chemical vapor deposition of

8/9/2019 Chemical Vapor Deposition Of

1/23

CHEMICAL VAPOR DEPOSITION OF

MOLYBDENUM METAL COATINGS ON

SILICON NITRIDE SUBSTRATES

Submitted by

Kamilah Turner

University of Michigan

to

University Research Alliance

Amarillo, Texas


2/23

2

Abstract

The storage of nuclear waste has been a problem for years. To reduce the amount and

time of nuclear waste storage, new methods are being developed to mutate spent nuclear

waste into less hazardous materials. One of the processes being developed results in an

equipment failure with the mutation of transuranic (TRU) nitride particles. To avoid this

failure, molybdenum metal as a coating has been evaluated to determine its effectiveness

in preventing the failure with the nitride particles. Two methods of deposition were

investigated; however, chemical vapor deposition was determined to be the method of

choice. Each method was used to coat a surrogate nitride material with molybdenummetal to determine feasibility of metal coating and deposition conditions. Molybdenum

coated samples were analyzed using x-ray photoelectron spectroscopy (XPS) and

scanning electrode microscopy (SEM). The molybdenum coatings were found to be

highly dispersed, resulting in a very uneven coating at each of the experimental

temperatures. Islands of molybdenum and oxygen of various sizes were found

throughout the sample surface. Molybdenum metal was also found to be extremely air

sensitive; passivation of the material before exposure to air and reduction to the metal

state was required to properly analyze the coating. The design of the deposition

apparatus was found to be ineffective in coating the molybdenum metal to the nitride

substrate. Further experimentation and development of coating apparatus must be

completed to successfully assess the effectiveness of molybdenum coating and deposition

conditions.


3/23

3

Introduction

One of the key challenges in the nuclear power industry is the storage of nuclear wastes.

The current United States approach to the disposal of high level nuclear waste is to store

the unreprocessed spent fuel in a geologic repository (1). By some estimates, the storage

areas will be completely filled with spent nuclear fuel by 2015 (1). This indicates that

other waste repositories will need to be identified in the near future, or a process that

could reduce the amount of nuclear waste being stored would have to be developed. In

1999, the U.S. Congress directed the Department of Energy (DOE) to develop a reduction

process under the Accelerator-Driven Transmutation of Waste (ATW) Program (1).

Thus far, Los Alamos National Labs (LANL) has led the development of ATW as an

alternative technological option to the disposition of nuclear waste (1). This technology

allows for spent fuel to be sent to an ATW site where transuranics (TRUs) and particular

long-lived fission products would be destroyed by fission or transmutation within the

facility. There are four key elements in the implementation of the ATW program: system

definition, accelerator, fuel treatment, and target/blanket (2). The focus of research

described in this report is related to the fuel treatment element.

Problem Statement

The ATW fuel treatment portion of the program is focused on the selection of a process

to recover un-transmuted transuranics from irradiated fuel for recycle through the ATW

recycle system. Currently, the ATW program is investigating different processes such as

the volatility process (1). In one process, an irradiated fuel, which consists of solid fuel

particles, is dispersed into a zirconium metal matrix to dilute the high level of fissile

material in the fuel particles. It is in this matrix that the particles will be transmuted to a

non-radioactive or less radioactive species by neutron bombardment.


4/23

4

The TRU fuel particles contain large amounts of fissile material. The heat generated

from the particles is so high, that there is a need to dilute the fuel. To dilute the particles,

they have been dispersed into the metal matrix. Zirconium is an attractive metal for the

matrix material because it has a low neutron absorption cross section, is compatible with

a variety of reactor coolants, has a reasonable thermal conductivity, and melts at high

temperature. The particle recovery process involves removing the TRUs and fission

products from the zirconium (3). In the case of TRU nitride fuel particles, there is a

concern for interactions between the fuel particle and the zirconium matrix at their

interface during normal operation of the fuel at temperatures greater than 1000C. The

interaction of concern is the formation of a zirconium nitride or oxide. This formation

could lead to a redistribution of the fissile material that could in turn lead to a failure of

the fuel element. To prevent the possible interactions between the nitride fuel

particles and the zirconium metal matrix, it was proposed that a refractory metal be

deposited onto the surface of the fuel particles.

The goal of research described in this report was to investigate the deposition of

molybdenum on nitride fuel particles. The following objectives were established to

achieve the project goal:

1) Determine a refractory metal for deposition

2) Determine the best conditions for deposition

3) Develop an initial schematic for deposition of metal on spherical particles

Refractory Metal Selection

The refractory metals initially chosen for this project were molybdenum, tungsten, and

niobium. These refractory metals were chosen because they were not expected to interact

with the fuel or zirconium, have good oxidation resistance at high temperatures, and

methods for their deposition have already been defined. Due to time constraints,

molybdenum became the refractory metal of choice. Molybdenum was the metal of


5/23

5

choice because the research group supporting the project has substantial expertise

regarding its properties and characteristics.

Deposition Conditions

There are a number of coating processes that can be used to deposit Mo including

chemical vapor deposition (CVD) and physical vapor deposition (PVD). The CVD

process allows for gases containing the deposition material to be introduced into a

reaction chamber and deposited on a substrate to form a coating (4). After evaluating

various methods and equipment availability, CVD was the chosen process for depositing

Mo. In determining a good coating method, it is important to identify the size of the

particles that the material will be deposited on. In the case of the nitride particles, the

optimum particles size is not defined. Work at Argonne National Labs (ANL) is

currently being done to determine the optimum size. The ideal particle size is probably in

the few-hundred micron range. A uniform dispersion needs to be achieved to minimize

the overlap of damage zones.

Determining the deposition conditions was done using flat wafers instead of actual

spherical particles. An important factor in CVD is the geometry of the system during the

coating process. Discovering the best geometry and deposition conditions

simultaneously is a major task; therefore, it has been decided to first determine the

deposition conditions, and then take on the task of determining methods to coat spherical

particles.

Deposition on Spherical Particles

After determining the conditions to uniformly coat Mo, an investigation was to be done

to formulate ideas on methods to coat spherical particles in the final system. A literature

search was used to accomplish this objective.


6/23

6

Experimental Design

Material Deposition

Two different methods deposition were involved in experimentation: salt deposition and

chemical vapor deposition (CVD). Salt deposition involves placing the nitride substrate

in a Mo salt solution and heating the water off to leave a Mo salt coating. The substrate

was then reduced in H2 at 800C leaving the Mo metal. The second deposition involves

chemical vapor deposition (CVD). In the CVD method, MoCl5 was reduced under H2 on

a heated substrate (900 -1300C) at atmospheric pressure (5). Another more common

method of coating Mo involves the use of a molybdenum hexacarbonyl, Mo(CO)6 (6,12).

Due to safety concerns, this precursor was not used.

The molybdenum precursor, molybdenum (V) chloride (Sigma-Aldrich, 98% purity), is

sensitive to air and moisture. A special reactor was designed to address this issue. This

is discussed in the next section with more detail. Also, for the purposes of

experimentation, instead of a uranium nitride substrate, a radioactive substance, silicon

nitride (Si3N4) wafers were used as a surrogate material. The wafers were made by

University of Michigan Professor, Erdogan Gulari. Silicon nitride was deposited on a

silicon substrate using low pressure CVD (LPCVD); the Si3N4 layer is approximately

2000 angstroms thick. The wafers were broken along its structural lines into 1-cm2

pieces for the reactor. Each wafer, before being placed in the reactor was cleaned with a

trichloroethylene/acetone/methanol/de-ionized (DI) H2O rinse to remove all

contaminants. The rinse process is detailed in Appendix A.

In the first method, a 10% ammonium heptamolybdate, (NH4)6Mo7O24-4H2O

(Mallinckrodt, crystals), in DI H2O solution was gently stirred and heated moderately

until the salt completely dissolved. The wafer was then placed in the solution and heated

up to100C until the water completely evaporated. This typically resulted in a very

uneven coating of the salt. The sample was then placed in a reactor rube heated by a PID-

controlled split tube furnace (Applied Test Systems, Inc.) for 3 hours at 800C. Due to


7/23

7

the large and apparent non-uniformity of the coating, this method was not further

investigated.

As stated above, the second deposition process is routed in conventional CVD. In a

heated reactor tube, the MoCl5 precursor and silicon nitride substrate were heated

simultaneously while passing H2, the reactant gas, and Ar, the carrier gas, through the

reactor (6).

HClMoHMoCl ++0

25

A complete diagram can be found of the reaction and reactor design in the Appendix B.

Due to extreme reaction temperatures, the reactor tubing was made of quartz. The reactor

tube has a diameter is 1.8 cm (0.71 in.) and a length of 30 cm (11.8 in.). The precursor

was kept in a small quartz boat. A small quartz easel-like stand was used for the

substrate. The reactor tube was assembled in a nitrogen-filled glove box (Vacuum

Atmospheres, MO-20) to avoid entry of air into the system. The reactor tube and system

were purged with argon before introducing hydrogen into the system. The furnace was

ramped to 450C, then heated to the final reaction temperature which was in the range of

900-1100C. At the reaction temperature, the H2 gas was introduced into the reactor a

varying flow rates (20-30cc/min). After a 3-hour reaction time, the H2 was shut off and

the reactor cooled. Once reaching temperatures less than 35C, the coated substrate was

passivated in a mixture of 1%O2 /He at 20 cc/min for 3 hours. A scrubber containing DI

water followed the reactor tube to react with the hydrogen chloride gas by-product before

emitting into the hood. Litmus paper (EM-Science, ColorpHast) was used before and

after each reaction to determine how much HCl(g) was converted to its acid form. After

every 4th

run, the reactor tube was disassembled and cleaned to remove residual Mo.

Characterization

X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were

used to characterize the molybdenum samples. XPS was carried out on a Perkin

(OPHU3+, XVLQJ D0J .. [UD\ VRXUH DW : N9 7KH LQVWUXPHQW ZDV


8/23

8

calibrated using the reference binding energies of Au 4f7/2 at 84.00eV, Cu 3p at 75.13

eV, and Cu 2p3/2 at 932.66 eV. A broad survey spectrum from 600-0eV was initially

taken. The spectra were charge-corrected to carbon 1s at 287 eV. Next, a high-resolution

scan of oxygen and molybdenum was obtained to quantify composition and to determine

the chemical states. Also, a gas phase reactor allowed samples to be heated and reduced

from the passivated state and moved to the XPS analysis chamber without exposure to

air. Curve fitting of the XPS spectra was performed with a nonlinear least squares

method using Gaussian distributions. SEM was carried out on a Phillips XL30 using a

thermally assisted Schottky (5kV) field emission gun for high intensity probe formation.

The system uses a zirconated tungsten filament and has a vacuum of ~10-6 torr.

Results and Discussion

Effect of Passivation

Experiments were done at various temperatures in the range of 900-1100C and flow rate

ratios of Ar to H2. Some of the initial runs were conducted at 1000 C. The passivation

effects on the Mo coating are shown in Figures 1-3. Table 1 lists binding energies and

characteristics of Mo and some of its oxidation states. These samples, based on substrate

weight change, were coated with approximately 125 microns of Mo metal.

Without passivation, analysis by XPS shown in Figure 4 indicates MoO3 was the only

form of Mo present on the substrate surface.

Compound Binding Energy (eV) Doublet/Singlet: Separation (eV)

Mo 227.7 Doublet: 3.15

MoO2 229.4 Doublet: 3.20

MoO3 232.7 Doublet: 3.20

Table 1. Binding Energies of Mo and MoOx compounds(7)


9/23

9

22 022523 023524 0

Binding Energy (eV )

N

(E

Figure 1. NON-PASSIVATED 1000C: MoOx (2


10/23

10

situ, the Mo multiplex shown in Figure 3 indicated a 66% increase in the Mo metal

content and a 27% decrease in the Mo oxide content.

220225230235240

Binding Energy

N(E

Figure 3. IN-SITU REDUCED: Mo metal, MoOx

This clearly shows there is definite Mo coverage on the wafer before passivation. It also

verifies that the intermediate MoOx species found before passivation is reducible. Longer

reduction times of approximately 5-6 hours are likely to show complete oxide removal

and Mo metal coating. In addition to the increase in Mo metal between the passivated and

reduced Mo multiplex, there was also a decrease in the oxygen atomic concentration,indicating reduction was successfully taking place. The time required to complete in-situ

reduction using the XPS facilities on each sample was too intensive for this study.

Consequently, the characterization of other samples was completed using the passivated

sample.

Oxygen Content

The initial survey scans showed large percentages of oxygen in the sample. As with Mo,

a multiplex scan was completed to further investigate the O1s peak. Oxygen is found in

the vicinity of 530 eV. The oxygen 1s peak was also found to have shoulder peaks. The

shoulder peak is indicative of the different forms of oxygen present. The separation

between the shoulder and the O1s peak is related to the form of MoOx found in the

MoOx

Mo


11/23

11

material. The number of shoulder peaks with the O1s peak and their location should

correspond with the number and types of molybdenum oxides present in the Mo

multiplex scan, as shown in Figures 4 and 5 and Table 2 below, for the samples prepared

at Ar:H2 ratio of 1:1.

% L Q G L Q J ( Q H U J \

1

(

&

&

&

Figure 4. Oxygen multiplex

22 0225230235240

Bind ing Energy (eV)

N

(E

1100C

1000C

900C

Figure 5. Mo multiplex

Temperature(C) # of Oxygen shoulder Peaks # of MoOx peaks

900 2 2

1000 2 2

1100 2 2

Table 2. Number of O shoulder and MoOx peaks


12/23

12

This observation holds for all samples and further confirms that the same type of Mo

species is present in each sample.

Surface Coverage

Three survey scans were taken of all samples. The Mo and O multiplex scans were taken

based on these initial surveys. As shown in the example in Figure 6, each of these initial

scans also contains Si and N peaks.

1 0 02 0 03 0 04 0 05 0 06 0 0

Binding Energy(eV)

N

(E

N 1s

BE 401eV

Si 2p3

BE 102 eV

Figure 6. IN-SITU REDUCED: Initial Survey

This indicates that there is not an even film of reduced Mo metal on the Si 3N4 substrate.

Scanning electron microscopy (SEM) was used further investigate this observation. The

following micrographs shown in Figures 7 and 8 represent an example of the Mo/MoOx

coating.


13/23

13

Figure 7. SEM Scan 1000X (900C ,1:1)

Figure 8. SEM Scan 5000x (900C, 1:1)

These micrographs clearly show how sparse the coating actually is. This type of coating

is believed to represent all samples. An elemental analysis, energy dispersive x-ray

spectroscopy (EDX), was done to evaluate the composition of adsorbate islands on the


14/23

14

substrate surface shown in Figure 8. The EDX found larger amounts of O and Mo in

these islands. The amount of O in comparison to Mo was large. This correlates to the

XPS data shown in Figure 6 as Mo has a much higher sensitivity than O, and is found in

much lower percentages than O. (Mo: 2.867, O: 0.711) EDX also showed very small

quantities of oxygen on the surface where there were no islands.

Achieving an even Mo coating has been a difficult task. The reason could be due to

system geometry, and temperature and flow profiles. The diameter of the reactor tube is

1.8 cm with a cross-sectional area of 5.67 cm2. The area of the wafer is 1 cm2. The

distance between the precursor boat and the wafer is ~2 cm. This distance may not be

enough to allow for controlled flow. The Reynolds number was calculated using

viscosity and density of argon, as this is the carrier gas. The Reynolds number was found

to be on the order of 106

indicating turbulent flow. (Viscosity was calculated at 1000C.)

Turbulent flow consists of large rotational eddies near walls, that degenerate

progressively into smaller eddies. These rotational eddies are likely to be preventing the

Mo from coating the substrate in an even and controlled manner. The Mo appears to be

splattering on the surface. This may be controlled by moving the boat further away from

the substrate, or by possible cooling down the substrate. In the reactor tubing past the

wafer, there was a significant amount of MoOx that condensed in the quartz tubing, aswell as in the Swagelok tubing. This shows that the Mo was definitely moving in the

vapor phase towards the wafer. The condensation in the tubing indicates that a reactor

design which integrates a cooling function could have resulted in a more even coating.

Effect of Temperature and Flow Rate Effects

Two series of runs were completed at temperatures of 900,1000, and 1100C using Ar:H2

ratios of 2:1, and 1:1. In the runs using a 2:1 ratio, Mo metal was found along with MoO x

in the 900C and 1000C runs, as shown in Figure 10. However, in the 1100C run, there

was no Mo metal observed using XPS. In these runs, data also showed an increase in the

weight change with increasing temperature after each run was completed.


15/23

15

Temperature(C) Weight Increase (mg) % Weight Increase

900 0.4 0.20

1000 0.6 0.29

1100 0.8 0.38

Table 3. Mo Deposition Uptake w.r.t. CVD Temperature

Table 3 shows that as reaction temperature increased there was more Mo and MoOx on

the substrate. As mentioned above, no reduced Mo metal was seen in the 1100C run. A

possible reason as to why Mo is not seen yet there is a weight change increase is related

to the passivation of the Mo coating before removing from the reactor. The passivation

reaction is believed to occur as follows:

xMoOOMo + 20

With the increase in weight change of the substrate, it is clear more Mo is being coated.

Because there is more Mo metal on the surface, there is a concentration increase in

reactant, and the reaction will move much more rapidly to form a thicker oxide product

layer. Also, the strength of interaction between the substrate and adsorbate is often lower

at higher loadings as shown in Figure 11. This could allow more reduced Mo metal to be

oxidized (8).

Figure 11. Surface and Adsorbate Interactions

Surface-Adsorbate Interactions

Adsorbate-Adsorbate Interactions

Surface-Adsorbate Interactions


16/23

16

At 1100C, it is proposed that MoOx forms completely over the Mo metal in a uniform

subsurface layer of approximately 10-20 angstroms, such that no Mo metal is within sight

of the XPS scans. In the 900C and 1000C runs at the 2:1 reactant gas ratio, there is less

Mo metal on the substrate, meaning lower local Mo concentration. As a results, the

substrate has a much stronger interaction with the adsorbate and oxidation occurs at a

slower rate over a smaller volume.

% L Q G L Q J ( Q H U J \ H 9

1

(

&

&

0 R0 R 2 0 R 2

Figure 10. Mo multiplex.

While in 900C there appears to be more Mo metal than molybdenum oxide, it is also

shown that the amount of oxide increases between 900C to 1000C. The ratio of Mo0

to

Mo+x

was calculated based on peak intensities for both runs. As shown in Table 4, the

ratio does not change much, even though there is an increase in weight change. Both of

these observations give more support to the idea that the amount of Mo on the surface

affects how well the passivation reaction will occur.


17/23

17

Temperature (C) Mo0:Mo

+x*

900 0.45

1000 0.42

1100 ---

*Note: 2


18/23

18

theory of passivation occurring on materials that have more Mo. The initial passivation at

1100C occurs faster, and more oxygen is able to diffuse into the passivation layer.

Deposition on Spherical Particles

The focus of this project was to determine how Mo metal coated to silicon nitride to give

an idea of how the metal would coat with uranium nitride. All experimentation was done

on flat wafer. The actual application occurs with spherical particles. The reactor design

used here is not applicable. The conventional method of coating spheres is through open

bounce pan configuration (9). In this method, the bounce is induced by piezoelectric

transducers that control lateral and transverse motions to the pan. Issues arise when the

capsule mass and diameter change due to the addition of the coating. This requires

constant calibration of the bounce pan and its motion. Another important issue with this

conventional method and other methods is the geometry of deposition source and

aperture (opening to chamber with substrate) size. Both of these vary with the deposition

that is being achieved.

A better method to coat Mo on nitride spheres uses pulsed-gas levitation (9).

Figure 12. Pulsed-gas levitation (9)

The capsule moves by oscillating a partial flow of the reactant gas beneath it. The

diameter of the pulsed-gas inlet tube is 70-80% of the capsule diameter. The flow of the

capsule

Gas inlet


19/23

19

pulsed-gas is varied throughout to account for weight changes and to have an even

coating. This flow variation can be determined by energy balances of forces and changes

in capsule lateral or transverse movements. Hydrogen and argon would travel through

the gas inlet, and argon would travel through the tubing to levitate the capsule. The

system must be air-tight and heated for the deposition to occur.

Conclusion

Molybdenum metal coating on Si3N4 was prepared using a basic CVD method.

Passivation of the coating is needed to prevent exposure to air from converting Mo

completely to MoO3. This method results in an non-uniform film. The Mo metal will

deposit on the nitride substrate, however it the results is a severely uneven coating in the

experimental reactor design. Further development of a more efficient reactor design may

results in a more uniform coating.


20/23

20

References

1. Venneri et. al, Disposition of Nuclear Wastes Using Sub-critical Accelerator-Driven Systems, The Uranium Institute 24th Annual Symposium, 1999, London.

2. A Roadmap for Developing Accelerator Transmutation of Waste, A Report toCongress, October, 1999.

3. Laidler, J. Bresee, J. Pyrochemical Processing of Irradiated Transmuter Fuel.

4. Holmberg K, Matthews A. Coatings Tribology: Properties, Techniques and Applications in Surface Engineering, Tribology Series 28, Dowson D. (ed.)Elsevier: Amsterdam, 1994; 3,7-29.

5. Galasso F., Chemical Vapor Deposited Materials, CRC Press, Inc. 1991

6.

Endler et. al, Chemical Vapour deposition of MoS2 coatings using the precursorsMoCl5 and H2S, Surface and Coatings Technology 120-121 (1999) 482-488.

7. Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics Division,Perkin Elmer Corporation, Minnesota, 1979.

8. Somorajai, Gabor, Introduction to Surface Chemistry and Catalysis:, John Wileyand Sons, Inc. 1994.

9. Jankowski et al., Chambered capsule coatings, Thin Solid Films, 398-399 (2001587-590.

10.Briggs D., Seah M.P. Practical Surface Analysis b Auger and X-rayPhotoelectron Spectroscopy, John Wiley & Sons, 1983.

11.Chemical Vapor Deposition, Surface Engineering Series Vol.2, ASMInternational, The Materials Information Society, 2001.

12.Chemical Vapor Deposition- Principles and Applications, Academic Press, 1993.

13.Ivanova et al., Investigation of CVD molybdenum oxide films, Materials Letters,53 (2002) 250-257).

14.Mikhailov et al., The behaviour of the molybdenum-CVD diamond interface athigh temperature, Diamond and Related Materials 4 (1995) 1137-1141.

15.Perrys Chemical Engineers Handbook, 6th edition, McGraw Hill, 1984.

16.Petigny et al., Molybdenum deposition on TiO2 (110) surfaces with differentstoichiometries, Applied Surface Science 142 (1999) 114-119.


21/23

21

17.Surface Characterization of Advanced Polymers, edited by L. Sabbatini and P.Zambonin, VCH Verlagsgesesllschaft mbH, D-6940 Weinheim, 1993.


22/23

22

Appendix A

TCE Rinse: Initial Wafer Clean

Procedure:

1) If fingerprints or other residue appears on the wafer surface, swab it clean with aQ-tip dipped in trichloroethylene.2) Immerse in warm TCE for 5 minutes. (Room temperature for new wafers.) Pour

used TCE slowly into a designated TCE waste bottle.3) Immerse in acetone for 3 minutes. This immersion removes the TCE residue and

acts as a further cleaning solvent. Pour used acetone into waste solvent container.

4) Immerse in methanol for 3 minutes. This rinse removes the acetone residue. Pourused methanol slowly into the waste solvent bottle.

5) Rinse in running DI water for 3 minutes.

Reference:http://mitghmr.spd.louisville.edu/sops/sop1.html


23/23

Appendix B

REACTION SCHEMATIC

Vent to

MoCl5

Si3N4

H2O

30 cm

Dimensions of furnace: dia: 3.0 cm, length =15 cm

Dimensions or rxtr. tube: dia: 1.8 cm, length = 30 cm

furnace

Metal caps

Valves

Thermocouple

DEPOSITION SCHEMATIC

Scrubber

Three way

vents

H2

Ar

O2/H2O traps

M

M3-way (vent)2-way

HeatHeat

MoCl5 vapor Reacts with H2

Mo

Not drawn to scale

chemical vapor deposition of

Documents