Boron doped diamond deposited by microwave plasma-assisted CVD at low and high pressures

https://doi.org/10.1016/j.diamond.2007.08.042Get rights and content

Abstract

Boron doped diamond is deposited over a range of pressures and chemistries including pressures from 35–120 Torr and gas chemistries including hydrogen–methane–diborane and argon–methane–hydrogen–diborane mixtures. The diamond deposition system is a 2.45 GHz microwave resonant cavity system. Diborane (B2H6) gas chemistry has been utilized with flow rates of 2.5–100 ppm. At low pressures of 35 Torr polycrystalline films are deposited using a feed gas mixture of hydrogen and 0.5% methane. At moderate pressures of 95 Torr, diamond films are grown using 60% Ar, 39% H2 and 1% CH4. For the high pressure experiments of 120 Torr, polycrystalline films are deposited using 98% H2 and 2% CH4. The deposition rate ranges from 0.3 to 1.6 μm/h. This investigation describes the relationship of the diborane flow rate and pressure versus the resulting film morphology, electrical properties, and morphology of the deposited films. The deposition of boron-doped polycrystalline diamond is done on 5 cm diameter silicon and silicon dioxide coated substrates. The resistivity spatial variation across the wafer was ± 5% indicating a good uniformity.

Introduction

Diamond has unique semiconductor properties like wide bandgap, high breakdown voltage, and high electron and hole mobilities, which makes it a suitable candidate for applications in high temperature electronics and MEMS-based devices. Because of the potential applications in high frequency, high power electronics, the growth and characterization of p-type semiconducting diamond is of considerable interest. It is also used as material for temperature and pressure sensors, and as an electrochemical electrode. Impurity doping of diamond can be achieved by introducing boron in the gas phase during the diamond deposition process. Boron doping affects the morphology, structure and electrical properties of the diamond films. It is known that high boron-doping levels can result in the formation of electronic defects in the coating [1], [2]. Consequently, highly controlled and efficient boron-doping is important.

This study investigates the variation in electrical conductivity and boron content of diamond films resulting from varying the boron concentration in the deposition plasma gas phase at different pressure regimes. The current work focuses on the deposition of polycrystalline boron-doped diamond from low and high pressure (35–120 Torr) with variation of the boron content in the feedgas. Specifically, the morphology, growth rates and electrical properties will be studied with varying boron contents in the film and at different deposition pressure regimes. The uniformity in the coating morphology and resistivity across 5 cm diameter substrates will be studied as well. This study particularly focuses obtaining high electrical conductivity in the diamond films for electrochemical electrode applications.

Section snippets

Experimental details

Diamond films are deposited using a microwave plasma enhanced chemical vapor deposition system. The diamond deposition system is a 2.45 GHz microwave resonant cavity system [3]. Boron doped polycrystalline diamond is deposited with various crystal sizes and electrical properties by adjustments in the feedgas and deposition pressure. The substrates used are 0.5 mm thick silicon (Si) and 1 μm silicon dioxide (SiO2) on 0.5 mm Si substrates. They are pre-cleaned ultrasonically in acetone, ethanol,

Results and discussions

In the initial set of experiments, the chamber pressure is maintained at 35 Torr, microwave power is 1.6 kW, gases used in the plasma are 99.5% H2, 0.5% CH4, and 2.5–100 ppm B2H6. The deposition times varies from 12–15 h. The plan-view SEM images in Fig. 1 show that the grain size of the B-doped diamond films decreases with increasing boron content in the diamond films. Grain size was measured using the SEM images. An image area of 2 μm × 2 μm is chosen and several lines are drawn across the

Conclusions

Boron-doped diamond was deposited on Si and SiO2 substrates at 35 Torr and 120 Torr in a H2/CH4 plasma and at 95 Torr in a Ar/H2/CH4 plasma. The diborane doping levels in the plasma gas flow were in the range of 2.5–100 ppm. With these conditions, the deposition plasma coated uniformly across 5 cm diameter substrates. The grain size and growth rate of the coatings decreased with increasing B2H6 levels in the gas phase. The minimum amount of diborane needed to obtain high (metallic-like)

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