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Low pressure chemical vapor deposition of molybdenum oxides from molybdenum hexacarbonyl and oxygen
Thin Solid Films 259
( 1995) 5-13
Low pressure chemical vapor deposition of molybdenum molybdenum hexacarbonyl and oxygen
oxides from
Jeffrey S. Cross”,*, Glenn L. SchradePb “Department
of’ Chemical
Engineering, Center _ftir Interfacial bUSDOE-Ames Laboratory, Received
Materials and CrystaNization, Iowa State University, Iowa State University, Ames, IA 50011, USA
4 February
1994; accepted
28 October
Ames,
IA 5001 I, USA
1994
Abstract Thin films of molybdenum oxides were deposited at 300-500 “C and 200-1014mTorr (26.6-135 Pa) from Mo(CO),, 02, H,O using an inductively heated low pressure chemical vapor deposition system. Two oxygen gas flow rates of 5 and 15 seem used. U-MOO, films were deposited at temperatures of 425-450 “C, pressures of 660-1014 mTorr (88.0-135 Pa), with an 0, rate of 5 seem; and at 450-500 “C, 300-500 mTorr (40.0-66.7 Pa), with an 0, flow rate of 5 seem. The polycrystalline films
and were flow
were deposited on silicon (100) wafers and exhibited preferred orientations. Gas phase decomposition of the precursor was significant with temperatures >400 “C and pressures >600 mTorr (88.0 Pa), with an 0, flow rate of 15 seem. Owing to decomposition of the precursor in the gas phase and low gas velocities, the films decreased in thickness in the direction of flow. Thermodynamic equilibrium calculations indicated that E-MOO, was the most stable phase for all deposition conditions. However, deposited only at high temperatures and pressures. A quadratic model of cc-Moo, formation was developed using design for the 5 seem deposition data as a function of temperature and pressure. Both parameters were significant in of U-MOO, films. The films were characterized using X-ray diffraction and X-ray photoelectron, Auger, and spectroscopies. Keywords:
Chemical
vapour
deposition;
Molybdenum
oxide; Organometallic
1. Introduction Molybdenum oxides include phases such as MOO,, “shear structures” MoL. 0, _ , , and several Moo,, where y = 4, 5, 8 and 9 [ 11. Solid state preparation of Mo,Ojy _ , shear structures is typically performed by mixing stoichiometric amounts of MOO, and MOO, in sealed ampules and heating at 500-870 “C for two days to several weeks [ 11. Important physical properties of the molybdenum oxides such as resistivity and melting point vary as a function of the oxygen content. Although much previous work has emphasized bulk materials, thin films of molybdenum oxide are also of interest. Potential applications for MOO, have focused primarily on solar cells [2]. MO, O,, _ , materials have *Present address: Fujitsu Laboratories Ltd., Inorganic Materials and Polymers Laboratory, 10-l Morinosato-wakamiya. Atsugi 243-01, Japan. 0040-6090/95/$9.50 .SSDf 0040-609OC
(0 1995 - - Elsevier 94)06427-X
Science
S.A. All rights
reserved
vapour
deposition;
Z-MOO, was experimental the formation laser Raman
Silicon
been studied because of low-dimensional conductivity [3] and optical anisotropy. Treated, amorphous MOO, films have recently achieved attention because of photochromic and electrochromic properties which could be useful in display or memory devices [4]. Z-MOO, has also been studied extensively because of its catalytic activity [ 51. Crystalline MOO, and a-Moo, films can be prepared by thermal oxidation of MO metal [6], by chemical vapor deposition [2] (CVD), and by exfoliation [7]. These techniques rely upon thermal oxidation at 470680 “C in order to transform the starting materials [MO [6], MOO, [8], and MoS, [7] to produce polycrystalline cc-Moo,. Reactive sputtering can be used to produce molybdenum oxides with different stoichiometries (Mo,O, MOO*, E-MOO,, and /?-Moo,) [9, lo]. Synthesis of the molybdenum oxide films using low pressure (LP) CVD has several advantages: synthesis time may be reduced compared with solid-state tech-
6
J. S. Cross, G. L. Schrader 1 Thin Solid Films 259 (1995) 5-13
niques [l] and LPCVD can produce better step coverage compared with line-of-sight processes such as sputtering or evaporation [ 111. Previous CVD research has focused primarily on producing molybdenum oxide films from molybdenum halides or molybdenum hexacarbonyl. Lander and Germer [ 121performed pioneering work on LPCVD using Mo(CO),; MO films were produced primarily. Preparation of cr-MOO, films using atmospheric CVD from Mo(CO), has been accomplished in a two-step process [8]. MOO, films were deposited at 300 “C and then oxidized at 500 or 600 “C [8]. The films were characterized by X-ray diffraction (XRD) and optical properties were evaluated. Molybdenum oxides have also been produced from Mo( CO), via laser-assisted CVD. Olson and Schrader [ 131 employed an ArF excimer laser to produce polycrystalline Mo,O, , ( < 1000 A). Amorphous molybdenum oxide films for electrochromic applications were produced via plasma enhanced CVD [ 141. The molybdenum oxygen stoichiometry of the light gray deposits was not determined. The above studies produced molybdenum oxides by a variety of techniques while neglecting the synergistic effects that the process parameters have on the deposit composition. The present study was undertaken to produce polycrystalline a-Moo, films from Mo(CO),, 02, and H,O using LPCVD in a single process, and to determine the effect of temperature, pressure, and precursor flow rate on the deposits. Extensive deposition characterization and statistical experimental design were used to develop a quadratic model relating the process parameters to the deposited film composition [ 151.
Pump
By-Pm Ar,Oz
Hz0
0~.Ar,H20 MO~C01~
Fig. 1. Inductively
heated
LPCVD
chamber.
bellow valve located in the roughing pump intake line. Chamber pressures of 71 mTorr (9.46 Pa) and 150 mTorr (20.0 Pa) were measured with O2 flow rates of 5 and 15 seem min-’ respectively with the throttle valve fully open. For several depositions water vapor was supplied to the chamber from a liquid water reservoir maintained at 0 “C. A UT1 1OOCmass spectrometer with an ISS-325 sampling system were used to analyze the gas effluent. 2.2. Characterization The deposited films were characterized with a Nicolet 60SX Fourier transform infrared (FTIR) spectrometer, Spex Triplemate laser Raman spectrometer, PHI scanning Auger electron spectrometer, PHI 5300 ESCA system, Siemens X-ray diffractometer (XRD), a JEOL scanning electron microscope (SEM), and a Tencor alpha step profilometer.
2. Experimental equipment and methods
2.3. Experimental methods
2.1. Deposition equipment
Prior to film depositions, the sample and susceptor were heated to 600 “C using the r.f. generator; evacuation by a thermolecular pump produced a base pressure of 8 x lop8 Torr (1.07 x IO-’ Pa). The substrates were 3 inch silicon ( 100) wafers which were cleaved in half. One half of a wafer was used for each deposition. No attempt was made to remove surface oxide. The r.f. generator was adjusted to achieve a susceptor temperature of 300-500 “C. The carrier gas was then admitted into the chamber for 1 h prior to deposition. After the desired deposition time (20 and 70 min), Ar was used to purge the chamber while the sample/susceptor was allowed to cool to room temperature. Films were deposited at O2 gas flow rates of 5 and 15 seem using both a one factor at a time (OFAT) deposition routine and statistical experimental design [ 151. Ranges of temperature, pressure, and Mo( CO), flow rate for the 15 seem depositions are listed in Table 1. The methodology used to compare the influence of process parameters on the deposit was to vary one variable while holding the other two constant. This
The films were prepared using a LPCVD chamber as depicted in Fig. 1. The 10 cm diameter quartz chamber was surrounded by an induction coil which was connected to a 5 kW Lepel, 200-450 kHz, r.f. generator. The generator heated an electrically isolated, SIC coated graphite susceptor; no discharge was observed. Feed mixtures of Ar (99.995%, Matheson), O2 (99.97%, Matheson), Mo(CO), (99%, Alfa), and water vapor were used. The 0, to Mo(CO), molar ratio in the gas stream was regulated by varying the quantity of gas flowing through a Mo(CO), sublimator maintained at 30 “C. The concentration of Mo(CO), in the gas phase was determined by collecting the vaporized precursor in a cold trap after the sublimator and correlating the mass of the sublimed Mo(CO), with gas flow rate. Brooks mass flow controllers were used to regulate the flow rate of oxygen and argon. The pressure in the system was measured with an MKS Instruments Inc. capacitance manometer and regulated by throttling a
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13 Table I Summary
of deposition
conditions
for oxygen
flow rate of 15 seem
Parameters
Settings
0, flow rate (seem) 02/Mo(CO), molar ratio Water flow rate (seem) Temperature ( -C) Pressure (Pa)
I5 25- 61 0.0-0.15 300- 450 26.6- I35
I
develop a model relating the deposited X-MOO, film percentage to the deposition temperature and pressure. The contribution of temperature and pressure to the model was determined by first standardizing into dimensionless variables as given below: x = (Temp. - 450) (1) X: = (Presss4!::4O,CJ (2)
Table 2 Deposition conditions rate at 5 seem Process
used for experimental
variables
design with oxygen
flow
Levels
Pressure (Pa) Temperature (‘C) Mo(CO), flow rate (mg min O2 flow rate (seem) Deposition time (min)
(26.6, 4O.o’r 3 53.3 3 66.7”“) (425 >450Cp1 475 1 500”9 I .44 5 30
‘)
where temperature is in ‘C and pressure is in Pa. Several different linear and quadratic regression models were developed relating the film composition data to the standardized values of deposition pressure and temperature using regression analysis with SAS [ 161, a statistical software package. The t-statistics were calculated for each of the regression model coefficients according to a t-distribution and the following expression (3)
ap, axial point;
seem, standard
cm’/min;
cp, center
point.
coincided with depositing films with a Mo(CO), flow rate of 4.5 mg min’, at a pressure of 660 mTorr (88.0 Pa) while varying the temperature from 300450 ‘:C and at 425 “C while varying the pressure from 217-1014 mTorr (28.9-135 Pa). Two levels of Mo( CO), flow rate (2.3 and 4.5 mg min’) were used at 425 “C and 470 mTorr (62.7 Pa). Ten films were deposited at an O2 flow rate of 5 seem using a two-factor (temperature and pressure) two-level factorial design with center and axial points [ 151(Table 2). The order in which the deposition runs were conducted was randomized and was noted as the sequence number in the tables. The ranges of temperature and pressure investigated were from 425-500 “C and 200500 mTorr (26.6-66.7 Pa). For both gas flow rates three responses were recorded for each deposition: deposition rate, the gas phase CO-CO, ratio, and U-MOO, percentage of the deposited films. The deposition rate was determined by weighing the films before and after the deposition and dividing the difference by the deposition time. The gas-phase CO-CO, amp signal ratio was determined by dividing the mass spectrometer mass to charge signal at 28 by the signal at 44 (approximately 0.02% of the 28 m/e- signal was attributed to the fragmentation of CO,). This ratio was reported because the actual signal intensities change with pressure. Some of the films contained segregated regions of cc-Moo, and reduced molybdenum oxides on the same wafer. When this occurred the film E-MOO, percentage was estimated by film characterization and by measuring the deposited film Z-MOO, surface area and dividing it by the total wafer area. The film X-MOO, percentage was used to
where Y is the variable average, ug is the mean, .Pis the standard deviation and n is the number of observations.
3. Results 3.1. Process uariahles at 15sccm
0,
3.1.1. Temperature effects As the film deposition temperature increased the deposition rate decreased (Fig. 2) which led to subsequent changes in film composition and crystallinity. Laser Raman spectroscopy revealed that films deposited at 300 “C had bands closely matching MOO, powder (Fig. 3(a) and Table 3). As the temperature was increased to 350 ‘C the Raman bands in the films closely matched those of Mo40,, [ 131 (Fig. 3(b)). Raman spectra of films deposited at 375 and 400 “C showed two broad bands which could not be assigned to any particular molybdenum oxide even after expanding the baseline (Fig. 3(c)). At 425 C and above, bands
0.30 1
;j
,,,,
275
1-L:::,,,,, r
300
325
3.50
375
400
425
450
475
Temperature (‘C) Fig. 2. Deposition rate versus temperature at a pressure with a Mo(CO), flow rate of 4.5 mg min I.
of 88.0 Pa
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) S- 13
8 Table 3 Raman spectra
bands (cm-‘)
of films deposited
at 300,350
300 “C
350 “C
425 “C
204 230 351 364 461 497 513 144
186 311 430 451 730 792 834 880 908
157 249 294 340 380 521 666 821 996
(s) (m) (sh) (s) (w) (s) (s) (s)
(s) (s) (m) (m) (w) (s) (s) (w) (s)
and 425 “C
(s) (w) (s) (m) (m) (w) (m) (vs) (s)
a) Intensities: shoulder.
vs, very
strong;
s, strong;
m, medium;
w, weak;
sh,
10.00 8.939
20.00 4.436
TWO -
THETA
--
30.00 2.976
40.00 2.252
d SPACING
51I.00 1 ,823
Fig. 4. XRD patterns of films deposited at 88.0 Pa and temperatures of (a) 300 “C, (b) 350 “C, (c) 375 “C, and (d) 425 “C.
4
IQXJ
375°C
800
600
400
200
num oxides [ 1, 171. Further increases in temperature produced oriented off-white colored films with the most intense peaks at 6.88, 3.44, and 2.13 8, which correspond to (020), (040), and (060) planes of a -MOO, [ 171. Further characterization of the films deposited at 425 “C and higher indicated that the deposits contain less than 1% C as measured by Auger electron spectroscopy. Electron spectroscopy for chemical analysis (ESCA) of the 425 “C film surfaces determined that the MO 3d,,, and 3d,,, electrons had binding energies of 232.7 and 235.9 eV respectively. These values are in close agreement with Mo(V1) binding energies of 232.6 and 235.8 eV reported in the literature for U-MOO, [ 181. Films deposited at 425-450 “C had a columnar structure and faceted surface (Fig. 5).
wavenumbers (cm-l) Fig. 3. Raman spectra of films deposited at 88.0 Pa and temperatures of (a) 300 “C, (b) 350 “C, (c) 375 “C*, and (d) 425 “C (* the ordinate in (c) was expanded 100 times compared with (d). The Raman band wavenumbers are listed in Table 3).
matching those of U-MOO, powder were observed (Fig. 3(d) and Table 3). XRD patterns revealed that the films had preferential orientation [ 171. The deposits at 300 “C resembled oriented MOO, (Fig. 4(a)). The peaks observed in the 350 “C film are broad and probably contain both chi and eta Mo,O,, [ 171. The films deposited at temperatures of 375 to 400 “C showed two highly oriented XRD peaks at 3.97 and 1.98 A. These two diffraction peaks were not significant enough to identify the material considering the large number of different molybde-
Fig. 5. SEM micrograph 425 “C and 88.0 Pa.
of surface
of U-MOO,
film deposited
at
J. S. Cross, G. L. Schruder / Thin Solid Films 259 (1995) S- 13
0.04-1
20
40
60
80 100 Pressure (Pa)
120
140
Fig. 6. Molybdenum oxide deposition rate versus pressure at temperature of 425 C with a Mo( CO), flow rate of 4.5 mg mm’ with an oxygen flow rate of I5 seem.
3.1.2. Pressure eflkcts
The deposition pressure strongly influenced deposition rate, composition, and uniformity. Increasing the pressure sharply decreased the average film deposition rate at 425 “C and Mo( CO), flow rate of 4.5 mg min-’ (Fig. 6). The films deposited over this range of pressures consisted of bluish-black reduced molybdenum oxides (MOO,, where .Y< 3) at pressures < 270 mTorr (36.0 Pa), mixed molybdenum oxides with pressures between 270-470 (36.0-62.7 Pa), and Z-MOO, with pressures > 600 mTorr (80.0 Pa) (Fig. 7). The mixed phase deposits consisted of two different regions where the reduced material deposited on the gas inlet side of the wafer and higher oxidized phase deposited on the gas outlet side of the sample. This also resulted in films of non-uniform thicknesses. Comparisons of film thicknesses deposited at 425 “C showed that the thickness varied across the wafer in the direction of flow. At the leading edge or gas inlet side of
an E-MOO, deposit, the film was 12.5 kA thick and decreased in thickness to 1.2 kA at the trailing edge. For a film deposited at 270 mTorr (36.0 Pa), the film was 16.0 kA thick at the leading edge and decreased to 4.1 kA at the opposite edge. Films deposited at lower temperature also showed thickness variations but to a lesser degree. As noted, reduced molybdenum oxides or shear structures, MOO.,, exist where x is 2.75 < x < 3.0 [ 1, 171. Because these materials have similar layered structures and O-MO bond lengths, XRD, Raman, FTIR, and X-ray photoelectron spectroscopy cannot quantitatively determine the stoichiometry of films, particularly for oriented materials (Figs. 3(c) and 4(c)). These reduced films were metastable and were easily converted into Z-MOO, by heating in air at 500 “C for 10 min. Previously reported results stated that the addition of water vapor to the gas stream results in films with little carbon [ 121. However, in the present case there was no quantifiable difference in film composition (less than 1% C) or morphology when the water vapor in the gas inlet stream was varied from 0.0 to 0.51 seem. 3.2. Process vrrriuhlesut 5sccnz 0, A second series of experiments were conducted at an oxygen flow rate of 5 seem while varying the pressure (26.6-66.7 Pa) and temperature (425500 “C) (Table 4). The same general trends on the deposit formation at the high ( 15 seem) 0, flow rate were observed at 5 seem. The films were characterized using Raman, FTIR, XRD, and X-ray photoelectron spectroscopy (XPS). The E-MOO, films were formed at temperatures between 450-500 ‘C and pressures of 53.3-66.7 Pa (Table 4 and Fig. 8). The T-MOO, films were poIycrystalline with preferred (OkO) orientation. XRD and Raman characterization revealed nearly identical spectra to those in Figs. 3(d) and 4(d). The morphology of the off-white colored x-Moo, deposits showed a columnar
Table 4 Experimental Si
d)
Run
I
IlO.
Seq. no.
cl
1I
b)
L4
10.0 8.84
n
20.0 30.0 4.44 2.98 28 (“) -- d-spacing
40.0 2.25 (A)
50.0 1.82
Fig. 7. XRD patterns of films deposited at a Mo(CO), flow rate of 4.5 mg mm’, 425 ‘C and pressures of (a) 28.0 Pa, (b) 26.6 Pa, (c) 49.3 Pa. and (d) 62.7 Pa.
9
1 2 2 3 4 5 5 5 6 7
8 3 6 2 4
I 5 10 7 9
settings and deposition results for 0: flow rate of 5 seem _____ .~ _._~_ Press. (Pa) _
Temp. ( C)
Deposit (‘VI,Z-MOO,)
amu 28/44” Dep. rate (A/A) (mg mm’) ~.._____ ~
26.6 26.6 26.6 53.3 53.3 40.0 40.0 40.0 40.0 66.7
425 475 475 425 475 450 450 450 500 450
0 2 3 39 100 25 8 IO 100 100
64.8 58.7 60.5 41.9 3s. I 44.9 44.0 45.3 40.1 29.1
“Mass spectrometer
signal
ratios
0.180 0.180 0.187 0.137 0.133 0. I53 0.167 0.160 0.187 0.123
of amu 28/44 (amps/amps).
IO
J. S. Cross, G. L. Schrader 1 Thin Solid Films 259 (1995) 5- 13 Table 5 Analysis of variance 5 seem 0, flow rate
2.5 \fby
Least squares
regression
Source 0.0 0.0
r”“l”“r”“l”“l”“l”“i”‘~f 0.5 1.0 1.5 2.0 2.5 Distance (cm)
3.0
3.5
for regression
and
Regression Error Total R2 = 0.977 Standardized
2 P 6:
60
(100) .
50
(1W .
(3% .
(100) .
(8,10,25) .
‘Ill
425
450
475
with
a
analysis
Degrees of Freedom
Sum of Squares
Mean Square
4 5 9
16952.3 393.8 11346. I
4238.0 78.8
53.8
parameter
__ 70
for depositions
F-Value
4.0
Fig. 8. Plot of U-MOO, content (‘l/o) of films versus temperature pressure with an 0, flow rate of 5 seem.
c &
model
estimates ---
Variable
Coefficient
Std. Error -___
2-Value
P>ltl
Intercept X, X, x,x,XT2
19.9 36.6 15.3 13.6 13.2
3.73 3.07 3.99 4.10 3.17
5.35 I I.91 3.82 3.31 4.16
0.0031 0.0002 0.0123 0.0212 0.0088
500
Temperature ( ‘C) Fig. 9. Film thickness measured from edge of wafer for two different films deposited with an oxygen flow rate of 5 seem.
structure. At lower temperatures and pressures bluishblack colored reduced oxides were produced. The deposition pressure had a pronounced influence on the composition, deposition rate, uniformity, and gas phase composition. Gradations in film thickness were also observed in the direction of flow (Fig. 9). The film thickness was influenced by the parameters as well as the physical geometry of the reactor. Increasing the pressure from 26.7 to 53.3 Pa at 475 “C produced a steeper gradation in film thickness (Fig. 9). It appeared that both the temperature and pressure were synergistically coupling to influence the composition which was determined by modeling the response. Several different models of the film a-Moo, percentage as a function of temperature and pressure were developed from the results in Tables 2 and 4 using regression analysis. The model listed below (Eq. (4)) statistically fits the data the best of those evaluated. It consisted of five terms including both linear and quadratic expressions,
would have an R* of 1.0. The F-value for the model is 53.8 which indicates that at a 95% confidence interval the model is statistically significant because it exceeds the minimum F(,.,, value of 5.19 [19]. Pearson correlation coefficients were evaluated for all the variables. Coefficients were calculated for the following pairs: pressureldeposition rate ( -0.91 I), pressurel[AMU 28/44] ( -0.927) and pressureju-Moo, (0.756) and indicate that the pressure is an influential process variable on all three responses. The negative values indicated that as pressure increased these values decreased. The predicted values from the model using Eq. (4) and the measured deposited a-Moo, film percentages were compared (Table 6). The residual was determined by subtracting the model predicted values from the film measured values; the values of the residuals varied from Table 6 Comparison of deposited U-MOO, film composition versus model predicted values (Eq. (4)) for depositions with an 0, flow rate of 5 seem Run no.
Sequence no.
Film LX-MOO, (“A,)
Predicted (!A)
I 2 2 3 4 5 5 5 6 7
8 3 6 2 4 I 5 10 7 9
0 2 3 39 100 25 8 10 100 100
-5.1 - 1.7 ~ 1.7 40.8 98.5 19.9 19.9 19.9 103.3 93.0 ______
U-MOO,
Residual (X/u)
YMoo3= 19.9 + 36.6X, + 15.3X, + 13.6X,X, + 13.2X; (4) All five terms are statistically significant at a 95% confidence interval [ 191. The significance of each term is indicated by the magnitude of the t-values and coefficients since the standard errors are similar (Table 5). The model fit of the data is expressed in two ways. The R* for the model is 0.977 (Table 5); this value indicates that 97.7% of the variability in the data is accounted for in the model. A model with a perfect fit
____-(Residual
= Film - Prediction)
5.1 3.7 4.7 - I.8 I.5 5.1 - Il.9 -9.9 -3.3 7.0
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13
7 to - 12 with the predicted values ranging from - 1.7 to 103.3% (Table 6).
II
agreement with most CVD processes which are controlled by the reaction kinetics [ 11, 231. 4.2. Statistical model
4. Discussion 4.1. Thermodynamics
To determine the most stable phase of molybdenum oxide that would deposit in the Mo( CO),, O?, and H,O thermal deposition process, a computer program, SOLGASMIX-PV [20], was used. The program calculated the thermodynamic equilibrium at constant pressure based upon minimization of the system Gibbs free energy and conservation of mass. Equilibria were calculated for both a three-component MO, 0, and C system consisting of 14 gas species and 9 solid compounds plus a four-component MO, 0, C, and H system consisting of 56 gas species and 9 solid compounds. For both three- and four-component systems the solid species consisted of C(graphite), MO, Mo,C, Mo(CO),, MOO,, Mo40,,, Mo,O,,, Mo,O,,, and Z-MOO,. The thermodynamic data used in the calculations were taken primarily from the JANAF tables [21, 221. Equilibria were calculated over a range of temperatures 127-527 “C, pressures 100-1000 mTorr (13.3133.3 Pa) and gas compositions. The O,/MO(CO)~ molar ratio was varied from l-200 and H,O/Mo( CO), molar ratio was varied from 0.0-3.0. Based upon equilibrium calculations for the above conditions, X-MOO, was the only solid component formed as long as the O,/Mo(CO), ratio was 5 or greater. When O2 was in excess, the carbon in the system consisted almost entirely of CO1 gas. Varying the water vapor composition in the gas phase had litt.le effect on the equilibrium. Although thermodynamically U-MOO, is the most stable solid phase in the presence of excess oxygen from 127-525 “C, it was deposited only at temperatures of 425-500 “C depending upon the system pressure and precursor flow rate. The conditions under which gMOO, formed was at temperatures > 425 “C, and pressures > 600 mTorr (80.0 Pa), with an 0, flow rate of 15 seem; and at pressures > 300 mTorr (40.0 Pa), and temperatures > 450 ‘C, with O2 at 5 seem. Equilibria calculations showed that the gas phase consisted of primarily CO, and 0,. During actual depositions mass spectrometry of the gas phase showed intense peaks at amu 28 (CO), 32 (0,) and 44 ( COZ) in decreasing order of intensity. The ratio of CO-CO2 in the gas phase or the 28/44 signal ratio was correlated to the film composition (Table 6); the amu 28/44 signal I-dtiO was the lowest (29.1-40.1) and thus CO* (or 44 amu signal) concentration highest when U-MOO, was deposited. Accordingly, analysis of both the gas and solid phase indicated the process was not limited by thermodynamics but by the reaction kinetics. This is in
Modeling the depositions with a quadratic expression yielded several interesting observations. First, the model showed that temperature and pressure act both independently and synergistically (Eq. (4)) as indicated by statistically significantly terms. Secondly, all the model coefficients were positive which indicated that increasing temperature and pressure yielded higher aMOO, deposits due to the reasons mentioned in Section 4.4. Comparisons of the a-Moo, percentage of the depositions with the predicted values for the model in some cases underestimated ( ~0) the response at low levels and overestimated ( > 100) the response at high levels (Table 5). Since the minimum response was 0 and the maximum response was 100, it appeared that a cubic expression may provide a better fit of the data. However, since the temperature and pressure range examined in this study was small, a quadratic expression appeared to adequately fit the data. It has been shown that the pyrolysis of various gaseous CVD precursors. based on percentage, versus temperature has a cubic response [23]. 4.3. Process variuble eflects As noted in Figs. 2-4 and 6-9 the decomposition process was strongly influenced by temperature and pressure as in Eq. (4). This is because the processing window in which the MOO, films were deposited was influenced by the precursor reaction with oxygen. This is more clearly demonstrated by examining the role of temperature on the reaction rate of Mo( CO),. Previous results showed that Mo(CO), begins to decompose at 150 “C and was completely decomposed by 450 “C in an evacuated chamber [24]. In the present study the material deposited uniformly on the substrate at 300 “C forming MOO,, but at 350 “C some of the material deposited on the walls of the chamber surrounding the susceptor. This trend increased as the temperature of the substrate was raised, which explained why the film deposition rate decreased with temperature (Fig. 2). This phenomenon, homogeneous gas phase nucleation, was due to the high thermal gradients surrounding the susceptor which caused the precursor to partially decompose before it reached the susceptor, resulting in wall deposits [25]. This is frequently encountered with metalorganic precursors at high substrate temperatures [ 11, 23, 251. As a result of the gas reactions the film depositing on the substrate varied in thickness across the surface at the high temperatures (Fig. 9). This was due to a
J. S. Cross, G. L. Schrader
12
decrease in the precursor concentration in the axial direction [26]. In the present study, the Mo(CO), content in the effluent was too low to measure with mass spectrometry when K-MOO, films were deposited. It has been reported in the literature that in order to form uniform deposits in a cold wall reactor, gas velocity and/or reactor geometry require optimization [26, 271. In this deposition system, where the gas flow rate was fixed, the pressure was regulated by adjusting a throttling valve in the pump foreline. For a simple flow-through system the mean residence time, z is equated to 7=-Z=-
-
PV
PV
e
PWA
which describes the relationship between pressure (p), volume (V), velocity (v), flow rate (Q) and number of gas molecules (n) in a flow-through system [28]. Pressure is proportionally related to r and inversely related to the gas velocity. Hence, varying the pressure resulted in a corresponding change in the residence time (gas velocity) with a constant gas flow rate. Flow-through reactor studies have shown that the residence time influences the film deposition rate [23, 291. From a generalized approach the change in composition as a function of temperature and pressure can be explained in terms of reaction and deposition rate. At low pressures (high gas velocities and low residence time) the deposited films were more uniform but reduced oxides. At these pressures ( > 80.0 Pa at 15 seem and >40 Pa at 5 seem) material was depositing on the surface faster than it could be fully oxidized and for adatom surface diffusion to an appropriate crystalline kink site [23]. At higher pressures (low velocities and high residence time) the material has sufficient time for reaction and surface diffusion before subsequent deposits. Also, less material was depositing on the substrate due to the gas phase reactions and film deposition on the chamber wall. For this molybdenum oxide system there was a trade off between material composition and deposition rate and uniformity. The influence of deposition rate and precursor flow rate on the a-Moo, content of the film was examined by depositing films at 425 “C and 62.7 Pa where both phases coexisted on the substrate at an O2 flow rate of 15 seem. Increasing the precursor flow rate from 2.3 to 4.5 mg mini decreased the a-Moo, content of the film from 85 to 50%. Comparison of these LPCVD results with molybdenum oxide films deposited by reactive sputtering showed similar results [9, lo]. The reactive sputtering results showed that the deposition rate directly influenced the oxygen to molybdenum stoichiometry [9, lo] and crystallinity. Similar results were also observed from the deposition of MOO, using a plasma enhanced
/ Thin Solid Films 259 (1995) 5-13
CVD process [30]. Therefore, controlling the pressure, temperature, and precursor concentration regulated the rate at which the precursor arrived at the substrate surface and reacted to form molybdenum oxides. Considering the reactivity of Mo(CO), in oxygen at high temperatures and the difficulty in obtaining uniform MOO, films in a one-step process at moderate pumping speeds, the advantage of separating the process into two steps as was previously done (where the films were first deposited at 300 “C and then subsequently oxidized at 500 or 600 “C) becomes apparent
PI. 5. Conclusions The feasibility of producing molybdenum trioxide films was demonstrated using a LPCVD system with oxygen gas flow rates of 5 and 15 seem. Thermodynamic equilibrium calculations of the system showed that solid N-MOO, was the most stable phase over a range of temperatures, pressures, and gas compositions. Actual depositions showed that the material deposited over a narrow range of conditions and its formation was influenced by temperature, pressure, and Mo(CO), flow rate. The deposit composition (oxygen to molybdenum ratio) and uniformity were more strongly controlled by transport phenomena and reaction kinetics than by thermodynamics. The film thickness was more uniform at lower pressures, however it was not possible to produce homogeneous a-Moo, films at low pressures. The significance of temperature and pressure on the depositions with an oxygen flow rate of 5 seem was determined using a quadratic model. The model indicated that temperature and pressure act both synergistically and independently on the deposits. Films deposited at an oxygen flow rate of 5 seem at pressures between 300-500 mTorr (40.0-66.7 Pa) required a slightly higher substrate temperature (25-50 “C) in order to produce a-Moo, films compared with the 15 seem results due to pressure- temperature interaction. The addition of water vapor to the gas stream had little effect on the film composition.
Acknowledgments
The assistance of Dr. R. W. Stephenson of the Iowa State University Statistics department was particularly valuable in developing the model and interpretation of the deposition data. Thanks are also extended to Mr. Alan Landon of the Iowa State Microelectric Research Center for his measurement of the film thickness, Mr. Jim Anderegg of the Ames Laborator for the film XPS measurements, and Dr. A. Bevel0 for the film Auger electron spectroscopy measurements.
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13
References Acta Chemica Stand., 13 (1959) 954. [2] C. E. Carver, A. Divrechy, S. Karbal, J. Robin and A. Donnadieu, Thin Solid Films, 94 ( 1982) 269. [3] C. L. Schlenker (ed.), Low Dimensional Electronic Properties of Molybdenum Bronzes and Oxides, Kluwer, Dordecht, 1989. [4] J. N. Yao. K. Hashimoto and A. Fujishima, Nature, 355 (1992) 624. [5] J. Haber, in C. Morterra, A. Zecchina and G. Costa (eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam. 1989. p. 441. [6] B. Mingot. N. Floquet, 0. Bertrand, M. Treillieux. J. J. Heizmann, J. Massardier and M. Abon, J. Catal., 118 (1989) 424. [7] W. M. R. Divigalpitiya, R. F. Frindt and S. R. Morrison, Thin Solid Films, 188 (1990) 173. [8] A. Abdellaoui, L. Martin and A. Donnadieu, Phys. Status Solidi (A). 109 (1988) 455. [9] P. F. Garcia and E. M. McCarron III. Thin Solid Films, 15.5 (1987) 53. [IO] A. R. Jankowski and L. R. Schrawyer, Thin Solid Films, 193/194 (1990) 61. [ I I] J. L. Vossen and W. Kern (eds.), Thin Film Processes II, Academic Press, San Diego, 1991. [ 121 J. J. Lander and L. H. Genner. Trans. AIME, 175 (1948) 648. ( 13) K. A. Olson and G. L. Schrader, in B. M. Clemens and W. L. Johnson (eds.), Thin Film Structures and Phase Stability, MRS Symp. Proc. Vol. 187, Materials Research Society, Pittsburgh, 1990, p. 167. [ 141 C. E. Tracy and D. K. Benson, J. Vat. Sci. Technol A, 4l.5) ( 1986) 2377.
[I] L. Kihlborg,
13
[IS] D. C. Montgomery, Design and Analysis of E.xperiments, 3rd edn., Wiley, New York, 1991. [ 161 SAS Institute, Cary, North Carolina. [ 171 JCPDS-International Centre for Diffraction Data, Swarthmore, PA, USA: MOO, (32-0671) Mo,O,, (5- 0337 and 13%0142) Mo,,O,, (1330345). Mo,O,, (12-0517). Mo,O,, (5p0339), Mo,O,, (12-0753 and 5-0441) and cc-Moo, (5-0508). [ 181 T. H. Fleisch and G. J. Mains, J. Chem. Phys., 76(2) ( 1982) 780. [ 191 L. Ott, An Introduction to Statistical Method.7 and Data Analysis, 3rd edn., PWS-Kent, Boston, 1988. [20] T. M. Besmann, Oak Ridge National Laboratory, TM-5775 ( 1977) Oak Ridge, TN. [21] JANAF Thermochemical Tables. 3rd edn., J. Phys. Chem. Ref Data, 14(l) (1985). [22] 0. Kubaschewski and C. B. Alcock, Metallurgical Thermochemistry, 5th edn., Pergamon Press, Oxford, 1979. [23] G. B. Stringfellow, Organometalhc Vapor-Phase Epitaxy: Theory and Practice. Academic Press, San Diego, 1989. [24] A. P. Patokin and V. V. Sagalovich, Russ. J. Inorg. Chem., .50(3) (1976) 370. [25] H. E. Carlton and W. M. Goldberger, J. Metals, 17 ( 1965) 61 I. [26] J. Van de Ven, G. M. J. Rutten, M. J. Raaijmakers and L. J. Giling, J. Cryst. Growth, 76 (1986) 352. [27] K. F. Jensen, in D. W. Hess and K. F. Jensen (eds.), Microelectronic Processing: Chemical Engineering Aspects, Advances in Chemistry Series 221, ACS, Washington, DC, 1989. [28] B. Chapman, Glow Discharge Processes, Wiley, New York, 1980, p. 16. [29] T. J. Donahue and R. Reif, J. Electrochem. Sot.. 133(8) (1986) 1691. [30] J. S. Cross (1993) 59.
and G. L. Schrader,
J. Chem. Vapor Depos., 2(l)
( 1995) 5-13
Low pressure chemical vapor deposition of molybdenum molybdenum hexacarbonyl and oxygen
oxides from
Jeffrey S. Cross”,*, Glenn L. SchradePb “Department
of’ Chemical
Engineering, Center _ftir Interfacial bUSDOE-Ames Laboratory, Received
Materials and CrystaNization, Iowa State University, Iowa State University, Ames, IA 50011, USA
4 February
1994; accepted
28 October
Ames,
IA 5001 I, USA
1994
Abstract Thin films of molybdenum oxides were deposited at 300-500 “C and 200-1014mTorr (26.6-135 Pa) from Mo(CO),, 02, H,O using an inductively heated low pressure chemical vapor deposition system. Two oxygen gas flow rates of 5 and 15 seem used. U-MOO, films were deposited at temperatures of 425-450 “C, pressures of 660-1014 mTorr (88.0-135 Pa), with an 0, rate of 5 seem; and at 450-500 “C, 300-500 mTorr (40.0-66.7 Pa), with an 0, flow rate of 5 seem. The polycrystalline films
and were flow
were deposited on silicon (100) wafers and exhibited preferred orientations. Gas phase decomposition of the precursor was significant with temperatures >400 “C and pressures >600 mTorr (88.0 Pa), with an 0, flow rate of 15 seem. Owing to decomposition of the precursor in the gas phase and low gas velocities, the films decreased in thickness in the direction of flow. Thermodynamic equilibrium calculations indicated that E-MOO, was the most stable phase for all deposition conditions. However, deposited only at high temperatures and pressures. A quadratic model of cc-Moo, formation was developed using design for the 5 seem deposition data as a function of temperature and pressure. Both parameters were significant in of U-MOO, films. The films were characterized using X-ray diffraction and X-ray photoelectron, Auger, and spectroscopies. Keywords:
Chemical
vapour
deposition;
Molybdenum
oxide; Organometallic
1. Introduction Molybdenum oxides include phases such as MOO,, “shear structures” MoL. 0, _ , , and several Moo,, where y = 4, 5, 8 and 9 [ 11. Solid state preparation of Mo,Ojy _ , shear structures is typically performed by mixing stoichiometric amounts of MOO, and MOO, in sealed ampules and heating at 500-870 “C for two days to several weeks [ 11. Important physical properties of the molybdenum oxides such as resistivity and melting point vary as a function of the oxygen content. Although much previous work has emphasized bulk materials, thin films of molybdenum oxide are also of interest. Potential applications for MOO, have focused primarily on solar cells [2]. MO, O,, _ , materials have *Present address: Fujitsu Laboratories Ltd., Inorganic Materials and Polymers Laboratory, 10-l Morinosato-wakamiya. Atsugi 243-01, Japan. 0040-6090/95/$9.50 .SSDf 0040-609OC
(0 1995 - - Elsevier 94)06427-X
Science
S.A. All rights
reserved
vapour
deposition;
Z-MOO, was experimental the formation laser Raman
Silicon
been studied because of low-dimensional conductivity [3] and optical anisotropy. Treated, amorphous MOO, films have recently achieved attention because of photochromic and electrochromic properties which could be useful in display or memory devices [4]. Z-MOO, has also been studied extensively because of its catalytic activity [ 51. Crystalline MOO, and a-Moo, films can be prepared by thermal oxidation of MO metal [6], by chemical vapor deposition [2] (CVD), and by exfoliation [7]. These techniques rely upon thermal oxidation at 470680 “C in order to transform the starting materials [MO [6], MOO, [8], and MoS, [7] to produce polycrystalline cc-Moo,. Reactive sputtering can be used to produce molybdenum oxides with different stoichiometries (Mo,O, MOO*, E-MOO,, and /?-Moo,) [9, lo]. Synthesis of the molybdenum oxide films using low pressure (LP) CVD has several advantages: synthesis time may be reduced compared with solid-state tech-
6
J. S. Cross, G. L. Schrader 1 Thin Solid Films 259 (1995) 5-13
niques [l] and LPCVD can produce better step coverage compared with line-of-sight processes such as sputtering or evaporation [ 111. Previous CVD research has focused primarily on producing molybdenum oxide films from molybdenum halides or molybdenum hexacarbonyl. Lander and Germer [ 121performed pioneering work on LPCVD using Mo(CO),; MO films were produced primarily. Preparation of cr-MOO, films using atmospheric CVD from Mo(CO), has been accomplished in a two-step process [8]. MOO, films were deposited at 300 “C and then oxidized at 500 or 600 “C [8]. The films were characterized by X-ray diffraction (XRD) and optical properties were evaluated. Molybdenum oxides have also been produced from Mo( CO), via laser-assisted CVD. Olson and Schrader [ 131 employed an ArF excimer laser to produce polycrystalline Mo,O, , ( < 1000 A). Amorphous molybdenum oxide films for electrochromic applications were produced via plasma enhanced CVD [ 141. The molybdenum oxygen stoichiometry of the light gray deposits was not determined. The above studies produced molybdenum oxides by a variety of techniques while neglecting the synergistic effects that the process parameters have on the deposit composition. The present study was undertaken to produce polycrystalline a-Moo, films from Mo(CO),, 02, and H,O using LPCVD in a single process, and to determine the effect of temperature, pressure, and precursor flow rate on the deposits. Extensive deposition characterization and statistical experimental design were used to develop a quadratic model relating the process parameters to the deposited film composition [ 151.
Pump
By-Pm Ar,Oz
Hz0
0~.Ar,H20 MO~C01~
Fig. 1. Inductively
heated
LPCVD
chamber.
bellow valve located in the roughing pump intake line. Chamber pressures of 71 mTorr (9.46 Pa) and 150 mTorr (20.0 Pa) were measured with O2 flow rates of 5 and 15 seem min-’ respectively with the throttle valve fully open. For several depositions water vapor was supplied to the chamber from a liquid water reservoir maintained at 0 “C. A UT1 1OOCmass spectrometer with an ISS-325 sampling system were used to analyze the gas effluent. 2.2. Characterization The deposited films were characterized with a Nicolet 60SX Fourier transform infrared (FTIR) spectrometer, Spex Triplemate laser Raman spectrometer, PHI scanning Auger electron spectrometer, PHI 5300 ESCA system, Siemens X-ray diffractometer (XRD), a JEOL scanning electron microscope (SEM), and a Tencor alpha step profilometer.
2. Experimental equipment and methods
2.3. Experimental methods
2.1. Deposition equipment
Prior to film depositions, the sample and susceptor were heated to 600 “C using the r.f. generator; evacuation by a thermolecular pump produced a base pressure of 8 x lop8 Torr (1.07 x IO-’ Pa). The substrates were 3 inch silicon ( 100) wafers which were cleaved in half. One half of a wafer was used for each deposition. No attempt was made to remove surface oxide. The r.f. generator was adjusted to achieve a susceptor temperature of 300-500 “C. The carrier gas was then admitted into the chamber for 1 h prior to deposition. After the desired deposition time (20 and 70 min), Ar was used to purge the chamber while the sample/susceptor was allowed to cool to room temperature. Films were deposited at O2 gas flow rates of 5 and 15 seem using both a one factor at a time (OFAT) deposition routine and statistical experimental design [ 151. Ranges of temperature, pressure, and Mo( CO), flow rate for the 15 seem depositions are listed in Table 1. The methodology used to compare the influence of process parameters on the deposit was to vary one variable while holding the other two constant. This
The films were prepared using a LPCVD chamber as depicted in Fig. 1. The 10 cm diameter quartz chamber was surrounded by an induction coil which was connected to a 5 kW Lepel, 200-450 kHz, r.f. generator. The generator heated an electrically isolated, SIC coated graphite susceptor; no discharge was observed. Feed mixtures of Ar (99.995%, Matheson), O2 (99.97%, Matheson), Mo(CO), (99%, Alfa), and water vapor were used. The 0, to Mo(CO), molar ratio in the gas stream was regulated by varying the quantity of gas flowing through a Mo(CO), sublimator maintained at 30 “C. The concentration of Mo(CO), in the gas phase was determined by collecting the vaporized precursor in a cold trap after the sublimator and correlating the mass of the sublimed Mo(CO), with gas flow rate. Brooks mass flow controllers were used to regulate the flow rate of oxygen and argon. The pressure in the system was measured with an MKS Instruments Inc. capacitance manometer and regulated by throttling a
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13 Table I Summary
of deposition
conditions
for oxygen
flow rate of 15 seem
Parameters
Settings
0, flow rate (seem) 02/Mo(CO), molar ratio Water flow rate (seem) Temperature ( -C) Pressure (Pa)
I5 25- 61 0.0-0.15 300- 450 26.6- I35
I
develop a model relating the deposited X-MOO, film percentage to the deposition temperature and pressure. The contribution of temperature and pressure to the model was determined by first standardizing into dimensionless variables as given below: x = (Temp. - 450) (1) X: = (Presss4!::4O,CJ (2)
Table 2 Deposition conditions rate at 5 seem Process
used for experimental
variables
design with oxygen
flow
Levels
Pressure (Pa) Temperature (‘C) Mo(CO), flow rate (mg min O2 flow rate (seem) Deposition time (min)
(26.6, 4O.o’r 3 53.3 3 66.7”“) (425 >450Cp1 475 1 500”9 I .44 5 30
‘)
where temperature is in ‘C and pressure is in Pa. Several different linear and quadratic regression models were developed relating the film composition data to the standardized values of deposition pressure and temperature using regression analysis with SAS [ 161, a statistical software package. The t-statistics were calculated for each of the regression model coefficients according to a t-distribution and the following expression (3)
ap, axial point;
seem, standard
cm’/min;
cp, center
point.
coincided with depositing films with a Mo(CO), flow rate of 4.5 mg min’, at a pressure of 660 mTorr (88.0 Pa) while varying the temperature from 300450 ‘:C and at 425 “C while varying the pressure from 217-1014 mTorr (28.9-135 Pa). Two levels of Mo( CO), flow rate (2.3 and 4.5 mg min’) were used at 425 “C and 470 mTorr (62.7 Pa). Ten films were deposited at an O2 flow rate of 5 seem using a two-factor (temperature and pressure) two-level factorial design with center and axial points [ 151(Table 2). The order in which the deposition runs were conducted was randomized and was noted as the sequence number in the tables. The ranges of temperature and pressure investigated were from 425-500 “C and 200500 mTorr (26.6-66.7 Pa). For both gas flow rates three responses were recorded for each deposition: deposition rate, the gas phase CO-CO, ratio, and U-MOO, percentage of the deposited films. The deposition rate was determined by weighing the films before and after the deposition and dividing the difference by the deposition time. The gas-phase CO-CO, amp signal ratio was determined by dividing the mass spectrometer mass to charge signal at 28 by the signal at 44 (approximately 0.02% of the 28 m/e- signal was attributed to the fragmentation of CO,). This ratio was reported because the actual signal intensities change with pressure. Some of the films contained segregated regions of cc-Moo, and reduced molybdenum oxides on the same wafer. When this occurred the film E-MOO, percentage was estimated by film characterization and by measuring the deposited film Z-MOO, surface area and dividing it by the total wafer area. The film X-MOO, percentage was used to
where Y is the variable average, ug is the mean, .Pis the standard deviation and n is the number of observations.
3. Results 3.1. Process uariahles at 15sccm
0,
3.1.1. Temperature effects As the film deposition temperature increased the deposition rate decreased (Fig. 2) which led to subsequent changes in film composition and crystallinity. Laser Raman spectroscopy revealed that films deposited at 300 “C had bands closely matching MOO, powder (Fig. 3(a) and Table 3). As the temperature was increased to 350 ‘C the Raman bands in the films closely matched those of Mo40,, [ 131 (Fig. 3(b)). Raman spectra of films deposited at 375 and 400 “C showed two broad bands which could not be assigned to any particular molybdenum oxide even after expanding the baseline (Fig. 3(c)). At 425 C and above, bands
0.30 1
;j
,,,,
275
1-L:::,,,,, r
300
325
3.50
375
400
425
450
475
Temperature (‘C) Fig. 2. Deposition rate versus temperature at a pressure with a Mo(CO), flow rate of 4.5 mg min I.
of 88.0 Pa
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) S- 13
8 Table 3 Raman spectra
bands (cm-‘)
of films deposited
at 300,350
300 “C
350 “C
425 “C
204 230 351 364 461 497 513 144
186 311 430 451 730 792 834 880 908
157 249 294 340 380 521 666 821 996
(s) (m) (sh) (s) (w) (s) (s) (s)
(s) (s) (m) (m) (w) (s) (s) (w) (s)
and 425 “C
(s) (w) (s) (m) (m) (w) (m) (vs) (s)
a) Intensities: shoulder.
vs, very
strong;
s, strong;
m, medium;
w, weak;
sh,
10.00 8.939
20.00 4.436
TWO -
THETA
--
30.00 2.976
40.00 2.252
d SPACING
51I.00 1 ,823
Fig. 4. XRD patterns of films deposited at 88.0 Pa and temperatures of (a) 300 “C, (b) 350 “C, (c) 375 “C, and (d) 425 “C.
4
IQXJ
375°C
800
600
400
200
num oxides [ 1, 171. Further increases in temperature produced oriented off-white colored films with the most intense peaks at 6.88, 3.44, and 2.13 8, which correspond to (020), (040), and (060) planes of a -MOO, [ 171. Further characterization of the films deposited at 425 “C and higher indicated that the deposits contain less than 1% C as measured by Auger electron spectroscopy. Electron spectroscopy for chemical analysis (ESCA) of the 425 “C film surfaces determined that the MO 3d,,, and 3d,,, electrons had binding energies of 232.7 and 235.9 eV respectively. These values are in close agreement with Mo(V1) binding energies of 232.6 and 235.8 eV reported in the literature for U-MOO, [ 181. Films deposited at 425-450 “C had a columnar structure and faceted surface (Fig. 5).
wavenumbers (cm-l) Fig. 3. Raman spectra of films deposited at 88.0 Pa and temperatures of (a) 300 “C, (b) 350 “C, (c) 375 “C*, and (d) 425 “C (* the ordinate in (c) was expanded 100 times compared with (d). The Raman band wavenumbers are listed in Table 3).
matching those of U-MOO, powder were observed (Fig. 3(d) and Table 3). XRD patterns revealed that the films had preferential orientation [ 171. The deposits at 300 “C resembled oriented MOO, (Fig. 4(a)). The peaks observed in the 350 “C film are broad and probably contain both chi and eta Mo,O,, [ 171. The films deposited at temperatures of 375 to 400 “C showed two highly oriented XRD peaks at 3.97 and 1.98 A. These two diffraction peaks were not significant enough to identify the material considering the large number of different molybde-
Fig. 5. SEM micrograph 425 “C and 88.0 Pa.
of surface
of U-MOO,
film deposited
at
J. S. Cross, G. L. Schruder / Thin Solid Films 259 (1995) S- 13
0.04-1
20
40
60
80 100 Pressure (Pa)
120
140
Fig. 6. Molybdenum oxide deposition rate versus pressure at temperature of 425 C with a Mo( CO), flow rate of 4.5 mg mm’ with an oxygen flow rate of I5 seem.
3.1.2. Pressure eflkcts
The deposition pressure strongly influenced deposition rate, composition, and uniformity. Increasing the pressure sharply decreased the average film deposition rate at 425 “C and Mo( CO), flow rate of 4.5 mg min-’ (Fig. 6). The films deposited over this range of pressures consisted of bluish-black reduced molybdenum oxides (MOO,, where .Y< 3) at pressures < 270 mTorr (36.0 Pa), mixed molybdenum oxides with pressures between 270-470 (36.0-62.7 Pa), and Z-MOO, with pressures > 600 mTorr (80.0 Pa) (Fig. 7). The mixed phase deposits consisted of two different regions where the reduced material deposited on the gas inlet side of the wafer and higher oxidized phase deposited on the gas outlet side of the sample. This also resulted in films of non-uniform thicknesses. Comparisons of film thicknesses deposited at 425 “C showed that the thickness varied across the wafer in the direction of flow. At the leading edge or gas inlet side of
an E-MOO, deposit, the film was 12.5 kA thick and decreased in thickness to 1.2 kA at the trailing edge. For a film deposited at 270 mTorr (36.0 Pa), the film was 16.0 kA thick at the leading edge and decreased to 4.1 kA at the opposite edge. Films deposited at lower temperature also showed thickness variations but to a lesser degree. As noted, reduced molybdenum oxides or shear structures, MOO.,, exist where x is 2.75 < x < 3.0 [ 1, 171. Because these materials have similar layered structures and O-MO bond lengths, XRD, Raman, FTIR, and X-ray photoelectron spectroscopy cannot quantitatively determine the stoichiometry of films, particularly for oriented materials (Figs. 3(c) and 4(c)). These reduced films were metastable and were easily converted into Z-MOO, by heating in air at 500 “C for 10 min. Previously reported results stated that the addition of water vapor to the gas stream results in films with little carbon [ 121. However, in the present case there was no quantifiable difference in film composition (less than 1% C) or morphology when the water vapor in the gas inlet stream was varied from 0.0 to 0.51 seem. 3.2. Process vrrriuhlesut 5sccnz 0, A second series of experiments were conducted at an oxygen flow rate of 5 seem while varying the pressure (26.6-66.7 Pa) and temperature (425500 “C) (Table 4). The same general trends on the deposit formation at the high ( 15 seem) 0, flow rate were observed at 5 seem. The films were characterized using Raman, FTIR, XRD, and X-ray photoelectron spectroscopy (XPS). The E-MOO, films were formed at temperatures between 450-500 ‘C and pressures of 53.3-66.7 Pa (Table 4 and Fig. 8). The T-MOO, films were poIycrystalline with preferred (OkO) orientation. XRD and Raman characterization revealed nearly identical spectra to those in Figs. 3(d) and 4(d). The morphology of the off-white colored x-Moo, deposits showed a columnar
Table 4 Experimental Si
d)
Run
I
IlO.
Seq. no.
cl
1I
b)
L4
10.0 8.84
n
20.0 30.0 4.44 2.98 28 (“) -- d-spacing
40.0 2.25 (A)
50.0 1.82
Fig. 7. XRD patterns of films deposited at a Mo(CO), flow rate of 4.5 mg mm’, 425 ‘C and pressures of (a) 28.0 Pa, (b) 26.6 Pa, (c) 49.3 Pa. and (d) 62.7 Pa.
9
1 2 2 3 4 5 5 5 6 7
8 3 6 2 4
I 5 10 7 9
settings and deposition results for 0: flow rate of 5 seem _____ .~ _._~_ Press. (Pa) _
Temp. ( C)
Deposit (‘VI,Z-MOO,)
amu 28/44” Dep. rate (A/A) (mg mm’) ~.._____ ~
26.6 26.6 26.6 53.3 53.3 40.0 40.0 40.0 40.0 66.7
425 475 475 425 475 450 450 450 500 450
0 2 3 39 100 25 8 IO 100 100
64.8 58.7 60.5 41.9 3s. I 44.9 44.0 45.3 40.1 29.1
“Mass spectrometer
signal
ratios
0.180 0.180 0.187 0.137 0.133 0. I53 0.167 0.160 0.187 0.123
of amu 28/44 (amps/amps).
IO
J. S. Cross, G. L. Schrader 1 Thin Solid Films 259 (1995) 5- 13 Table 5 Analysis of variance 5 seem 0, flow rate
2.5 \fby
Least squares
regression
Source 0.0 0.0
r”“l”“r”“l”“l”“l”“i”‘~f 0.5 1.0 1.5 2.0 2.5 Distance (cm)
3.0
3.5
for regression
and
Regression Error Total R2 = 0.977 Standardized
2 P 6:
60
(100) .
50
(1W .
(3% .
(100) .
(8,10,25) .
‘Ill
425
450
475
with
a
analysis
Degrees of Freedom
Sum of Squares
Mean Square
4 5 9
16952.3 393.8 11346. I
4238.0 78.8
53.8
parameter
__ 70
for depositions
F-Value
4.0
Fig. 8. Plot of U-MOO, content (‘l/o) of films versus temperature pressure with an 0, flow rate of 5 seem.
c &
model
estimates ---
Variable
Coefficient
Std. Error -___
2-Value
P>ltl
Intercept X, X, x,x,XT2
19.9 36.6 15.3 13.6 13.2
3.73 3.07 3.99 4.10 3.17
5.35 I I.91 3.82 3.31 4.16
0.0031 0.0002 0.0123 0.0212 0.0088
500
Temperature ( ‘C) Fig. 9. Film thickness measured from edge of wafer for two different films deposited with an oxygen flow rate of 5 seem.
structure. At lower temperatures and pressures bluishblack colored reduced oxides were produced. The deposition pressure had a pronounced influence on the composition, deposition rate, uniformity, and gas phase composition. Gradations in film thickness were also observed in the direction of flow (Fig. 9). The film thickness was influenced by the parameters as well as the physical geometry of the reactor. Increasing the pressure from 26.7 to 53.3 Pa at 475 “C produced a steeper gradation in film thickness (Fig. 9). It appeared that both the temperature and pressure were synergistically coupling to influence the composition which was determined by modeling the response. Several different models of the film a-Moo, percentage as a function of temperature and pressure were developed from the results in Tables 2 and 4 using regression analysis. The model listed below (Eq. (4)) statistically fits the data the best of those evaluated. It consisted of five terms including both linear and quadratic expressions,
would have an R* of 1.0. The F-value for the model is 53.8 which indicates that at a 95% confidence interval the model is statistically significant because it exceeds the minimum F(,.,, value of 5.19 [19]. Pearson correlation coefficients were evaluated for all the variables. Coefficients were calculated for the following pairs: pressureldeposition rate ( -0.91 I), pressurel[AMU 28/44] ( -0.927) and pressureju-Moo, (0.756) and indicate that the pressure is an influential process variable on all three responses. The negative values indicated that as pressure increased these values decreased. The predicted values from the model using Eq. (4) and the measured deposited a-Moo, film percentages were compared (Table 6). The residual was determined by subtracting the model predicted values from the film measured values; the values of the residuals varied from Table 6 Comparison of deposited U-MOO, film composition versus model predicted values (Eq. (4)) for depositions with an 0, flow rate of 5 seem Run no.
Sequence no.
Film LX-MOO, (“A,)
Predicted (!A)
I 2 2 3 4 5 5 5 6 7
8 3 6 2 4 I 5 10 7 9
0 2 3 39 100 25 8 10 100 100
-5.1 - 1.7 ~ 1.7 40.8 98.5 19.9 19.9 19.9 103.3 93.0 ______
U-MOO,
Residual (X/u)
YMoo3= 19.9 + 36.6X, + 15.3X, + 13.6X,X, + 13.2X; (4) All five terms are statistically significant at a 95% confidence interval [ 191. The significance of each term is indicated by the magnitude of the t-values and coefficients since the standard errors are similar (Table 5). The model fit of the data is expressed in two ways. The R* for the model is 0.977 (Table 5); this value indicates that 97.7% of the variability in the data is accounted for in the model. A model with a perfect fit
____-(Residual
= Film - Prediction)
5.1 3.7 4.7 - I.8 I.5 5.1 - Il.9 -9.9 -3.3 7.0
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13
7 to - 12 with the predicted values ranging from - 1.7 to 103.3% (Table 6).
II
agreement with most CVD processes which are controlled by the reaction kinetics [ 11, 231. 4.2. Statistical model
4. Discussion 4.1. Thermodynamics
To determine the most stable phase of molybdenum oxide that would deposit in the Mo( CO),, O?, and H,O thermal deposition process, a computer program, SOLGASMIX-PV [20], was used. The program calculated the thermodynamic equilibrium at constant pressure based upon minimization of the system Gibbs free energy and conservation of mass. Equilibria were calculated for both a three-component MO, 0, and C system consisting of 14 gas species and 9 solid compounds plus a four-component MO, 0, C, and H system consisting of 56 gas species and 9 solid compounds. For both three- and four-component systems the solid species consisted of C(graphite), MO, Mo,C, Mo(CO),, MOO,, Mo40,,, Mo,O,,, Mo,O,,, and Z-MOO,. The thermodynamic data used in the calculations were taken primarily from the JANAF tables [21, 221. Equilibria were calculated over a range of temperatures 127-527 “C, pressures 100-1000 mTorr (13.3133.3 Pa) and gas compositions. The O,/MO(CO)~ molar ratio was varied from l-200 and H,O/Mo( CO), molar ratio was varied from 0.0-3.0. Based upon equilibrium calculations for the above conditions, X-MOO, was the only solid component formed as long as the O,/Mo(CO), ratio was 5 or greater. When O2 was in excess, the carbon in the system consisted almost entirely of CO1 gas. Varying the water vapor composition in the gas phase had litt.le effect on the equilibrium. Although thermodynamically U-MOO, is the most stable solid phase in the presence of excess oxygen from 127-525 “C, it was deposited only at temperatures of 425-500 “C depending upon the system pressure and precursor flow rate. The conditions under which gMOO, formed was at temperatures > 425 “C, and pressures > 600 mTorr (80.0 Pa), with an 0, flow rate of 15 seem; and at pressures > 300 mTorr (40.0 Pa), and temperatures > 450 ‘C, with O2 at 5 seem. Equilibria calculations showed that the gas phase consisted of primarily CO, and 0,. During actual depositions mass spectrometry of the gas phase showed intense peaks at amu 28 (CO), 32 (0,) and 44 ( COZ) in decreasing order of intensity. The ratio of CO-CO2 in the gas phase or the 28/44 signal ratio was correlated to the film composition (Table 6); the amu 28/44 signal I-dtiO was the lowest (29.1-40.1) and thus CO* (or 44 amu signal) concentration highest when U-MOO, was deposited. Accordingly, analysis of both the gas and solid phase indicated the process was not limited by thermodynamics but by the reaction kinetics. This is in
Modeling the depositions with a quadratic expression yielded several interesting observations. First, the model showed that temperature and pressure act both independently and synergistically (Eq. (4)) as indicated by statistically significantly terms. Secondly, all the model coefficients were positive which indicated that increasing temperature and pressure yielded higher aMOO, deposits due to the reasons mentioned in Section 4.4. Comparisons of the a-Moo, percentage of the depositions with the predicted values for the model in some cases underestimated ( ~0) the response at low levels and overestimated ( > 100) the response at high levels (Table 5). Since the minimum response was 0 and the maximum response was 100, it appeared that a cubic expression may provide a better fit of the data. However, since the temperature and pressure range examined in this study was small, a quadratic expression appeared to adequately fit the data. It has been shown that the pyrolysis of various gaseous CVD precursors. based on percentage, versus temperature has a cubic response [23]. 4.3. Process variuble eflects As noted in Figs. 2-4 and 6-9 the decomposition process was strongly influenced by temperature and pressure as in Eq. (4). This is because the processing window in which the MOO, films were deposited was influenced by the precursor reaction with oxygen. This is more clearly demonstrated by examining the role of temperature on the reaction rate of Mo( CO),. Previous results showed that Mo(CO), begins to decompose at 150 “C and was completely decomposed by 450 “C in an evacuated chamber [24]. In the present study the material deposited uniformly on the substrate at 300 “C forming MOO,, but at 350 “C some of the material deposited on the walls of the chamber surrounding the susceptor. This trend increased as the temperature of the substrate was raised, which explained why the film deposition rate decreased with temperature (Fig. 2). This phenomenon, homogeneous gas phase nucleation, was due to the high thermal gradients surrounding the susceptor which caused the precursor to partially decompose before it reached the susceptor, resulting in wall deposits [25]. This is frequently encountered with metalorganic precursors at high substrate temperatures [ 11, 23, 251. As a result of the gas reactions the film depositing on the substrate varied in thickness across the surface at the high temperatures (Fig. 9). This was due to a
J. S. Cross, G. L. Schrader
12
decrease in the precursor concentration in the axial direction [26]. In the present study, the Mo(CO), content in the effluent was too low to measure with mass spectrometry when K-MOO, films were deposited. It has been reported in the literature that in order to form uniform deposits in a cold wall reactor, gas velocity and/or reactor geometry require optimization [26, 271. In this deposition system, where the gas flow rate was fixed, the pressure was regulated by adjusting a throttling valve in the pump foreline. For a simple flow-through system the mean residence time, z is equated to 7=-Z=-
-
PV
PV
e
PWA
which describes the relationship between pressure (p), volume (V), velocity (v), flow rate (Q) and number of gas molecules (n) in a flow-through system [28]. Pressure is proportionally related to r and inversely related to the gas velocity. Hence, varying the pressure resulted in a corresponding change in the residence time (gas velocity) with a constant gas flow rate. Flow-through reactor studies have shown that the residence time influences the film deposition rate [23, 291. From a generalized approach the change in composition as a function of temperature and pressure can be explained in terms of reaction and deposition rate. At low pressures (high gas velocities and low residence time) the deposited films were more uniform but reduced oxides. At these pressures ( > 80.0 Pa at 15 seem and >40 Pa at 5 seem) material was depositing on the surface faster than it could be fully oxidized and for adatom surface diffusion to an appropriate crystalline kink site [23]. At higher pressures (low velocities and high residence time) the material has sufficient time for reaction and surface diffusion before subsequent deposits. Also, less material was depositing on the substrate due to the gas phase reactions and film deposition on the chamber wall. For this molybdenum oxide system there was a trade off between material composition and deposition rate and uniformity. The influence of deposition rate and precursor flow rate on the a-Moo, content of the film was examined by depositing films at 425 “C and 62.7 Pa where both phases coexisted on the substrate at an O2 flow rate of 15 seem. Increasing the precursor flow rate from 2.3 to 4.5 mg mini decreased the a-Moo, content of the film from 85 to 50%. Comparison of these LPCVD results with molybdenum oxide films deposited by reactive sputtering showed similar results [9, lo]. The reactive sputtering results showed that the deposition rate directly influenced the oxygen to molybdenum stoichiometry [9, lo] and crystallinity. Similar results were also observed from the deposition of MOO, using a plasma enhanced
/ Thin Solid Films 259 (1995) 5-13
CVD process [30]. Therefore, controlling the pressure, temperature, and precursor concentration regulated the rate at which the precursor arrived at the substrate surface and reacted to form molybdenum oxides. Considering the reactivity of Mo(CO), in oxygen at high temperatures and the difficulty in obtaining uniform MOO, films in a one-step process at moderate pumping speeds, the advantage of separating the process into two steps as was previously done (where the films were first deposited at 300 “C and then subsequently oxidized at 500 or 600 “C) becomes apparent
PI. 5. Conclusions The feasibility of producing molybdenum trioxide films was demonstrated using a LPCVD system with oxygen gas flow rates of 5 and 15 seem. Thermodynamic equilibrium calculations of the system showed that solid N-MOO, was the most stable phase over a range of temperatures, pressures, and gas compositions. Actual depositions showed that the material deposited over a narrow range of conditions and its formation was influenced by temperature, pressure, and Mo(CO), flow rate. The deposit composition (oxygen to molybdenum ratio) and uniformity were more strongly controlled by transport phenomena and reaction kinetics than by thermodynamics. The film thickness was more uniform at lower pressures, however it was not possible to produce homogeneous a-Moo, films at low pressures. The significance of temperature and pressure on the depositions with an oxygen flow rate of 5 seem was determined using a quadratic model. The model indicated that temperature and pressure act both synergistically and independently on the deposits. Films deposited at an oxygen flow rate of 5 seem at pressures between 300-500 mTorr (40.0-66.7 Pa) required a slightly higher substrate temperature (25-50 “C) in order to produce a-Moo, films compared with the 15 seem results due to pressure- temperature interaction. The addition of water vapor to the gas stream had little effect on the film composition.
Acknowledgments
The assistance of Dr. R. W. Stephenson of the Iowa State University Statistics department was particularly valuable in developing the model and interpretation of the deposition data. Thanks are also extended to Mr. Alan Landon of the Iowa State Microelectric Research Center for his measurement of the film thickness, Mr. Jim Anderegg of the Ames Laborator for the film XPS measurements, and Dr. A. Bevel0 for the film Auger electron spectroscopy measurements.
J. S. Cross, G. L. Schrader / Thin Solid Films 259 (1995) 5-13
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