Plasma-Assisted Processing of Materials

Timothy A. Grotjohn  (grotjohn@egr.msu.edu)

1) Plasma-Assisted Diamond Synthesis and Processing for Diamond Electronics

Fig. 1: Free standing diamond plates. Each plate has been plasma-assisted chemical vapor deposition grown and then was laser cut from the seed substrate. Left: Boron-doped diamond with a size of approximately 3mm x 3mm x 2 mm thick. The single crystal boron-doped diamond is blue and a frame of polycrystalline diamond (gray) is around the edge. Right: A 1.2 carat diamond plate.  The marks on the ruler are in 1 mm increments.

 

 
 


          

The field of diamond synthesis and applications is undergoing a spectacular period of transformation due to advances in the ability to deposit high quality single crystal diamond (SCD). SCD’s exceptional semiconductor properties would be transformative and have enormous potential for use in high power electronics technology with applications to transportation, manufacturing, and energy sectors, as well as, for high power, high frequency electronics. For instance, diamond-based power electronics would operate safely at elevated temperatures and would not require the extensive cooling and circuit protection that is found in today’s high-power systems. Direct connection of diamond power electronics to the electrical transmission systems of the power grid would facilitate more efficient distribution of alternative energy sources such as wind and solar to the power grid. SCD-based power devices will enable high voltage switching with high efficiency and high current levels.

Among the wide bandgap materials, diamond has the best properties for power electronic devices as shown in Table 1 and has the potential to be the next wide bandgap material exploited as SiC and GaN materials reach their theoretical limits.

 

Property

Si

GaAs

6H-SiC

4H-SiC

GaN

Diamond

Bandgap, Eg (eV)

1.12

1.43

3.03

3.26

3.45

5.45

Electric Breakdown Field, Ec (kV/cm)

300

400

2500

2200

2000

10000

Thermal Conductivity, l (W/cm×K)

1.5

0.46

4.9

4.9

1.3

22

                             Table 1: Physical characteristics of some semiconductors.

 

Given the superlative properties of diamond, as applied to several electronic applications, a natural question that arises is why diamond has not seen widespread use. The two bottlenecks that have limited the use of diamond are (1) lack of substrates/wafers of sufficient area size, low cost and high quality for fabrication processes, and (2) lack of shallow acceptors/donors for both p and n type semiconductor material doping. Work is underway at MSU to increase the size and quality of diamond substrates. In terms of doping, p-type diamond is readily achieved with boron doping, however boron has a deep acceptor energy level of 0.37 eV. N-type diamond has been a more difficult challenge to solve with the best solution to date being phosphorus with a deep energy level of 0.58 eV. Approaches to dealing with the deep donor and acceptor levels is to (1) utilize heavily doped regions when possible so that the dopant activation energy is reduced giving low resistivity diamond material and/or (2) operate the diamond device at higher temperatures where more dopants are active. The performance of diamond electronics is shown in Fig. 2 as compared to electronics fabricated in SiC and GaN materials. Fig. 2 shows that diamond could have superior performance to these other materials at temperatures above 100 C.

The Michigan State University diamond research effort has worked on single crystal diamond for a number of years and has diamond deposition equipment and processes for growing intrinsic, p-type and n-type diamond. Additional processes and equipment are also available for microfabrication of diamond electronic devices such as plasma-assisted etching, laser cutting, polishing and electrical contact formation. A goal of this research is to design, fabricate and characterize high voltage, high power diamond electronic devices including diodes and transistors.

 

Fig. 2: Comparison of power loss in diodes of equal breakdown voltage and current flow versus temperature. [T. Grotjohn, Advanced Diamond Science and Technology Topical Conference, Detroit, Oct. 2011.]

 

2) Design of Microplasma Source and Their Applications

                Microwave powered plasma sources that produce plasma discharges of less than one millimeter in size are designed and the microplasma discharges are studied. Applications of the microplasma sources to materials processing including deposition, etching and surface treatment are being investigated. Modeling and diagnostics of the plasma discharges is also being done.

 

Recent Selected Publications

1) J. Asmussen, T. A. Grotjohn, T. Schuelke, M. Becker, M. Yaran, D. King, S. Wicklein, and D. K. Reinhard, “Multiple substrate microwave plasma-assisted chemical vapor deposition single crystal diamond synthesis,” Applied Physics Letters 93, 031502-1, 2008.

2) R. Ramamurti, M. Becker, E. Schuelke, T. A. Grotjohn, D. K. Reinhard, J. Asmussen,“Deposition of thick boron-doped homoepitaxial single crystal diamond by microwave plasma chemical vapor deposition,” Diamond and Related Materials 18, pp. 704-706, 2009.

3) D. T. Tran, C. Fansler, T. A. Grotjohn, D. K. Reinhard, and J. Asmussen, “Investigation of mask selectivities and diamond etching using microwave plasma-assisted etching,” Diamond and Related Materials 19, 778-782, 2010.

4) K.W. Hemawan, T.A. Grotjohn, D.K. Reinhard, and J.Asmussen, “High pressure microwave plasma assisted CVD synthesis of diamond”, Diamond and Related Materials 19, 1446-1452, 2010.

5) Y. J. Gu, J. Lu, T. Grotjohn, T. Schuelke and J. Asmussen, “Microwave plasma reactor design for high pressure and high power density diamond synthesis,” Diamond and Related Materials 24, 210-214, 2012.

6) T. Schuelke and T. A. Grotjohn, “Diamond polishing,” Diamond and Related Materials 32, 17-26, 2013.

 

Graduate Students

Ayan Bhattacharya, Shannon N. Demlow, Peiyao Liu, Suwanmonkha Nutthamon, Steve Zajac