, consist ofthe same material.
A comparison of the relative intensities of the absorption bands between the
spectra a-c suggests a strong correlation between the quantitative composition
ofthe film and the HF-to-N02 mole ratio. When the HF/N02 ratio is increased
the absorptions at 2120, 2090 and 904 cm>' increase in intensity and those
at 700, 3310, 1438, 1090 and 733 cm-1 diminish. The absorption band
at 3700 cm-1 is ascribed to 'V(OH) of an isolated SiOH group and the
band at 3310 cm-1 to 'V(OH) of hydrogen-bonded SiOH and/or chemisorbed H20. The bands at 2120 and 2090 cm-1 can be readily assigned
to Y(Si-H). The band at 1438 cm-1 is in the frequency region of NOl' ions.
Wavenumber (cm-I) _
4000
2
2000
4
1600
6
1200
8
---
1000
BOO
700
10
12.
14
Wavelength (/1)
Fig. 5. I.r. absorption spectra of stain films produced under various conditions,
Symbol
HF
N02
B surface
concentration
concentration
concentration
(mole %)
(mole %)
(at.cm=")
a
8
1
< l.l016
b
2
1
< l.l016
c
1
4
< l.l016
d
2
1
R:I 2.1018
e
2
1
R:I 7.1019
~~~~~~~~~~~~~~~~~~~~~~~~~~---~---
CHEMICAL
PROPERTIES
OF STAIN FILMS
ON SILICON
305
Since N02 - ions give two absorptions of comparative strength at 1380-1320
cm"! and 1250-1235 cm-I, the band at 1438 cm-I can reasonably be attributed
to v(N03 -) which produces strong absorption at 1410-1340 cm-1 and weak
absorption at 860-800 cm-I. The broad but weak band at 1090 cm-I may be
assigned to v(Si-O). Identification of the bands at 904 and 733 cm-I is rather
difficult. The deformation vibration ofthe SiH3 group and the valence vibration
ofthe Si-F bond have been reported to give an absorption band near 940 cm-I
and at 900-800 cm-I, respectively. The fact that the band at 904 cm-1 is intensified with increasing HF /N02 ratio in the staining gas supports the surmise that
both v(SiH3) and v(Si-F) are possible causes of this absorption band. In the
thermal-decomposition study of the stain films as described below, however,
the band at 904 cm-I was eliminated by heat treatment at 125 oe in vacuum,
while no change of the band at 2120 and 2090 cm-I was detected. The band
at 904 cm-I is therefore thought to be caused by v(Si-F). The band at 733 cm-I
can be ascribed to both (j(SiH3) and (j(N03 -). The observed behaviour of the
band at 733 cm-I, namely a decrease of intensity with increasing HF/N02
ratio, suggests that this band can be assigned to (j(N03 -). These results are .
summarized in table I.
TABLE I
Assignment of i.r. absorption band
frequency
(cm-I)
assignment
remarks
3700
3310
2260
2120
2090 (Sh)
1438
1130
v(O-H)
v(O-H)
isolated SiOH
H-bonded SiOH and/or chemisorbed H20
produced by irradiation with u.v. light
1090
1080
v(Si-O)
v(Si-O)
1060
904
885
v(Si-O)
:v(Si-F)
v(Si-F)
733
(j(N03-)
v(Si-H)
v(Si-H)
v(Si-H)
v(N03-)
v(Si-O)
produced as a shoulder by heat treatment at
100 oe and as a peak at 125 oe
produced by heat treatment at 600 oe or by
irradiation with u.v. light
produced by heat treatment at 100-150 oe
produced by heat treatment at 125
irradiation with u.v. light
oe or by
306
Satoshi YOSHIOKA
3.2.2. Thermal
decomposition
of the stain
film
Spectral changes of the stain film after heat treatments in air are shown in
fig. 6. The absorption bands caused by SiOR, N03 - and Si-F disappeared after
heat treatment at lOO-150°C. The bands due to v(Si-R) were broadened at
100-150°C and then eliminated at ISO-200°C. The band due to v(Si-O) at
1090 cm-1 increased in intensity with temperature above 90°C and shifted
simultaneously to 1060 cm-1 at lOO-150°C. This band again shifted in the
direction of higher frequency above 150°C, reaching the wavenumber of
1080 cm"" at 600°C. A shoulder at 1130 cm-1 was produced at lOO-150°C
and eliminated at 150-200 °C. These changes in the absorption bands may
have resulted both from thermal decomposition and from oxidation of the
film.
Wavenumber
4000
(cm-I)
_
2000 1600
1200
7000
800
700
d-----V
2
4
6
8
---
10
12
14
Wavelength (p)
Fig. 6. Change of i.r. absorption spectrum by heat treatments in air;
a: immediately after preparation;
b: after, heat treatment at 100 °C for 1 hour;
c: afterj heat treatment at 150°C;
d: after heat treatment at 200°C.
In order to minimize the effect of O2 in the ambient atmosphere on spectral
changes the sample was heat-treated in a vacuum chamber (fig. 4), while the
spectra were recorded for several sample temperatures. The pressure in the
chamber was about 1.10-2 Torr. Figure 7 shows the changes of spectra after
heat treatment of the film in a vacuum. Elimination of the bands due to SiOR
and N03 - was observed at about 125°C. The 'J.'(Si-F)band shifted to 885 cm"?
at 125°C and disappeared at 150 °C. No change in the shape and intensity of
the v(Si-R) bands was detected in vacuum as long as the temperature was lower
than 175°C. A decrease in the intensity of these bands occurs at 175-200 °C,
while neither a shift nor broadening was observed until they disappeared.
CHEMICAL PROPERTIES
Wavenumber (cm-I) _
4000
2000 1600
OF STAIN FILMS
1200
1000
ON SILICON
800
307
700
a
b-------..
c----.. /"--~- .....
;.:;_-......,,----
d----'r--------------------e ----..-,--------
--------
f----._,------------...;..._--
g-------2
4
6
8
-
10
12
14
Wavelength (il)
Fig. 7. Change of i.r. absorption spectra by heat treatments in vacuum;
a: immediately after preparation;
b: after heat treatment at 100°C for half an hour;
c: after heat treatment at 125°C;
d: after heat treatment at 150°C;
e: after heat treatment at 175°C;
f: after heat treatment at 200°C;
g: after heat treatment at 200 °C in vacuum and exposure to air at room temperature.
The band at 1130 cm-i appeared as a shoulder at 100 oe, became a large
independent absorption peak at 125 oe and then disappeared at 150 oe. The
v(Si-O) band at 1090 cm-i decreased in intensity above 150 oe and disappeared
at 200 oe in a vacuum. When air was admitted into the vacuum chamber at
room temperature after the film had been heat-treated at 200 oe, a new absorption band due to v(Si-O) was produced at 1080 cm-i.
3.2.3. Photochemical
decomposition
of the stain
film
The i.r. absorption spectra of the stain films changed after irradiation with
U.v. light at room temperature as shown in fig. 8. A new absorption band at
2260 cm-i due to 'J.r(Si-R)was produced and those at 2120 and 2090 cm-i
decreased in intensity simultaneously. Further irradiation produced in this
frequency region a broad band, which was finally eliminated. The bands due
to v(OR) and v(N03 -) decreased gradually under irradiation. The band due
to v(Si-]~<')
at 904 cm-i shifted to 885 cm-i after a short irradiation, but further
irradiation caused no observable change.
Absorption of light by the stain film was studied by a reflective method in
the wavelength range between 260 and 500 mfL in order to obtain further
information on the mechanism of photochemical decomposition of the film
material. The intensity ratio of the reflected light from the specimens to that
from a silicon slice was measured using a hydrogen-discharge lamp as a light
308
Satoshi YOSHIOKA
Wavenumber
4000
(cm-I)
2000
a--
_
1600
1200
,,---
b--~,nr------
1000
__
----=-
a'
4
6
8
---
700
-...._...-__
c------~,,'---------d-------------~
2
BOO
10
12
14
Wavelength (p)
Fig. 8. Change of i.r. absorption spectra of stain films by irradiation with u.v, light; the samples
for a-d and a'-d' were produced in gaseous mixtures containing 8% HF, 1% N02 and 1 %
HF, and 4% N02, respectively;
a, a': immediately after preparation;
b, b': after irradiation with U.v. light for 1 hour;
c:
after irradiation with u.v, light for 2 hours;
d, d': after irradiation with u.v. light for 10 hours.
300
400
-
500
600
Wavelength {mu}
Fig. 9. Change of u.v. absorption spectrum of stain film produced in an HF-rich mixture;
a: immediately after preparation;
b: after irradiation with U.v. light for 10 hours;
c: after heat treatment at 600 °C in air for 16 hours.
source. A broad and strong absorption extending from 250 to 300 mIL was
observed for the stain films produced in an HF-rich gaseous mixture, but only
a weak absorption was detected for those produced in N02-rich gases. Figure 9
shows a typical result obtained before and after irradiation and heat treatment
of the film.
CHEMICAL
PROPERTIES
OF STAIN FILMS ON SILICON
309
4. Study of the chemical properties of the stain film by means of omegatron mass
spectroscopy
4.1. Omegatron mass analysis
The chemical properties of stain films on silicon have been studied mostly
by means of i.r.-absorption spectroscopy. The i.r.-absorption technique, however, could not definitely determine the mechanism of the chemical reactions
in the stain films; this technique has a rather low sensitivity and cannot detect
the gaseous reaction product. In order to remove ambiguity the evolution of
gases from the film was studied with an omegatron mass spectrometer. This
instrument possesses high sensitivity and is capable of reasonable resolution;
it can detect a partial pressure of 10-11 Torr. It provides a unique tool for
investigating the chemical raections ofthin films which evolve gaseous products.
The principle of operation of the omegatron has been dealt with in several
papers 8). The ion current i+ is given by an equation
i+ = a sap i- exp (-Lp.);
(3)
a : collection efficiency of the produced ion,
s : path length of the electron beam,
a : ionization probability of the molecule,
p : partial pressure,
t=: electron current for ionization,
path length for the resonant ion,
A : mean free path.
In this equation a and s are the characteristic values of the tube structure
employed. The values of a, i- and exp (-LjA) are determined by the gas
molecule and the operating condition of the omegatron. The ion current will
therefore be proportional to the partial pressure of the gas component if these
values are kept constant. The proportionality between the partial pressure and
the ion current has been proved experimentally to exist as long as the total
pressure inside the omegatron is less than 10-5 Torr 9). When a mass spectrum
is obtained by a flow method in which the gas evolved from the specimens
is exhausted through a control valve, the evolution rate Q of a gas component
in the sample chamber is
L:
Q
R:1
Cp,
(4)
where C is the conductance of the valve. When C is a constant value, eqs (3)
and (4) give
Q
=
si-,
(5)
where K is a constant. Equation (5) shows that the observed ion current gives
310
Satoshi YOSmOKA
the relative evolution rate of the corresponding molecule under the above condition.
4.2. Experimental set-up
The vacuum system employed is shown in fig. ID. The sample chamber is
made of quartz and is connected to the vacuum system by means of a graded
seal. Before each measurement the entire system upstream from the liquidnitrogen cold trap was baked out at about 300°C; the sample chamber was
baked out at about 1000 °C. A pressure of 10-9_10-10 Torr was attained in
this way. The baking-out procedure eliminated the hydrocarbon background
in the mass spectrum. The pressure of the gases in the omegatron analyzer was
varied by adjusting the conductance of the control valve. The conductance was
kept constant during measurement to ensure validity of the linear relationship
between thé ion current and the evolution rate of the corresponding gas. The
pressure in the omegatron was determined with a Bayard-Alpert ionization
gauge. The ion current was recorded using a vibrating-reed electrometer; the
sample temperature was measured with an alumel-chromel thermocouple. The
resolution of the omegatron analyzer used was about 30 (at M = 30); two
kinds of ions with mass numbers of 30 and 31 could be recorded as two independent ion-current peaks.
5
1 2 3
Fig. 10. Schematic diagram of vacuum system for omegatron mass analysis;
1. sample chamber,
6. omegatron analyzer,
2. sample,
7. Bayard-Alpert gauge,
3. furnace,
8. control valve,
4. chrornel-alumel thermocouple,
9. liquid-Nj cold trap.
5. capillary,
The specimen consisted of stain films about 2500 A thick on both faces ot
silicon slices having p-type resistivities of 100 and 0·005 Q cm respectively. If
was placed in the sample chamber, which was then welded to the vacuum
system. The sample chamber was baked out in vacuo, the specimen being
moved out of the chamber with a magnet in order to prevent its temperature
from rising. For investigation of the thermal decomposition of the stain film
the temperature of the specimen was raised at a rate of 2 °C per minute up to
500°C. For study of the photochemical decomposition the film was irradiated
with U.v. light through the chamber wall of quartz; a mercury lamp was used
as 'the light source. Recording of a mass spectrum in the mass-number range
between 2 and 100 took about 7 minutes.
CHEMICAL PROPERTffiS
311
OF STAIN FILMS ON SILICON
4.3. Results
4.3.1. Background-gas
analysis
The residual gas in the vacuum system and the gas evolved from the silicon
substrate were analyzed, the temperature of the sample chamber being deliberately raised to determine the background gas. Typical mass spectra are
shown in fig. 11. The mass numbers of the observed ion-current peaks reveal
that the predominant background gases are H20, CO, CO2 and H2• The total
pressure change in the omegatron analyzer on modifying temperature suggests
that the gas desorption becomes noticeable above 250 oe and reaches a peak
at about 330 oe, as shown in fig. 12. The ion-current-temperature curves for
each ion (fig. 13) indicate that the desorption rates of H20, CO and CO2 and
that of H2 increase noticeably above 250 oe and 400 oe respectively.
~
a}
t:
a
..../§
-------I~J--
b)
Fig. 11. Omegatron mass spectra of background gas; (a) at 240°C, 3'4.10-9 Torr; (b) at 330°C,
1,2.10- 8 Torf.
6
Pressure
(Torr)
5
(\
\
7
-~
~
2
__...cr'
7
8
5
-,
bi?
/
...............
--
2a_
9
100
~
/
;:7
200
300
-
-;
~
<,
400
500
Tempercriure (Oe)
Fig. 12. Change in total pressure of background gas and evolved gas with temperature;
gas, -0- gas evolved from the stain film.
-e- background
312
Satoshl YOSHIOKA
Fig. 13. Ion current vs temperature for the background gas;
MIe = 2 (Hz+),·
• MIe = 16 (0+),
l::,. MIe = 17 (OH+),
Á MIe = 18 (HzO+),
MIe = 28 (CO+),
• MIe = 44 (COz +).
o
o
4.3.2. Thermal
decomposition
of the stain
films
A typical change in the total pressure of the evolved gas during heat treatment of the stain film is shown in fig. 12. The large pressure increase on heating
the film and the difference of the pressure-temperature curves between Ca) and
Cb) in fig. 12 indicate considerable gas evolution from the film. Mass spectra
Fig. 14. Omegatron mass spectra of gas evolved from the stain film; (a) film temperature
120°C, (b) film temperature 240 -c,
CHEMICAL
PROPERTIES
OF STAIN FILMS ON SILICON
313
of the gases evolved from the film at 120 and 240 oe are shown in fig. 14. The
mass number of the observed ion-current peaks and the corresponding ionic
species are listed in table II. Figure 15 shows the changes with temperature of
the ion current due to some representative ionic species. These results establish
that the principal gas evolution occurs in two temperature regions: in the first
temperature region between 70 and 150 oe several kinds of gases, including
H20, N204 and HF, are evolved, and in the second region between 200 and
300 oe H2 and SiHn fragment ions are evolved. No effect due to the resistivity
of the substrate silicon was observed on these results.
Determination of the ionic species corresponding to the observed ion-current
peaks is fairly easy. For a few ions, however, it is complicated, because two
different ions having the same mass number are recorded as an ion-current
peak. The presence of OH+ and NH3 + ions as the ionic species corresponding
to the observed mass number of 17 is possible, because organic species do not
have to be considered here. The pattern coefficient of H20 in an omegatron
mass spectrum is stated to be 100, 21 and 2 for H20+, OH+ and 0+ ions respectively 10). In fig. 14a, however, the ion current due to a species of mass
number 17, i1 7+, is larger than i18+ (equal to iH20+), and therefore the species
is probably a mixture of more than two ions, but chiefly H20+ and NH3 +.
To confirm this deduction the ion-current ratios i16+/i18+ and i17+/i18+ were
calculated for both the background gas and the gases evolved from the film,
and plotted against temperature (fig. 16a). The ratios for the background gas
are constant with temperature; this result suggests that the ions of mass 17
and 16 in the background gas are OH+ and 0+, respectively. The calculated
pattern coefficients of H20+, OH+ and 0+, namely 100, 30 and 10, are in
agreement with the published values mentioned above. The ion-current ratio
for the evolved gas, however, changes a great deal with temperature; this result
reveals that ions other than OH+ and 0+ are present in the evolved gas. If it
is assumed that the ion currents i17+, i16+ and i15+ obtained for the evolved
gas are due to OH+
NH3 +, 0+
NH2 + and NH+, then the following
equations result:
.
+
+
i17+
=
i16+
i15+
= io+
=
iOH+
+
+
iNH3 +s
iNH2+,
(7)
iNH+.
The values of iOH+ and io+ were calculated from i18+ for the evolved gas using
the pattern coefficients of H20+, OH+ and 0+, namely 100, 30 and 10; the
values of iNH3+' iNH2+ and iNH+ are found from eq. (7). The ion-current ratios
iNH2+/iNH3+
and iNH+/iNH3+ were calculated in this way and plotted in fig. 16b.
These ratios are constant with temperature, and the calculated pattern coefficients for NH3 +, NH2 + and NH+, namely 100, 75 and 10, are in good agreement with the published data: 100, 70-80 and 5 10). These results establish
314
Satoshi YOSHIOKA
TABLE II
Correlation between the evolved gas and the molecule in the film
ionic species
MIe
evolved gas
groups and ions
H2
sur,
NH3
H2O
N03SiOH
HF
N2
SiH
SiH2
SiH3
SiH4
Si-F
N03SiHn
SiHn
SiHn
N02
N204
N03N03-
Fig. IS. Ion current vs temperature for the evolved gas;
H2+,
• SiH+,
... H20+,
0 HF+,
6. OH+,NH3+,
• N204+ •
H2+
2
15
16
17
18
19
20
28
29
'30
31
32
44
46
-92
0+
OH+
H2O+
H3O+
HF+
N2+
SiH+
SiH2+
SiH3+
SiH4+
NH+
NH2+
NH3+
(CO+) *)
NO+
sra,
(C02 +) *)
N02+
N204+
*) Residual gas in the vacuum system.
--.. TO~
'f
S!
'oj..>
c:
(IJ
'':;,
lJ
T02
c:
.!:!
1
o
TO
CHEMICAL
;g
PROPERTIES
OF STAIN FILMS
315
ON SILICON
2'0
ft
e
.....
c:
~ 1·5
-5
lJ
1
'0
e
.....
\\
/
0·5
:g
\
I
c:
~
c: 1·0
~
lJ
~
~
~
lOO
I~
--
~
300
400
500
Temperaturel=C)
200
-
r
v
0
I
~
n
0
c:
00
100
_
200
300
Temperature (OC)
b}
a}
Fig. 16. Ion-current ratio vs temperature, B.G.: evolved gas, B.G.: background gas;
(a) 0 i17+/;18+ (B.G.),
!:::,. ;16+/;18+ (B.G.),
• i17+/;18+ (B.G.),
... il6+/i18+ (B.G.);
(b) 0 (i16+-;0+)/(;17+-;OH+) (B.G.), !:::,. ;15+/(il7+-;OH+) (B.G.).
lOO
-..
$
'03
1
...._
.....
~
lJ
c:
i
2·6
2·8
3·0
-1/T(.103oW'}
1·6
Fig. 17. Ion current vs l/T;
o H2+,
... H20+,
•
SiH+,
•
N204+, !:::,. SiH2+, 0 HF+,
X SiH3+.
that NH3 is evolved from the heat-treated stain film.
.
The ion current must be proportional to the evolution rate of the corresponding molecule as described in sec. 4.1. On the basis of this fact rough estimates of the activation energies for the evolution of the gas molecules from the
.316
Satoshi YOSHIOKA
stain film can be made from the ion-current vs l/T plot. The plot and the calculated values are presented in fig. 17 and table Ill, respectively.
TABLE III
Activation energy for gas evolution
-E (kcal/mole)
gas molecule
16-17
H2' SiH, SiH2, SiH3, SiH4
N204, N02, N2, NH3
11-12
8-9
4-5
H20
HF
4.3.3. Photochemical
decomposition
of the stain
film
Mass spectra of the gases evolved from the stain film were recorded before
and during irradiation of the film with u.v. light. Typical spectra are shown in
fig. 18; the increase in ion current due to H2 +, Sifî,+ fragment ions, H20+
and N204 + was observed. The ion current due to H2 + increased and reached
a saturation value soon after irradiation of the specimens as shown in fig. 19.
When a fluorescent lamp was used as a light source instead of a mercury lamp
a similar ion-current-time curve was obtained for H2 evolution .
.....c:
<IJ
a)
t::;,
I
t.J
c:
~
161718
28
-z92.
~
bJ
Fig. 18. Omegatron mass spectrum of
gas evolved from the stain film under
irradiation with u.v. light;
(a) before irradiation,
(b) under irradiation.
t
o
3
-
Time {min}
Fig. 19. Evolution rate of H2 from the stain
film under irradiation with u.v, light,
CHEMICAL
PROPERTIES
OF STAIN FILMS ON SILICON
317
5. Discussion
5.1. Reaction kinetics of stain-film growth
The growth of stain films on silicon in an HF-N02 gaseous mixture obeys
a parabolic rate law, being similar to the growth in an HP-HN03 solution 4).
This fact suggests that the rate-determining factor in the film growth is the
diffusion of some ionic or molecular species in the grown film.
On the basis of the experimental results described in the previous sections,
which showed that the stain film is composed of silicon hydride, SiOH, Si-P,
N03 - and probably elemental silicon, possible chemical reactions are given by
the following equations for the preparation of the film material:
2HP
~HP2-
Si
~ Si+
H+
+e-
+ H+,
+ e-,
~tH2,
+ HF2- ~SiHF2,
SiHP2 + 2 N02~ SiH + 2 N02P,
SiHP2 + H20 ~ SiOH + 2 HP,
2 N02 + 2 HP ~ 2 N02P + H2.
Si+
(8)
(9)
(10)
(11)
(12)
(13)
(14)
Increase of the rate constant k with N02 and HF concentration under a growth
condition of relatively low HF concentration is accounted for by eqs (11) and
(12). The rate-constant equation
(2)
which is obtained for the HF-rich staining gas is explained qualitatively as
follows. When HP is abundant in relation to N02, the N02 concentration will
decrease with increasing HP near the substrate surface as a result of the
chemical reaction (14); the rate constant k thus decreases with increasing HF
concentration. The quantitative analysis of eq. (2) requires further investigation.
5.2. Chemical composition of the stain film
The experimental results obtained by means of two different techniques,
i.r.-absorption measurement and omegatron mass analysis, demonstrate that
increase in the ion current due to H2 + and Sil-l,+ fragment ions and elimination of the Si-H valence band occur in the same temperature region. The values
of ion current due to the Sifl, + are one order of magnitude larger than those
due to the SiH4 +. This fact suggests that the SiHn+ fragment ions are produced
318
Satoshl Y08HIOKA
not by electron bombardment of the SiH4 molecules in th~ omegatron analyzer
but by the ionization of the fragment molecules ejected directly from the stain
film. These results prove that the stain film is composed of silicon hydride
which has SiH, SiH2 and SiH3 groups.
Disappearance of the weak broad band due to Si-O bonds at 1095 cm-1
after heat treatment in a vacuum above 200 oe implies that these Si-O bonds
do not form the skeletal structure of the stain film and are ejected, probably
as a siloxane gas, when the silicon hydride is decomposed pyrolytically. Nitrogen peroxide in the staining gas may play an important role in the formation
of Si-O bonds during and/or after the film growth. This guess agrees with the
observed fact that the intensity of the Si-O band decreases with increasing
HF /N02 ratio in the staining gas. The conclusion is that the stain film produced
in a gaseous mixture consists of silicon-hydride groups connected by Si-Si bonds
similar to those produced by an electrochemical method in an HF solution 5).
The fact that the Si-F absorption band is eliminated and the evolution rate
of HF increases reveals that Si-F bonds are present in the stain film and that
HF is evolved as a result of bond rupture.
Appearance of the absorption bands due to SiOH and N03 - in the i.r.
spectrum of the as-grown stain film is a characteristic feature of these films
prepared in an HF-N02 gaseous mixture. Increase of the HF/N02 concentration ratio, however, diminishes these bands and gives a spectrum similar to
those which are recorded for stain films produced by other methods 5). From
this result it may be concluded that there is no qualitative difference of composition between the stain films on silicon produced by the various methods.
5.3. Thermal decomposition of the stain film
The experimental results of the thermal-decomposition study of the stain
films by means of i.r. absorption spectroscopy are in good agreement with
those obtained from the omegatron mass analysis. The assignments of the
observed absorption bands and the chemical reactions expected from the
behaviour of these· bands were confirmed by analysis of the gas molecules
evolved from the film and the change of evolution rates with temperature. The
elimination of the absorption bands due tb SiOH, N03 - and SiF may correlate
with the large increase of the total pressure in the sample chamber at 100-150 oe,
and that due to Si-H bonds may correlate with the pressure increase at250-350 oe.
The groups and ions expected from the i.r. absorption bands and the corresponding molecules in the evolved gas are given in table 11.
The broadening of the absorption bands due to 'V(Si-H)shown in fig. 6 may
be related to the increase in intensity of the 'V(Si-O)band, because this phenomenon was not observed in vacuo. Generally, broadening of the band occurs
as a result of different bands overlapping or of fluctuation of the atomic
distance. The broadening observed is probably caused by appreciable fluctua-
CHEMICAL
PROPERTIES
OF STAIN FILMS ON SILICON
319
tions of the atomic distance in the Si-H bonds as a result of the increasing
number of Si-O bonds in the silicon hydride.
The shift of the 'V(S1-0) band in the direction of lower frequency and the
increase in intensity after heat treatment of the film at lOO-150°C (fig. 6)
suggest that the bonds between Si and OH, N03 - and F- are ruptured pyrolytically and that oxygen in the air is adsorbed onto the Si atoms, forming
weak Si-O bonds. The shift of the 'V(Si-O)band in the opposite direction and
the increase in intensity caused by heat treatment above 200 °C probably
resulted from an increase in the Si-O-bond strength and further oxidation of
the chemically active silicon atoms produced by the thermal decomposition of
silicon hydride. The elimination of the 'V(Si-O)bands by heat treatment of the
film above 200°C in vacuo may be accounted for by the formation of gaseous
siloxane during the thermal decomposition of the silicon hydride, as mentioned
in the previous section.
The absorption band at 1130cm-1 appears during heat treatment of the film
at lOO-150°C both in air and in a vacuum (figs 6, 7). Because its absorption
occurs in the frequency region of Si-O valence vibration, the band should be
assigned to 'V(Si-O),and its relatively large wavenumber may be explained by
the inductive effect of certain substituents. Formation of a silicon oxyfluoride
by hydrolysis of Sif', molecules is possible, because the large electronegativity
of F atoms can explain the shift of the 'V(Si-O)band by the inductive effect.
The shift of the 'V(Si-F)band in the direction of lower frequency (fig. 6) is
possibly explained by the inductive effect of adjacent 0 atoms when a silicon
oxyfluoride is formed. We were unable to find any source in which the inductive
effect of 0 atoms on the 'V(Si-F)band is described. Nevertheless, this explanation is reasonable because the 'l{Si-Cl) band of CI(CH3hSiSi(CH3hCI at 519
and 471 cm "! has been found to shift to 486 and 460 cm-1 owing to the formation of CI(CH3hSiOSi(CH3)2CIll).
The omegatron-mass-analysis data reveal that NH3 is evolved together with
N204, N02 and N2 from the stain film by heat trea~ent at about 125°C in
vacuo. Evolution of these gases at the same temperature suggests that they are
produced by the various steps in the reduction ofN03- in the film; the reducing
reagents are probably the silicon-hydride groups or active hydrogen gas ejected
from the film.
5.4. Photochemical decomposition of the stain film
The shift ofthe 'V(Si-H)bands to 2260 cm-1 after irradiation ofthe stain film
with U.v. light is probably caused by a change in the force constant of the Si-H
bond due to the inductive effect of an adjacent atom or group. The increase
of Si-O bonds in the film, as shown in fig. 8, may be responsible for this shift
because of the high electronegativity of the 0 atom. The broadening of the
'V(Si-H)band would be accounted for by the fluctuation of the Si-JI atomic
320
Satoshl YOSHIOKA
distance in the same way as described in a previous section dealing with thermal
decomposition. The elimination of the v(Si-H) band after irradiation with u.v.
light suggests a photochemical decomposition of silicon hydride. The ion current due to N204 + and H20+ in fig. 18 suggests that the increase in the temperature of the sample would be less than 40 oe even if all their evolution is
assumed to have resulted from the thermal decomposition of the stain film.
The evolution of H2 and SiHn molecular fragments therefore confirms the
photochemical decomposition of the silicon hydride in the stain film. The experimental results, particularly the marked absorption shown by the stain film
at 250-300 mu and the decreased intensity of this absorption after irradiation
with u.v. light, justify a model in which H atoms are excited by the irradiation
resulting from the photochemical decomposition of the silicon hydride. A polysilane Ph(Me2Si)nPh (n = 2 - 6) is reported to have an absorption at
240-270 mu 12). The upper limit of the dissociation energy of the Si-H bond
has been found to be 24 680 cm-1 13). These facts support the proposed model.
The decrease in intensity ofthe v(N03 -) band at 1438 cm-1 after irradition
with u.v. light suggests a possible photochemical decomposition of N03 - in
the film. Photolysis of a solid nitrate has been reported to occur in accordance
with the presumable equation 14)
The result ofthe omegatron mass analysis implies the evolution ofN204 during
irradiation and supports the photochemical decomposition of N03 - •
The shift of the band due to v(Si-F) at 904 cm-1 in the direction of lower
frequency would be explained by the formation of a hydrogen bridge with
OH or of a silicon-oxyfluoride molecule SiOFn. The band due to v(OH), however, decreases in intensity after irradiation with u.V. light, as shown in fig. 8.
The latter mechanism may therefore be more probable.
Acknowledgement
The author thanks Dr S. Takayanagi for his continuing interest and encouragement, Mr Y. Yaegashi for assisting with the omegatron measurement
and Mr N. Uemura for assisting with the absorption measurement. The author
also thanks Professor J. Osugi for valuable discussions.
Research Laboratory, Matsushita Electronics Corp.,
Takatsuki, Osaka, Japan
December 1968
CHEMICAL
PROPERTffiS
OF STAIN FILMS ON SILICON
321
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