Microwave Plasma Assisted Synthesis of Single Crystal Diamond at High
Pressures and High Power Densities
J. Lu a, Y. Gu a, D. K. Reinhard
a, b,
T. A. Grotjohn a, b, and J. Asmussen
a, b
Michigan State University, Department of Electrical & Computer Engineering, East Lansing, MI 48824
b Fraunhofer USA, Center for Coatings and Laser Applications, East Lansing, MI 48824
ABSTRACT
EXPERIMENTAL RESULTS
Current microwave plasma assisted chemical vapor deposition (MPACVD)
diamond synthesis theory suggests that CVD synthesized diamond quality and
growth rates can be improved by using high power density microwave discharges
operating at pressures above 160 Torr [1]. Thus we are experimentally exploring
single crystal diamond (SCD) synthesis under high pressure and high power density
conditions that take advantage of the improved deposition chemistry and physics that
may exist in the high pressure (180-300 Torr) regime. Here in this poster paper the
experimental results for SCD synthesis are presented using a recently improved 2.45
GHz microwave plasma assisted CVD reactor design [2].
The SCD synthesis was carried out using H2/CH4 input gas chemistries over the
180-300 Torr regime. SCD growth rates versus pressure, input gas chemistry (with
and without N2 addition), and substrate temperature are presented, and output SCD
quality was evaluated by Micro-Raman, IR-UV transmission spectrometry and SIMS
analysis. SCD growth rates increase as pressure, methane concentration and
discharge power density increase. Linear growth rates of 70-80 micron/hr are
achieved without N2 addition. SCD growth rates additionally increase with small
amounts of N2 additions (10-200 ppm) to the feed gas. Under similar growth
conditions, i.e. temperature, methane concentration, etc., growth rates increase faster
vs. N2 addition at higher pressures than at lower pressures. A SCD growth window
was observed between 950 - 1300 °C, the details of which have not been published
by other investigators up to now. Micro-Raman spectroscopy, IR-UV transmission
spectrometry and SIMS measurements showed that the synthesized SCD was of
excellent quality (type IIa, gem quality or better) within the growth window of 1030
– 1250 °C with high growth rates of 40-45 micron/hr.
SCD synthesis is a multi-variable optimization problem – when
holding substrate size and flow rate (~400 SCCM) constant the
major variables are pressure, CH4/H2, substrate temperature, and
reactor design. A multi-dimensional plot is necessary to evaluate
different reactor performance.
§ Growth rates versus pressures for Reactor A, B, and C
(N2 = 0).
§ Reduction of powered electrode area and the increase in
pressure increases the deposition rate.
Growth Rate by Linear Encoder (µm/hr)
80
60
40
Reactor B
Ts ~ 1050 – 1200°C
30
20
10
0
160
Ts ~ 1050°C
180
45
35
30
25
20
15
10
5
35
950
1000
1050
1100
1150
1200
1250
q Independent Process Variables
q Reactor
20
15
10
5
0
950
1000
§
§
§
§
§
§
§
§
Pressure: 160 - 300 Torr
Absorbed power: 1.4 - 2.1 kW
Gas feed composition: variable CH4/H2 = 3-9%
Total gas flow rate constant ~ 400sccm
Input gas purity: 6N - H2; 5.5N – CH4
Deposition time: 6 - 24 hours
Substrate type: 3.5 x 3.5mm2 HPHT diamond
substrates
q Internal
§
§
§
Variables
Substrate temperature (Ts): 900-1400 C
Plasma volume: 4-10 cm3
Absorbed power density Pabs / plasma volume:
200-650 W/cm3
§
§
§
Reactor size and Configuration: Reactors A, B,
and C
Substrate holder design: pocket holders
Substrate position: Δz = L2- L1
Electromagnetic excitation mode: Hybrid
TM013/TEM mode
q Reactor
§
§
Performance and Diamond Quality
Linear growth rate (μm/hr)
SIMS and RAMAN measurements
REFERENCES
[1] D. G. Goodwin, J. Appl. Phys. 74, 6888 (1993).
[2] K. W. Hemawan, et al., Diamond and Related Materials, 19, 1446-1452 (2010).
[3] Y. Gu, et al., 2nd MIPSE (2011).
Growth Rate by Linear Encoder (µm/hr)
Growth Rate by Linear Encoder (µm/hr)
1100
1150
1200
1250
1300
Growth rate versus temperature for Reactor
B (240 Torr, N2 = 0)
160 Torr 7% CH4 Reactor A
220 Torr 5% CH4 Reactor B
240 Torr 5% CH4 Reactor B
50
1050
Temperature(C)
Temperature(C)
40
30
20
10
0
Growth rate by linear encoder
Nitrogen Content
5
40
4
30
3
2
20
1
10
0
0
0
100
200
300
400
0
N2 Concentration (ppm)
50
100
150
200
N 2 Content in Gas Phase (ppm)
Growth rate & N content versus N2 concentration
Growth Rate by Linear Encoder (µm/hr)
Pocket substrate holder cross sectional
view. The design of the pocket holder
is varied by varying dimensions A, B
and C.
Geometry Variables
300
25
1300
Growth rate versus pressure for Reactors
B and C (240 Torr, N2 = 0)
SUBSTRATE HOLDER DESIGN
EXPERIMENTAL VARIABLES
280
N Content in Crystal by SIMS(ppm)
900
Reactor C [3]
A
260
30
45
B
240
4% CH 4
5% CH 4
6% CH 4
40
0
DIAMOND QUALITY
C
220
45
Reactor B
Reactor C
40
Growth Rate by Linear Encoder(µm/hr)
Growth Rate by Linear Encoder(µm/hr)
§ For both reactors and various
Methane concentration, SCD
growth window is observed
approximately from 950 to
1300 °C.
§ The higher discharge power
density in Reactor C yields
higher deposition rate.
§ The growth rate exhibits a
maximum between 1125 –
1225 °C, which shifts as
methane concentration varies.
Growth rate versus N2 concentration
Reactor B
200
Pressure (Torr)
50
Reactor A
Ts ~ 1150 – 1300°C
50
Reactor A
§ SCD growth rates increase with
N2 addition to the feed gas.
§ Under similar growth
conditions, growth rates increase
faster at higher pressures than at
lower pressures respect to N2
concentration.
§ The Raman FWHM and N
content by SIMS of the
synthesized SCD both increase
with N2 addition to the feed gas.
REACTOR DESIGNS
Reactor C
3% CH4
5% CH4
7% CH4
9% CH4
70
2.8
Growth Rate
FWHM
40
2.6
35
2.4
30
25
2.2
20
FWHM
a
2.0
15
HPHT Seed: FWHM = 1.88
1.8
10
5
1.6
Element Six: FWHM = 1.64
0
950
1000
1050
1100
1150
1200
1250
1300
Temperature (C)
§ Thick diamond plates were fabricated by removing the SCD
layer from the seed by laser cutting.
§ The plates were mechanically polished and the edges were
laser trimmed.
§ The large size SCD plate on the right is near colorless, type IIa,
1.1 carat.
For Reactor B (240 Torr, 6% CH4, N2 = 0)
§ Synthesized SCD was characterized by micro-Raman
spectroscopy , IR-UV transmission spectrometry and
SIMS.
§ A good quality SCD growth window was observed
between 1030 – 1250 °C.
§ The Raman FWHM ranged from 1.65 – 2.0 cm-1.
SIMS analysis shows less than 300 ppb N and Si in
the synthesized SCD. IR-UV transmission
measurement also indicates type IIa diamond.
SUMMARY
At high process pressures of 180-300 Torr ,
q SCD growth rates increase as pressure, methane concentration and discharge power density increase
and linear growth rates of 70 - 80 microns/hr are possible without N2 addition.
q A SCD growth window was observed between 950 – 1300 °C, the details of which have not been
published by other investigators up to now.
q Micro-Raman spectroscopy, IR-UV transmission spectrometry and SIMS measurements showed that
the synthesized SCD was of excellent quality (type IIa: gem quality or better) within the growth
window of 1030 – 1250 °C with high growth rate.
q SCD growth rates increase with N2 addition to the feed gas. Under similar growth conditions, i.e.
temperature, methane concentration, etc. , growth rates increase faster at higher pressures than at
lower pressures.
q The trends of the results of diamond quality and growth rate are consistent with the predictions from
the simple theory of diamond growth and quality from Harris and Goodwin [1].