Experimentally defining the safe and efficient, high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis

Diamond & Related Materials 37 (2013) 17–28

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Experimentally defining the safe and efficient, high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis J. Lu a, Y. Gu a, T.A. Grotjohn a, b, T. Schuelke b, J. Asmussen a, b,⁎ a b

Michigan State University, Department of Electrical & Computer Engineering, 2120 Engineering Building, East Lansing, MI 48824, United States Fraunhofer USA, Center for Coatings and Laser Applications, East Lansing, MI 48826, United States

a r t i c l e

i n f o

Article history: Received 5 December 2012 Received in revised form 17 April 2013 Accepted 18 April 2013 Available online 2 May 2013 Keywords: Microwave plasma assisted CVD Single crystal diamond synthesis Microwave CVD reactor design Diamond synthesis process window

a b s t r a c t The detailed experimental behavior of a microwave plasma assisted chemical vapor deposition (MPACVD) reactor operating within the high, 180–300 torr, pressure regime is presented. An experimental methodology is described that first defines the reactor operating field map and then enables, while operating at these high pressures, the determination of the efficient, safe and discharge stable diamond synthesis process window. Within this operating window discharge absorbed power densities of 300–1000 W/cm3 are achieved and high quality, single crystal diamond (SCD) synthesis rates of 20–75 μm/h are demonstrated. The influence of several input experimental variables including pressure, N2 concentration, CH4 percentage and substrate temperature on SCD deposition is explored. At a constant pressure of 240 torr, a high quality, high growth rate SCD synthesis window versus substrate temperature is experimentally identified between 1030 and 1250 °C. When the input nitrogen impurity level is reduced below 10 ppm in the gas phase the quality of the synthesized diamond is of type IIa or better. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Since Matsumoto's [1] first experimental demonstration of diamond synthesis by microwave plasma assisted chemical vapor deposition (MPACVD), new microwave plasma reactor designs have emerged and evolved. MPACVD diamond synthesis processes have been demonstrated within the 30–180 torr pressure regime [2–15]. While all these reactor systems have synthesized diamond there is still a need to further improve the MPACVD performance, especially as the MPACVD diamond synthesis technologies move into a manufacturing environment. Specific performance issues that require improvement are: (1) increased diamond growth rates, (2) improved diamond growth efficiencies, especially the electrical growth efficiency, while still (3) synthesizing high quality diamond (4) during long and stable synthesis process recipes. It has been known for some time that diamond synthesis rates can be increased by increasing the operating pressure and the discharge power density [3,6]. Thus moving the MPACVD diamond synthesis processes to a higher pressure regime, i.e. 180–300 torr, offers the benefit of higher growth rates with improved diamond quality. However the MPACVD synthesis of diamond in this regime requires the design of reactors that operate efficiently and robustly under high pressure and high power density conditions. It also requires the development of new process methods to enable long and stable process runs. ⁎ Corresponding author at: Michigan State University, Department of Electrical & Computer Engineering, 2120 Engineering Building, East Lansing, MI 48824, United States. E-mail address: [email protected] (J. Asmussen). 0925-9635/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.diamond.2013.04.007

MPACVD polycrystalline diamond (PCD) and single crystal diamond (SCD) synthesis above 180 torr has been reported by several groups [16–28]. These experiments employed several microwave reactor technologies: (1) Seki Technotron reactors [17–19], (2) LIMHP reactors [3,24,25], (3) conventional microwave cavity plasma reactors (MCPR) [16,20,21] and (4) millimeter wave plasma assisted CVD reactors [22,23]. These experiments have reported on the growth rates, the properties and the quality of the synthesized diamond. While the general experimental conditions employed for the individual experiments were reported, many experimental details of the behavior of the different reactors and the associated microwave discharges at high pressure have not been described. The behavior of the discharge, such as the shape, the position, even the definition of the volume of the discharge, etc. versus pressure and power requires improved experimental definition. Undesirable reactor wall heating when operating at these high pressures and power densities has been reported [3,25] suggesting that the reactors are operating with low microwave coupling efficiencies. The benefit of controlling the position of the discharge with respect to the substrate position also has been pointed out [26]. Even among similar reactor designs diamond synthesis efficiencies, such as kW-h/carat, have not been carefully investigated. Thus there is the need to experimentally measure and report a considerably more detailed description of the operation and performance of the various MPACVD reactors as they operate in the high pressure (180–300 Torr) and the high discharge power density regime (100–1000 W/cm 3). In particular there is a need to identify the efficient, safe and operationally robust operational conditions for the higher pressure (>180 torr) regime.

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J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

Recently, in order to investigate MPACVD diamond synthesis at high growth rates the conventional microwave cavity plasma reactor (MCPR) [5–7], identified here as Reactor A, was redesigned to enable operation at pressures above 180 torr [26,27]. Two new MCPR designs, Reactor B and Reactor C, were developed and then were experimentally evaluated at pressures of 180–300 torr [26–28]. Reactors B and C first demonstrated their performance by synthesizing polycrystalline diamond (PCD) [26,27]. Then their SCD synthesis performance was compared with each other and with the performance of Reactor A [28]. The here reported investigation is directed toward providing a more detailed experimental description of the MCPR reactors as they operate under high pressure and high power density conditions and thereby improve the experimental understanding of microwave plasma reactor behavior at pressures of 180–300 torr. A special focus of this investigation is on defining at the high operating pressures the high growth rate, the electrically efficient and operationally robust, long-term stable and safe operating conditions for the two new MCPR designs [26,28]. Specifically additional new operational procedures and associated detailed experimental behavior of the MCPRs are presented as they are operated in the higher pressure regime. First an experimental methodology is described that searches for and identifies the experimental operating conditions that enable the safe and efficient reactor operation without wall material process contamination. In particular, the experimental high pressure and high power density microwave discharge behavior versus pressure and input power is presented. This experimental behavior is revealed via measuring the substrate temperature variation versus input power and pressure. A series of photographs of the microwave discharge are recorded as the reactor operation is varied over the high pressure and high power density regime. The result is an experimentally determined reactor “operating field map” that describes the reactor behavior over a 60–300 torr pressure and 1.2–2.5 kW input power regime. The operating field map curves and the associated discharge photos relate the operating pressure and the input power to the substrate temperature, the absorbed power density, etc. Then, within this high pressure regime, a desirable experimental multivariable process parameter space, i.e. a safe, efficient and high quality SCD process window, is experimentally identified. Thus, a SCD diamond synthesis process regime is carved out of the vast multi-dimensional experimental input/output process variable space. Finally, as an example of the new reactors experimental performance, high quality SCD synthesis is demonstrated as the reactors operate within the desirable, high pressure, and high discharge power density variable space. 2. Background The diamond synthesis performance of MPACVD reactors is a complex function of many external input variables and internal reactor variables [29]. Among these the following MPACVD process parameters are important external input variables: (1) the power absorbed by the microwave discharge, Pabs, (2) the operating pressure, p, (3) the methane concentration, CH4/H2, (4) the total input gas flow rate, ft, (5) the deposition time, t, and (6) the numerous reactor geometry variables such as reactor size, microwave electromagnetic mode excitation, substrate position with respect to the discharge [26], etc. These external variables can be divided into two groups; (a) the independent input variables (1)–(5) and (b) the reactor design variables (6). Additionally there is a group of important internal reactor variables, such as (7) substrate temperature, Ts, (8) discharge gas temperature, Tg, (9) discharge volume, Vd, (10) discharge absorbed power density including the average discharge absorbed power density b Pabs > =Pabs/Vd and the spatially variable microwave absorbed power density D


 ! ⇀
E 1 
2 en0 r vm 2 ⇀  ⇀ ; Pabs r ¼ E r  mvm 2 ω 2 þ vm 2


 ⇀ ⇀ where r is the spatial position
 vector, E r is the magnitude of the ⇀ impressed electric field, n0 r is the electron density, ω is the radian microwave excitation frequency, m is the electron mass, e is the charge of an electron, and vm is the effective electron neutral collision frequency. There are also additional internal variables (11) such as the variation of various discharge radical species concentrations versus position within the reactor [30,31]. At the substrate surface CVD diamond synthesis is understood via knowledge of several important internal variables such as the H and CH3 species concentrations and the gas Tg and substrate Ts temperatures [32]. The important challenge is to design and operate MPACVD reactors, i.e. to vary the controllable input variables 1-6, so that the appropriate internal variable conditions 7-11 are efficiently and robustly created at the substrate surface. Clearly, optimal and efficient MPACVD diamond synthesis requires a multi-dimensional reactor design and experimental optimization process.

3. The microwave cavity plasma reactor and the associated microwave systems 3.1. Reactor design and operational principles The experiments described in Sections 5 and 6 employed two new MCPR designs. These designs have already been described in detail elsewhere [26–28,33], are similar to earlier MCPR designs [5–7,34] and incorporate several important design principles [35]. The design principles are: (1) single mode electromagnetic excitation, (2) internal applicator matching, (3) the placement of the substrate on a movable water-cooled stage, (4) the scalability of the design versus excitation frequency, and (5) the ability for flexible process control via several tunable reactor geometry parameters. As synthesis pressure is increased there is another important principle that must be imposed on the reactor design and experimental optimization process. It is associated with the fundamental behavior of microwave discharges as the pressure and the absorbed density are increased. In the higher pressure, higher absorbed density regime the discharge position, size and shape vary, i.e. the discharge constricts and becomes more intense, as input variables such as pressure and input power are varied. The discharge at times may become freely floating and assume shapes that are related to the shape of the impressed electromagnetic fields [36,37]. At very high pressures microwave discharges become very non-uniform, intense and “arc-like”. They may become unstable and even move about the discharge chamber as they react to the impressed electromagnetic fields and to buoyant and convective forces caused by the gas flows around and within the discharge. They are thermally inhomogeneous with peak neutral gas temperatures in excess of 3000 K. This behavior has been observed earlier in the development of other microwave plasma devices such as electrothermal thrusters [38,39] and adds to the complexity of optimizing the MPACVD synthesis process at high pressures. When operating at pressures exceeding 180 torr, the control of the discharge position, size and shape is absolutely necessary for robust high pressure MPACVD diamond synthesis. Since it was also desirable to eliminate or at least minimize reactor wall heating and plasma-wall interactions and to maximize substrate growth reactions, an additional principle was established for high pressure reactor operation. It is to control the hot discharge position so that it is not in direct contact with either the metal reactor walls or the quartz dome walls. Additionally in order to enhance the diamond synthesis rates it is also necessary to control and position the discharge so that it is in direct contact with the cooled substrate. This later condition insures that the high density discharge/substrate boundary layer, i.e. the important discharge region for CVD diamond synthesis [30–32], is always next to and in good contact with the substrate.

J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

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Table 1 The main differences between Reactor A, B and C. Applicator dimensions (cm)

Reactor A (conventional MCPR [5–7]) Reactor B (new design [26,27]) Reactor C (new design [28])

Cooling stage dimensions (cm)

R1

R2

R3

R4

8.9 8.9 15.2

7.0 7.0 10.2

4.1 1.9 1.9

5.1 3.2 3.2

Therefore at high pressures a sixth reactor design principle is established. It is to always locate the discharge away from the reactor walls and next to and in good contact with the substrate. This then also becomes an additional experimental operational principle

Substrate position (Zs)

Quartz dome dimensions (cm)

Fixed Variable Variable

Excitation mode

Radius

Height

13.0 13.0 21.6

9.5 9.5 10.9

TM013 Hybrid TM013 + TEM001 mode Hybrid TM0 + TEM001 mode

locating the reactor walls further from the discharge. Since most of the experimental measurements presented in this paper utilize Reactor B, this reactor is described in detail below. As is shown in Fig. 1, the electromagnetic excitation region consists of a cylindrical waveguide (z > 0) and a coaxial waveguide section (z b 0). The substrate is placed on a molybdenum substrate holder, which in turn is placed on top of and in good thermal contact with the water-cooled stage. In comparison to Reactor A [5–7], the new reactor designs have reduced substrate holder and inner conductor cooling stage radii, i.e. R4 and R3 respectively, and thereby have reduced substrate holder areas by about a factor of 4.5. The redesigns also incorporated variable positioning of the substrate, i.e. L1 and L2 are adjustable. This modification enables the focusing of the electromagnetic energy onto the top of the substrate holder. Consequently it increases the axial electric field intensity and the associated electromagnetic displacement current density at the substrate surface. The cylindrical/coaxial waveguide configuration, shown in Fig. 1, allows the reactor to be excited in a hybrid TM013 + TEM001 electromagnetic mode. In order to achieve the hybrid excitation the top (z > 0) cylindrical section of length Ls and the coaxial section (z b 0) of length L2 have to be adjusted to the proper lengths. Then

3.2. Specific MCPR reactor descriptions Compared to the earlier, conventional versions of the MCPR [5–7] (Reactor A) the new reactors (Reactors B and C) were designed to provide reliable and safe operation at higher discharge power densities and higher pressures [26–28]. A cross section of Reactor B is shown in Fig. 1 and the major differences between the three reactor designs are reviewed in Table 1. All of the reactors have a similar cylindrical, phisymmetrical geometry, but have different cavity applicator radii, R1, R2 (see Fig. 1), and substrate holder/cooling stage radii, R3 and R4. The substrate holder/cooling stage for Reactors B and C are identical and their radii have been reduced from the radii of Reactor A. The substrate position is fixed in Reactor A while it is variable in Reactors B and C. All of the reactors are excited with a TM electromagnetic mode. A major difference between Reactor B and C is that in the Reactor C design both the applicator and quartz dome size have been increased thereby

Z axis coaxial variable probe

sliding short

Lp

R1

quartz dome Ls

substrate

plasma discharge

cavity bottom Z

R4

Z=0

L1

L2 R2 quartz tube holder

R3

conducting short plate water-cooled stage

Fig. 1. Cross sectional schematic view of Reactor B. The position of the substrate with respect to the z = 0 plane is given by Zs = L1–L2.

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J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

the electromagnetic field focus at and above the substrate (around the z = 0 plane) can be controlled and varied by length tuning L1 and L2 [26,28]. When a discharge is present these length variations also change and control the location, the shape and the absorbed power density of the discharge [26]. In the experiments discussed in this paper, Ls and Lp were adjusted to excite and match the TM013 mode in the cylindrical section of the applicator; i.e. Ls ≅ 21.6 cm and Lp ≅ 3.6 cm, and the TEM001 mode is excited in the coaxial section of the cavity applicator. The length L2 was held constant at 5.86 cm while L1 was varied between 5.27 cm and 6.5 cm. Thus the top surface of the substrate holder, defined as the substrate position Zs = L1–L2, could be varied approximately between +6 mm to about −6 mm around the z = 0 plane. 3.3. The external microwave system and microwave coupling efficiency A typical external experimental microwave plasma reactor system is shown in Fig. 2. It consists of a microwave power supply, circulator and a matched dummy load, incident and reflected power measuring meters, and the matching system such as tuning stubs and a sliding short, and a microwave plasma loaded reactor. The coupling circuit includes a matching network, which often utilizes external tuning stubs, sliding shorts and waveguides, and the reactor region consisting of the microwave applicator and the discharge. Microwave power is matched at the input plane of the microwave coupling system, which consists of the plasma reactor plus the matching network. The input power delivered to the input plane is Pt. Pt is related to the experimentally measured incident power, Pinc, the reflected power, Pref, and the power absorbed by the microwave discharge Pabs, by the following relationship Pt = Pinc − Pref = Ploss + Pabs. The Ploss term represents the microwave power absorbed by the conducting walls and the dielectric materials that are located between the matching input plane and the discharge. There are two important microwave power losses present in the microwave system: (1) power unused and lost, i.e. reflected power, Pref, that is absorbed in the matched load attached to the circulator, and (2) power losses in the metal walls and in any lossy dielectric materials located to the right of the input matching plane shown in Fig. 2. In general, the power absorbed by the microwave discharge is given by Pabs = Pinc − Pref − Ploss. The overall microwave coupling efficiency to the discharge, Effcoup, is given by the following equation [5,36,39–41], Effcoup = 1 − (Ploss + Pref) / Pinc × 100%. In an optimally designed and operated microwave plasma processing system there is little if any reflected power and there also are low ohmic wall and dielectric losses within the microwave coupling network. Under these conditions the microwave coupling efficiency is very high. However in the typical MPACVD diamond synthesis system the matching network is often located external to the reactor and is many standing wavelengths from

the discharge. Thus in some MPACVD reactor systems there may be significant waveguide wall losses present, i.e. Ploss may be significant, and the coupling efficiency may be less than 80%. The MCPR designs minimize the coupling wall and dielectric losses, since the reactor is “internally matched/tuned”, and the surface areas of the coupling circuit metal walls between the matching input plane and the discharge are minimized. In all MCPR systems the microwave power supply, circulator, and power measurement systems are located very close to the input matching plane. An example of a MCPR reactor system is shown in Fig. 3. It can be operated in such a way that there is essentially very low reflected power (b 50 W) from the cavity applicator input plane [37,39–42]. Under all experimental conditions presented in this paper the MCPR is excited in an understood electromagnetic mode and is operated in a matched condition where little or no power is reflected from the z = Ls input plane (See Fig. 1). Then there are no standing electromagnetic waves in the coaxial and rectangular coupling waveguides external to the MCPR. Standing electromagnetic waves only exist in the short cylindrical cavity applicator sections, i.e. between z = Ls to z = −L2. This enables high coupling efficiencies. Earlier MCPR experimental investigations that excited and matched a well-established, plasma loaded, electromagnetic mode inside a MCPR demonstrated high coupling efficiencies (i.e. > 95% and usually > than 98%) under both low and high pressure discharge coupling conditions [5,36,39–42]. The experiments described in this paper were performed under similar matched reactor conditions and excessive reactor wall heating has not been observed. When the reactor is matched in this manner, i.e. internally matched, microwave coupling to the high pressure discharge is high. In our operating field map measurements and discharge power density calculations it is assumed that Ploss is usually b 2% of Pinc, and hence Ploss has been neglected in the data presented in Section 5, i.e. Pt ≅ Pabs for the absorbed power density calculations. More specifically all the experimental and power density data points displayed in Figs. 5–7 below were obtained under well matched conditions, i.e. Pref b 5% of Pinc, and thus for simplicity in the power density calculations Pref and Ploss have been assumed to be zero. 4. Experimental techniques The general details of reactor operation, such as discharge ignition and general discharge performance have already been explained elsewhere [6,7,26–29]. The experiments for determining the operating field maps for each reactor employed 25.4 mm diameter and 1.5 mm Incident Reflected Power Power Meter Meter

Input Plane

2.45 GHz Microwave Power Supply

Pinc Pt

Pref Tuning Stubs

Circulator

Sliding Short

2.45 GHz Microwave Power Supply

Microwave discharge

Matched Load Incident Power Meter

Reflected Power Meter

Microwave Applicator

Fig. 2. External microwave system employed for MPACVD diamond synthesis.

Circulator

Pref

Matched Load

Pinc

Input Plane

Microwave Applicator with Internal Tuning

Pt Microwave Discharge

Fig. 3. The external microwave system used with the MCPR. Note that reactor matching takes place at the input plane of the cavity applicator which is just one to one and a half wavelengths away from the discharge.

J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

thick n-type silicon wafers as substrates. Each wafer was located, as shown in Fig. 4(a), on a molybdenum substrate holder, which was placed on the cooling stage as shown in Fig. 1. Before each experimental run, each silicon wafer was nucleation seeded by mechanical polishing using natural diamond powder of size b0.25 μm. In addition to operating field map measurements, Reactors B and C were also used to evaluate single crystal diamond (SCD) synthesis at the high pressures. SCD homoepitaxial synthesis was performed using H2/CH4 input gas chemistries over commercially available, 3.5 × 3.5 mm 2, high pressure high temperature (HPHT) seed crystals. The SCD synthesis experiments employed the same cooling stage and same reactor configuration as shown in Fig. 1. The SCD diamond seed was placed in the pocket of the molybdenum holder as shown in Fig. 4(b) and the holder thicknesses E and F were adjusted to achieve the proper range of substrate synthesis temperatures. It was noted earlier [26] that the operating field map also varies with the reactor substrate position geometry variable Zs. Thus here the description of the experimental methodology process has been simplified by assuming an experimentally “optimized” substrate position Zs = − 5.7 mm, which guaranteed a stable plasma discharge position in close proximity and contact with the substrate surface. This position was held constant for all operating field map measurements. During each experimental measurement the reactor was well matched (Pref b 5% Pinc) by adjustment of Ls and Lp. The H2 and CH4 input gases had purity grades of N5.5 (99.9995%) and N5 (99.999%) respectively and unless specified no additional nitrogen was added to the gas input system. The input nitrogen gas impurities were estimated from the known impurities of nitrogen in the feed gases and the impurities from any small leaks in the vacuum system. For experiments performed without nitrogen addition, the input gas impurities were less than 10 ppm. All the SCD substrates were wet-chemically cleaned before each experiment. The pre-treatment for substrates also included a one-hour H2 plasma etching process at the same pressure as was used in the ensuing deposition process. After H2 etching, methane gas was introduced into the chamber to start the diamond deposition. For both the silicon wafer and the SCD synthesis experiments the substrate temperature, Ts, was monitored by using an optical emission pyrometer (IRCON Ultimax Infrared Thermometer), which measured one fixed wavelength (“one-color”) at 0.96 μm. The emissivities used were 0.1 and 0.6, for single crystal diamond deposition and for polycrystalline diamond deposition on silicon, respectively. Twocolor pyrometer temperature measurements were also taken. Under the same operating conditions the two-color measurements were approximately 100 K higher than the one-color measurements. All the substrate temperature measurements reported here are from onecolor temperature measurements. It is understood that due to the intense emission from the discharge the measurement of the absolute value of the substrate temperature by a pyrometer may not be accurate. However the measurements presented here are repeatable and thus serve from run to run as a relative experimental substrate temperature measurement.

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During the reactor operating field map measurements, a series of photographs were taken of the discharge hovering over the substrate. The photographs were taken through a screened window located on the cavity side wall just above the top of bell jar. A CANON EOS 20D camera was used. In order to take photographs with similar background and comparable fixed reference position, the camera was located at a fixed position on a tripod outside the reactor. For purposes of comparison between the photographs, the camera exposure setting was kept constant at a given pressure. However since the discharge intensity varied with pressure, the exposure time was adjusted between different pressures so that the photographed discharge appeared similar to that seen by the naked eye. The discharge volume, Vd, was determined from size calibrated photographs of the discharge. The absorbed microwave discharge power is given by Pabs = Pt and the discharge average absorbed power density was calculated from bPabs > =Pabs/Vd. It's recognized that this method to determine the discharge's volume is not precise, since it depends on photographs taken with different exposure settings and photographs are sensitive only to visible emission lines from the plasma. The photographs do not directly measure the exact volume of ionized plasma regime. However at this time it is the best method to estimate the plasma's volume. SCD film thicknesses were measured by weight gain and also by a Solartron DR600 linear encoder. The linear encoder data are reported since they yield a more precise linear growth rate. The grown diamond film thickness of one sample was determined as the average of 5 measurements points from the top surface of the diamond. One measurement point was in the center of the grown SCD and the other four points were measured near the corners. Output SCD quality was evaluated by Raman spectroscopy, IR–UV transmission measurements, and SIMS analysis. Raman spectroscopy was performed using a Jobin Yvon system. This Raman system was equipped with a 514.5 nm Ar ion laser with a spot size of 20–30 μm and resolution of 0.2 cm −1. A spectrometer slit width of 50 μm was chosen to achieve a relatively high signal to noise ratio and to sufficiently reduce the laser light that passes through the slit. The Raman peak full-width-at-half-maximum (FWHM) measurements were compared to the measured FWHM from two reference samples: (1) a type IIIa diamond reference sample and (2) a HPHT diamond substrate. The type IIIa sample was purchased from Element Six as an optical-low absorption plus low birefringence grade crystal. This crystal had a nitrogen impurity level of ~ 30 ppb and a Raman peak FWHM at 1332 cm −1 of 1.57 cm −1. 5. Experimentally determining the efficient and safe experimental reactor operating regime In order to experimentally determine the safe and efficient operating regime for a given MCPR design, the vast potential input variable operating space was explored and reduced to a subset of the input variable space. Using Reactor B, the experimental (methodology) evaluation process was initiated (1) with the reactor adjusted to the initial positions of L2 = 5.86 cm and L1 = 5.29 cm, (2) by applying an input power

Silicon wafer

(a)

E F

(b)

SCD substrate D E F

Fig. 4. Side cross-sectional view of substrate holders used for (a) polycrystalline diamond, and (b) single crystal diamond deposition. The substrate holder cross sections are shown in black and the substrate cross sections are shown in grey.

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Pabs b 2 kW, (3) by operating at a pressure of 10–30 torr and (4) with a H2 input gas flow rate of 400 sccm. The substrate position was held constant at Zs = −5.7 mm. These starting conditions were determined from experiments already described in [26] and ensured the excitation of a single TM013 + TEM001 hybrid mode, the matching of the reactor, and the breakdown and the formation of a discharge on top of the substrate holder. Once the discharge was created a methane gas flow of 12 sccm was added and the pressure was increased to a specific operating pressure condition. The operation of the reactor was then experimentally evaluated by varying the reactor input variables. These experimental procedures resulted in a set of operating field map curves. Examples of the operating field map curves are shown in Figs. 5–7 and are described in more detail below. When operating with a given fixed reactor design the substrate temperature is a function of both the pressure and the absorbed power. Thus given a fixed input gas mixture and flow rate the major independent input experimental variables, i.e. absorbed microwave power, Pabs, and pressure, p, have an experimentally repeatable, nonlinear relationship with the substrate temperature, Ts [29,43]. When explored over the vast input experimental space this relationship is not only nonlinear but is also characterized by operating discontinuities and hysteresis as the plasma size varies and moves around the discharge chamber. The experimental data presented in Figs. 5–7 and in the paragraphs below only describe experimental performance when the discharge remains in close contact and attached to the substrate. Two typical reactor experimental performance curves that display the variation of the substrate temperature Ts, versus absorbed power, Pabs, are shown in Fig. 5. The lower curve displays the substrate temperature, Ts, versus Pabs behavior for a constant operating pressure of 60 torr while the upper curve displays similar reactor operating behavior for a constant operating pressure of 240 torr. For each curve shown in Fig. 5 the maximum input power, Pabs, is that input power that produces a discharge that is slightly larger than the one inch diameter silicon wafer substrate. Thus the discharge covers the processing region but is still far from the reactor walls. Next to each curve are a series of photographs of the discharge versus absorbed power. These photographs were taken at a constant pressure, and display the discharge's

1.4 kW

1.8 kW

2.1 kW

visual appearance as the input power is varied. The power listed above each photo is the absorbed power, Pabs, that maintains the discharge. Several observations can be made from the data shown in Fig. 5. Each curve shows that as input power is increased from ~ 1.1 kW to 1.7–2.8 kW both discharge size and substrate temperature increase. The discharge intensity and color are also different at the two pressures. The discharge is purple at low pressures and it becomes more intense and white-green at high pressures. The substrate temperature increases faster with input power at higher pressure than at lower pressure, i.e. the slope ΔTs/ΔPabs is higher at high pressures than at low pressures. At constant absorbed power, for example ~ 1.7– 1.8 kW, the discharge at 240 torr, which is shown in second left picture at the top of Fig. 5, is much smaller than the discharge at 60 torr, shown in the far right picture at the bottom. Thus the discharge power density is much higher at high pressures than at low pressures. This behavior is consistent with the discharge behavior that has been observed earlier in other high pressure microwave discharges [6,7,26,27,36,39]. A complete family of experimental curves, where the behavior has been measured for many constant pressure conditions, has been identified as the “operating field map” for the reactor [7,29,43]. These experimental performance curves together with the discharge photographs define the experimental behavior of the reactor and describe the associated discharge appearance as the operating pressure and power are increased. Fig. 6 displays an example of such a family of curves for Reactor B when it is operating with the specific, fixed CH4/H2 = 3%, ft ~ 412 sccm input gas chemistry. Also plotted in Fig. 6 is the average discharge absorbed density versus input power for a number of constant pressure experimental runs. As expected the discharge absorbed power density increases as pressure increases. However, at each constant pressure operating condition the absorbed power density only slightly changes as the input power is increased (see the expanded power density insert in Fig. 6). Under these operating conditions as the input power increases the discharge volume also increases and thus the discharge absorbed power density remains approximately constant or decreases slightly as the input power is increased.

2.3 kW

2.7 kW

1200

Substrate Temperature (C)

1100 1000

240 torr 900 800 700

1.1 kW

600 500

1.2 kW

1.4 kW

1.5 kW

1.7 kW

60 torr

400 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

Absorbed Power (kW) Fig. 5. Examples of the operating field map curves along with the associated discharge photographs for 60 torr and 240 torr for Reactor B. Experimental conditions: ft = 412 sccm, and CH4/H2 = 3%, Zs = −5.7 mm. As shown in the photo inserts, the discharge is hovering over a 25.4 mm diameter silicon wafer.

J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

23

1000 180 Torr 120 Torr 100 Torr 80 Torr 60 Torr

Substrate Temperature (C)

800

900 800 700

700 600 500

600

400 500

300

Power Density (W/cm3)

900

200 400 100 300

0 0.5

1.0

1.5

2.0

2.5

Absorbed Power (kW) Fig. 6. Operating field map and the corresponding power density curves for Reactor B. Experimental conditions: ft = 412 sccm, and CH4/H2 = 3%, Zs = −5.7 mm.

If the discharge were to entirely fill the space between the substrate and the quartz walls (this data is not shown in Fig. 6) the discharge power density would then begin to increase versus any additional increases in input power. This is an undesirable operating condition because if the discharge touches the reactor walls the

diamond synthesis process itself is altered due to wall reactions, and as the reactor walls heat up they become an additional thin film deposition surface. At these higher input power levels the quartz walls can even erode thereby contaminating the synthesis process. Additionally, depending on the level of the input power, the discharge

1100 240 Torr 180 Torr

Substrate Temperature (C)

1000

120 Torr 100 Torr 80 Torr

900

60 Torr

800

Efficient and safe experimental processing regime

700

600

500 1.0

1.5

2.0

2.5

3.0

Absorbed Power (kW) Fig. 7. Operating field map curves and the identification of the efficient and safe experimental diamond synthesis regime for Reactor B. Experimental conditions: ft = 412 sccm, and CH4/H2 = 3%, Zs = −5.7 mm.

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may even attach itself to the quartz walls or to surfaces below the substrate in the coaxial section of the applicator. These are identified here as undesirable discharge conditions and are a function of pressure, substrate position and input power. The experimental data presented in Fig. 6 define the operating conditions that enable discharge stable, and wall reaction free discharge operating conditions. Operation where the discharge is stably attached to the substrate and also where the wall reactions are minimized is identified here as the efficient, safe and robust method of operation. All the undesirable experimental situations described above are avoided when operating the reactor within the operating field map curves shown in Fig. 6. Fig. 7 displays an expanded and more detailed version of the reactor operating field map based on the same data set shown in Fig. 6. Superimposed on the operating field map is an enclosed operating region defined by the green dashed-dot lines. The left green dashed-dot line represents the minimum power required to form a 25.4 mm diameter hemispherical discharge over a 25.4 mm diameter silicon substrate. Input powers of less than indicated by this line will not produce uniform deposition over 25.4 mm diameters. The right green dashed-dot line represents the input power at which the plasma expands beyond 25.4 mm diameter, and if power is increased further beyond this limit the reactor then is operating in an inefficient condition. If the input power is increased further the discharge may touch the quartz dome. Large input powers result in inefficient operation and will cause dome heating and may possibly cause other undesirable discharge and dome wall interactions. Thus the operating space enclosed between the left and right dashed-dot lines represents the allowable operating input power levels that yield useful diamond deposition over a 25.4 mm silicon substrate. Since the reactor is also operating in a matched condition this enclosed region represents the efficient as well as the safe SCD plasma processing region for the reactor. Once the safe, well matched, and efficient operating region has been established diamond synthesis can be performed reliably, robustly and safely versus time. All of the experiments reported for SCD synthesis below were performed within this experimental multivariable input volume approximately along and around the thick green dashed line shown in Fig. 7. It is also useful to note that in the SCD synthesis experiments described below the molybdenum holder dimensions E and F (see Fig. 4b) were changed to vary the substrate temperature at a constant pressure. This change in substrate holder configuration shifted the operating roadmap curves either up or down versus Ts. However the safe and efficient operating regions remain approximately the same as indicated in Figs. 6 and 7. 6. Experimental evaluation of single crystal diamond synthesis 6.1. Diamond growth with nitrogen addition It is widely understood that the addition of a small amount of nitrogen will increase the diamond growth rate [13,44]. Thus a set of 8 h experiments were performed in Reactor B that investigated the relationship between the diamond growth rate versus increasing nitrogen concentration in the gas phase. Zs was held constant at approximately − 5.7 mm. The experimental results are displayed in Fig. 8(a) and as expected at high pressures (i.e. ~ 240 torr) the growth rate increases from about 8 μm/h to over 45 μm/h as the nitrogen content in the gas phase is increased from about 10 ppm to 200 ppm. The quality of the synthesized SCD versus nitrogen addition was also determined from Raman FWHM and SIMS measurements. The nitrogen content in the synthesized diamond as measured by SIMS and the Raman FWHM are plotted in Fig. 8(a) and (b). As nitrogen is added to the feed gas the diamond growth rate increases, the nitrogen incorporation in the diamond increases, and the diamond quality decreases. The SIMS analysis of the synthesized SCD indicates that there is less than 300 ppb (below the detection limit of the SIMS

measurements) of both nitrogen and silicon in the synthesized diamond for nitrogen gas phase concentrations of b 10 ppm. These experiments demonstrated the sensitivity at high pressures of the diamond growth rate to nitrogen concentration in the gas phase. Since a goal of the experiments was to synthesize high quality diamond all the remaining experiments presented below, with the exception of those described in Fig. 14, were performed without nitrogen addition in the gas phase. 6.2. Growth rate vs. multivariable space CVD diamond deposition is a multivariable problem. Thus impurity nitrogen concentrations in gas phase were, as described above, experimentally reduced to the very low input level of less than 10 ppm. However, there are still many other important experimental variables, such as reactor geometry, substrate position, pressure, substrate temperature, methane concentration, etc. that influence the diamond synthesis process. An example of the experimentally measured overall growth performance versus, Ts, p, and percent methane for Reactors A, B, and C is shown in Fig. 9. Growth rates of 55–75 μm/h are possible when operating Reactor C in the 240–300 torr pressure regime. It is useful to note, as others have [45], that at these high operating pressures any increases in the methane concentration beyond ~ 10% often results in the formation of soot within the discharge and in the gas surrounding the hot discharge. This phenomenon, which also occurs in ultrananocrystalline diamond synthesis [46], is poorly understood and should be investigated further. As is shown in Fig. 9 the growth rates are scattered in the multidimensional space and the relationship between the diamond growth rate and these variables is complex. Thus in order to explore each variable’s role in the SCD deposition process it is necessary to simplify the problem and cut cross sections through this multi-variable space along one constant cross sectional variable plane at a time. Here Fig. 10 specifically displays examples of the growth rate versus pressure for Reactors B and C. All data displayed in Fig. 10 for Reactor B were collected with a constant Zs = − 4.5 cm and for a deposition time of 8 h. The reactor operating conditions were located within the efficient and safe experimental operating regime that is shown in Fig. 7 and were also performed within a narrow substrate temperature range of 1050–1080 °C. Reactor C data were taken within its safe and efficient operating regime with a substrate temperature of approximately 1000 °C. As pressure increases, the SCD growth rate also increases. For example, using 5% methane, as pressure increases from 180 torr to 320 torr, the deposition rate increases 4 times. This trend is true for all methane concentrations. It is well known from lower pressure experiments [6,29] that the growth rate is dependent on the substrate temperature. Thus a series of experiments of SCD growth rate versus substrate temperature were performed on Reactor B and the results are displayed in Figs. 11 and 12. The pressure is held constant at 240 torr and the input chemistry is varied from CH4/H2 = 4% to 7%. The variation of substrate temperature shown in Figs. 11 and 12 was achieved either by (1) adjusting the substrate dimensions E and F before each run and/or (2) by slightly varying the input power during the run. During each 8 h deposition experiment the absorbed power was also slightly adjusted in order to maintain an approximately constant substrate temperature versus time, and Zs was also varied slightly from run to run from −4.5 to −5.7 mm. As is shown in Fig. 11 the growth rate increases as methane content is increased and at each constant methane concentration there is a growth rate maximum versus temperature. The data clearly show that there is a high growth rate window between 950 and 1300 °C. An example of the diamond quality within the high diamond growth rate window is presented in Fig. 12. The quality and associated growth rate of the synthesized diamond is plotted versus substrate temperature for a series of experiments where the methane concentration was held constant at 6%, the pressure was held constant at

J. Lu et al. / Diamond & Related Materials 37 (2013) 17–28

5

40

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(b) 50

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(a)

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Fig. 8. (a) Growth rate and nitrogen content in crystal vs. total nitrogen concentration in the gas phase (240 torr, CH4/H2 = 5%, Zs = −5.7 mm), (b) FWHM and nitrogen content in crystal vs. nitrogen concentration in the gas phase for same samples.

240 torr and the deposition time was 8 h. The synthesized SCD was characterized by micro-Raman and SIMS measurements. The black square data points represent the growth rate for each deposition experiment and each of the red circle data points show the corresponding Raman FWHM of the diamond sample. There are two horizontal dashed lines in Fig. 12, which display Raman FWHM data from two diamond reference samples. The lower dashed line represents a type IIIa SCD sample from Element Six with a FWHM of 1.57 cm −1 and (2) the upper dashed line represents a typical type Ib HPHT seed containing nitrogen impurity with FWHM of 1.88 cm −1. Compared with the two reference SCD samples, an excellent SCD quality and high growth rate window was observed between 1030 and 1250 °C. This is displayed in Fig. 12 as the region between the two green dash-dot vertical dashed lines. Within this window the synthesized SCD has very good quality and has a high growth rate as well. Additionally, SIMS measurements of the synthesized diamond show

that all diamond samples in this figure are below the detection limit of 300 ppb for N and Si in the SCD. 6.3. Diamond synthesis efficiency The diamond synthesis energy efficiency of Reactors B and C can be estimated by dividing the synthesized diamond volume per unit time by the absorbed power, Pabs [10]. This figure of merit is then expressed in mm 3/kW-h. Both reactors are capable of synthesizing diamond over a 25 mm diameter area. With a Pabs of around 2 kW the observed growth rates (see Figs. 9 and 10) in the 200–320 torr pressure regime vary from 20 to 75 μm/h resulting in energy synthesis efficiencies of 6-25 mm 3/kW-h. These efficiencies are excellent considering that they are calculated for experimental conditions that produce high quality diamond with little or no input nitrogen (b10 ppm) in the gas phase. These efficiencies are comparable to

Fig. 9. Growth rate in multi-variable space for Reactors A, B and C. The diameter of and the number on each data point indicates the methane concentration.

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Growth Rate by Linear Encoder (µm/hr)

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Pressure (torr) Fig. 10. Growth rate vs. pressure for different methane concentration for Reactor B (Zs = −4.5 mm, Ts ≈ 1050–1080 °C), and Reactor C (Zs = −4.8 mm, Ts ≈ 1000 °C).

the efficiencies that were calculated with 100 ppm or more nitrogen in the gas phase [10]. 6.4. Characterization of diamond plates Since the quality of synthesized SCD samples was excellent additional IR-UV transmission measurements were performed. This measurement technique required the fabrication of diamond plates. The synthesis process for these diamond plates was as follows. The diamond plates were synthesized via a multistep process which was carried out within the “efficient, safe and excellent diamond synthesis window” as had been identified using the methodology described above using either Reactor A, B or C. During the synthesis process the substrate temperature was held approximately constant by controlling the reactor adjustable input variables such as input power, pressure, substrate position, etc. SCD diamond plates were created by laser cutting the MPACVD diamond from the diamond seed and then mechanically polishing the final plate. The resulting plates had a thickness of

Growth Rate by Linear Encoder (µm/hr)

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2-3 mm and depending on the initial seed, the substrate size had a cross sectional area of 3.5 × 3.5 mm2 to 7 × 7 mm2. Examples of these diamond plates are shown in Fig. 13. Note that the edges of the plates were also laser trimmed and polished. The UV to IR transmission measurements were performed on several SCD sample plates that were synthesized in Reactors A, B and C and with different concentrations of nitrogen addition. Examples of the results are shown in Fig. 14. Sample 1 was synthesized in Reactor A at 160 torr, 5% methane, and with 11 ppm of nitrogen addition, while samples 2 and 3 were synthesized in Reactor C at 240 torr, 5% methane and with 5 ppm and zero nitrogen addition respectively and sample 4 was synthesized in Reactor B with no nitrogen addition. The IR transmission spectra for all samples (not shown in Fig. 14) were similar to those observed for type IIa diamond. However as is shown in Fig. 14 the sub-band gap ultraviolet absorption coefficients are different for the four samples. The absorption coefficient increases with the addition of nitrogen in the gas phase. The absorption coefficients at 250 nm for the diamond plates synthesized in Reactor C and B are between 4 and 7 cm −1. Thus the diamond plates have transmission spectra that are similar to that of type IIa diamond [47]. 7. Summary In order to achieve safe, efficient and stable operation in the higher pressure regime an additional reactor design and operational principle was established. This principle is to design and operate the reactor so that the discharge is always located away from the reactor walls and is next to and in good contact with the substrate. This principle was incorporated into the design and operation of Reactors B and C. The implementation of this operational principle controlled the position of the discharge, restricted the discharge location away from the reactor walls, enabled good discharge contact with the substrate, and controlled the size of the discharge. When operating under these conditions, discharge power density is increased by increasing operating pressure and reactor wall reactions are minimized. These reactor design and operational principles enable the safe, efficient and low maintenance operation of the reactor over a wide range of operating conditions. An experimental methodology was presented that determined for each reactor the efficient and safe operating diamond synthesis regime. This methodology first defines the nonlinear relationships between the input power, discharge average power density, pressure and substrate temperature; i.e. it establishes an operating field map at high pressure and power densities for each reactor. Then the safe and efficient reactor operating variable space over the 180–300 torr pressure regime is defined within this operating field map. At each operating pressure the operating field map: (1) restricts and defines the upper and lower input power limits of the reactor, (2) determines the discharge absorbed power density behavior versus input power and given a particular substrate holder configuration, (3) establishes the relationship between substrate temperature and absorbed power density versus substrate holder position, Zs. The experimental discharge and operating field map data are also invaluable experimental data that provide an improved understanding of the reactor performance at high pressure. The operating field map measurements along with the associated discharge photographs are also expected to be useful benchmark experimental data for evaluating numerical reactor models. For each reactor the operating field map and the safe and efficient operating regime was specifically defined for SCD synthesis. While operating within a safe and efficient reactor operating window, SCD synthesis was demonstrated from 160 to 300 torr. At a constant pressure of 240 torr a high quality, high growth rate SCD substrate temperature window was experimentally identified between 1030 and 1250 °C. In particular using feed gases with nitrogen impurity levels of less than 10 ppm SCD was synthesized with growth rates

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Temperature(C) Fig. 12. Growth rate and Raman FWHM vs. substrate temperature. No extra nitrogen addition. (Experimental conditions: Pressure = 240 torr, CH4/H2 = 6%, Zs = −5.1 to −5.7 mm.)

Prime novelty statement The MPACVD synthesis of diamond is explored in the 180 - 300 torr pressure regime. An experimental methodology is described that defines the safe and efficient experimental operational regime. Single crystal diamond synthesis is demonstrated when reactors are operated within this regime. In particular: 1. The operating field map for microwave plasma reactors in the high pressure regime is experimentally determined. 2. The influence of several input experimental variables on microwave plasma assisted CVD single crystal diamond synthesis is explored, and 3. A high quality high growth rate single crystal diamond synthesis window between 1030 and 1250 ˚C is identified.

25

Absorption coefficient (cm-1)

of 20–75 μm/h, and synthesis efficiencies of 6–25 mm 3/kW-h or 9.5– 2.3 kW-h/carat were achieved over a one-inch diameter deposition surface. The SCD synthesis experiments demonstrated that as the pressure and discharge absorbed power density are increased the diamond deposition rate increases. Diamond synthesis rates and quality surpass those that were achieved by synthesizing SCD at lower pressure and with earlier reactor technologies, i.e. Reactor A operating at 160 torr. As pressure is increased the experimental variable window to grow high quality diamond also expands and larger methane concentrations (5-9%) can synthesize high quality diamond. When nitrogen impurity levels are reduced below 10 ppm in the gas phase the quality of the synthesized diamond is of type IIa or better. After laser cutting and polishing high quality diamond blocks and plates were synthesized. These experiments support the hypothesis that MPACVD diamond synthesis rates and diamond quality increase and improve respectively as the operating pressure increases. Two new reactor designs have demonstrated robust SCD deposition at high pressures and power densities. By a combination of process control via reactor input adjustment and improved reactor design, SCD has been synthesized robustly both in the short term (one to many days) by producing a very stable discharge in good contact with the substrate and in the long term (one to several years) over many hundreds of separate experiments with little reactor maintenance. For example Reactor C has been operated for over 4000 h of deposition time using just two quartz domes which were alternated from run to run or were subjected to plasma cleaning after each deposition run.

20

sample 1 (Reactor A - 11 ppm N2) sample 2 (Reactor C - 5 ppm N2) sample 3 (Reactor C - no N2) sample 4 (Reactor B - no N2)

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Fig. 13. Examples of polished SCD samples.

Fig. 14. Transmission measurement results. Growth conditions: 160 torr, CH4/H2 = 5% (Reactor A); 240 torr, CH4/H2 = 6% (Reactor B); 240 Torr, CH4/H2 = 5% (Reactor C).

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