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The effect of water vapor on the sticking coefficient of lead on silicon monoxide
SURFACE
SCIENCE 14 (1969) 270-273 8 North-Holland
THE EFFECT
OF WATER
COEFFICIENT
VAPOR
Publishing Co., Amsterdam
ON THE
STICKING
OF LEAD ON SILICON MONOXIDE * Received 30 October 1968
Several authors1sz*3) have noted that water vapor may effect thin film deposition. The effect of water vapor to decrease the sticking coefficient of lead on silicon monoxide has been measured and is discussed here. The sticking coefficient of lead on silicon monoxide in dried and in waterladen atmospheres was measured with the quartz crystal microbalance technique described by Bachmann and Shin4). The experimental apparatus, illustrated in fig. I, utilizes one crystal of a quartz crystal microbalance
shutter
---
control Ier head
radiation shield
----
_ cvoporation source
dcssicont tuba
Fig. 1.
Experimental
apparatus.
(Westinghouse Electric Corporation, Pittsburgh, Pa.) as the experimental substrate. The crystal holder consists of a brass, water-cooled, cylindrical * Supported by the U.S. Atomic ATt49-7) 2883.
Energy Commission,
270
Washin~on,
DC,
Contract
THE EFFECT
block
with one end facing
OF WATER
271
VAPOR
the evaporation
source.
A 10 MHz,
AT-cut,
quartz crystal, with a deposition area of 1.02 cm2, is mounted on each end of the holder. A recirculating water bath maintains the holder at 30”. The molecular beam flux is generated by a resistance-heated molybdenum boat and is controlled by a deposit-rate controller (Sloan Instruments Corporation, ONMI-I, Santa Barbara, Calif.) Much larger frequency changes of crystal oscillation are caused by thermal radiation from the evaporation boat than by deposited mass buildup on the crystal. These effects have been minimized with a 1-rpm chopper, and the frequency measurements are recorded during the closed part of the chopper cycle. t 20
I
. A
1
+ confldrncc level
.
.
loo
50 DEPOSITION
Fig. 2.
TIME
(8)
Mass buildup of Pb on SiO as a function of time in: (A) dry atmosphere, humid atmosphere.
(B)
272
P. C. HARRIS
AND
E. W. BLOORE
The experiment was initiated by evacuating the chamber and depositing a 200 A-thick layer of silicon monoxide onto the quartz crystal in an ambient pressure of less than 1 x lo- 5 Torr. The diffusion pump valve was closed and air was introduced into the vacuum chamber, through a dessicant tube filled with freshly activated Drierite, until atmospheric pressure was reached. The substrate was allowed to remain at atmospheric pressure for 3 hours. The chamber was evacuated to a pressure of 8 x lop6 Torr, and lead was evaporated at a flux of 1.3 x 1014 atom cmm2 see-’ onto the silicon monoxide surface. The mass buildup of lead on the silicon monoxide as a function of time is shown by curve A in fig. 2. An increase in the slope of the curve indicates an increase in the sticking coefficient. A 200 A-thick layer of silicon monoxide was again deposited onto the quartz crystal in the same manner. After closing the diffusion pump valve, air was introduced into the chamber, through a tube containing water, until the chamber was at atmospheric pressure. The amount of water introduced into the chamber, 789 mg, was determined by adding the weight difference of the tube of water to the residual water content of the room air as calculated from standard humidity tables5). Since the volume of the chamber is 46.4 liters, the air contained 1.4 percent water. After the substrate remained in
A
t confidence level
0
5 MEAN
Fig. 3.
C&&IT
15 THICKNESS
20 (8,
Sticking coefficient of Pb on SiO as a function of mean Pb thickness in: (A) dry atmosphere, (B) humid atmosphere.
THE EFFECT
OF WATER
VAPOR
273
this atmosphere for 3 hours, the chamber was evacuated to a pressure of 8 x 10m6 Torr. Lead was deposited at the same flux as before, and the results are shown by curve B in fig. 2. The slope of curve 2A is constant after a thickness of approximately 15 A of lead has been deposited. The assumption is made that this straight line portion of the curve represents lead depositing on lead with a sticking coefficient of unity. This assumption was supported by deposition experiments conducted to 120 A-thickness in which no increase in the sticking coefficient was observed. Sticking coefficients during the course of each deposition were calculated for the individual data points and are presented in fig. 3. In most earlier sticking coefficient measurements, the results have not been precise enough for meaningful differential plots as very low mean thicknesses. For the quartz crystal microbalance measurements, however, the precision is high. The differential plots in fig. 3 indicate an apparent periodicity greater than the magnitude of expected errors, corresponding to about a 4 A mean deposit thickness (i.e., roughly one atomic diameter). Whether this periodicity results from peculiar coincidence, experimental difficulty, or chemical reality is not clear at this time. It has been shown that exposing silicon monoxide substrates to 1.4 percent water at atmospheric pressure will reduce the initial sticking coefficient by a factor of 10, i.e., from 0.4 to approximately 0.04. Even when the mean deposit thickness reaches 15 A and the sticking coefficient in dry air is approximately 1.0, the influence of water vapor reduces the sticking coefficient by 35 to 40 percent to approximately 0.65. PHILLIPC. HARRISand ERNESTW. BLOORE U.S. Army NucIear Defense Laboratory, Edgewood Arsenal, Maryland 21010, U.S.A.
References 1) 2) 3) 4) 5)
C. A. 0. Henning, Surface Sci. 9 (1968) 277. J. W. Matthews and E. Grunbaum, Appl. Phys. Letters 5 (1964) 106. H. H. Uhlig, Corrosion Sci. 7 (1967) 325. L. Bachmann and J. J. Shin, J. Appl. Phys. 37 (1966) 242. C. D. Hodgman, Ed., Handbook of Chemistry and Physics, 44th ed. (Chemical Rubber Publishing Co., Cleveland, Ohio, 1962) pp. 2205, 2582.
SCIENCE 14 (1969) 270-273 8 North-Holland
THE EFFECT
OF WATER
COEFFICIENT
VAPOR
Publishing Co., Amsterdam
ON THE
STICKING
OF LEAD ON SILICON MONOXIDE * Received 30 October 1968
Several authors1sz*3) have noted that water vapor may effect thin film deposition. The effect of water vapor to decrease the sticking coefficient of lead on silicon monoxide has been measured and is discussed here. The sticking coefficient of lead on silicon monoxide in dried and in waterladen atmospheres was measured with the quartz crystal microbalance technique described by Bachmann and Shin4). The experimental apparatus, illustrated in fig. I, utilizes one crystal of a quartz crystal microbalance
shutter
---
control Ier head
radiation shield
----
_ cvoporation source
dcssicont tuba
Fig. 1.
Experimental
apparatus.
(Westinghouse Electric Corporation, Pittsburgh, Pa.) as the experimental substrate. The crystal holder consists of a brass, water-cooled, cylindrical * Supported by the U.S. Atomic ATt49-7) 2883.
Energy Commission,
270
Washin~on,
DC,
Contract
THE EFFECT
block
with one end facing
OF WATER
271
VAPOR
the evaporation
source.
A 10 MHz,
AT-cut,
quartz crystal, with a deposition area of 1.02 cm2, is mounted on each end of the holder. A recirculating water bath maintains the holder at 30”. The molecular beam flux is generated by a resistance-heated molybdenum boat and is controlled by a deposit-rate controller (Sloan Instruments Corporation, ONMI-I, Santa Barbara, Calif.) Much larger frequency changes of crystal oscillation are caused by thermal radiation from the evaporation boat than by deposited mass buildup on the crystal. These effects have been minimized with a 1-rpm chopper, and the frequency measurements are recorded during the closed part of the chopper cycle. t 20
I
. A
1
+ confldrncc level
.
.
loo
50 DEPOSITION
Fig. 2.
TIME
(8)
Mass buildup of Pb on SiO as a function of time in: (A) dry atmosphere, humid atmosphere.
(B)
272
P. C. HARRIS
AND
E. W. BLOORE
The experiment was initiated by evacuating the chamber and depositing a 200 A-thick layer of silicon monoxide onto the quartz crystal in an ambient pressure of less than 1 x lo- 5 Torr. The diffusion pump valve was closed and air was introduced into the vacuum chamber, through a dessicant tube filled with freshly activated Drierite, until atmospheric pressure was reached. The substrate was allowed to remain at atmospheric pressure for 3 hours. The chamber was evacuated to a pressure of 8 x lop6 Torr, and lead was evaporated at a flux of 1.3 x 1014 atom cmm2 see-’ onto the silicon monoxide surface. The mass buildup of lead on the silicon monoxide as a function of time is shown by curve A in fig. 2. An increase in the slope of the curve indicates an increase in the sticking coefficient. A 200 A-thick layer of silicon monoxide was again deposited onto the quartz crystal in the same manner. After closing the diffusion pump valve, air was introduced into the chamber, through a tube containing water, until the chamber was at atmospheric pressure. The amount of water introduced into the chamber, 789 mg, was determined by adding the weight difference of the tube of water to the residual water content of the room air as calculated from standard humidity tables5). Since the volume of the chamber is 46.4 liters, the air contained 1.4 percent water. After the substrate remained in
A
t confidence level
0
5 MEAN
Fig. 3.
C&&IT
15 THICKNESS
20 (8,
Sticking coefficient of Pb on SiO as a function of mean Pb thickness in: (A) dry atmosphere, (B) humid atmosphere.
THE EFFECT
OF WATER
VAPOR
273
this atmosphere for 3 hours, the chamber was evacuated to a pressure of 8 x 10m6 Torr. Lead was deposited at the same flux as before, and the results are shown by curve B in fig. 2. The slope of curve 2A is constant after a thickness of approximately 15 A of lead has been deposited. The assumption is made that this straight line portion of the curve represents lead depositing on lead with a sticking coefficient of unity. This assumption was supported by deposition experiments conducted to 120 A-thickness in which no increase in the sticking coefficient was observed. Sticking coefficients during the course of each deposition were calculated for the individual data points and are presented in fig. 3. In most earlier sticking coefficient measurements, the results have not been precise enough for meaningful differential plots as very low mean thicknesses. For the quartz crystal microbalance measurements, however, the precision is high. The differential plots in fig. 3 indicate an apparent periodicity greater than the magnitude of expected errors, corresponding to about a 4 A mean deposit thickness (i.e., roughly one atomic diameter). Whether this periodicity results from peculiar coincidence, experimental difficulty, or chemical reality is not clear at this time. It has been shown that exposing silicon monoxide substrates to 1.4 percent water at atmospheric pressure will reduce the initial sticking coefficient by a factor of 10, i.e., from 0.4 to approximately 0.04. Even when the mean deposit thickness reaches 15 A and the sticking coefficient in dry air is approximately 1.0, the influence of water vapor reduces the sticking coefficient by 35 to 40 percent to approximately 0.65. PHILLIPC. HARRISand ERNESTW. BLOORE U.S. Army NucIear Defense Laboratory, Edgewood Arsenal, Maryland 21010, U.S.A.
References 1) 2) 3) 4) 5)
C. A. 0. Henning, Surface Sci. 9 (1968) 277. J. W. Matthews and E. Grunbaum, Appl. Phys. Letters 5 (1964) 106. H. H. Uhlig, Corrosion Sci. 7 (1967) 325. L. Bachmann and J. J. Shin, J. Appl. Phys. 37 (1966) 242. C. D. Hodgman, Ed., Handbook of Chemistry and Physics, 44th ed. (Chemical Rubber Publishing Co., Cleveland, Ohio, 1962) pp. 2205, 2582.