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Design of a UHV reactor for microwave plasma deposition of diamond films
Vacuum/volume45/number 5/pages 499 to 506/1994 Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
0042-207X/9457.00+ .00
Design of a U H V reactor for m i c r o w a v e plasma deposition of d i a m o n d films M G Jubber
and J I B Wilson,
Department of Physics, Heriot-Watt University, Riccarton,
Edinburgh EH14 4AS, UK and I C Drummond,
P J o h n a n d D K M i l n e , Department of Chemistry, Heriot-Watt University, Riccarton,
Edinburgh EH14 4AS, UK
A UHV compatible deposition system has been constructed for the growth of diamond films by 2.45 GHz microwave plasma chemical vapour deposition (CVD). The system comprises a Ioadlock and a spherical deposition chamber where the heated 100 mm diameter substrate is exposed to a reactive plasma environment. The design provides ports for in-situ monitoring by ellipsometry, optical emission spectrometry, mass spectrometry, and laser reflectometry and allows for the later addition of analysis chambers such as XPS. Computer control is provided for all major components and operations, including pumps, valves, gas flows, pressure and temperature adjustment. The system has four pumping groups, two for the main growth chamber providing base vacuum and for pumping the process gases, one for evacuating the Ioadlock and the fourth for the mass spectrometer. Microwaves enter the chamber via an antenna-based microwave applicator with a water-cooled quartz window. A key feature of this design is the ability to have a free standing ball plasma which touches neither the chamber walls nor the substrate.
I. Introduction In recent years the growth of diamond films at low pressures and temperatures has received increasing attention. Such films have a wide range of potential industrial applications based on diamond's unique combination of properties ~. A wide variety of growth techniques have been used to deposit diamond, including microwave CVD systems 2, oxyacetylene torches 3, filament systems 4 and plasma torches 5. The oxyacetylene torch methods have been shown to produce reasonable quality diamond at high growth rates 6, but suffer from small sample sizes, high surface temperatures and poor uniformity. A further problem with such systems is contamination of the film with nozzle material 7. Of the CVD systems, the two most commonly used techniques are the hot filament method and the microwave plasma CVD method. The hot filament method has been used to produce complex 3D shapes such as diamond tubes 8 and also to grow doped epitaxial films for use as Schottky diodes or prototype thyristors 9. However, in common with the torch methods, it suffers from inhomogeneous growth over large areas and for metal filaments invariably produces deposits contaminated with the filament material ~0. In terms of deposit purity, the microwave plasma CVD method is superior, since it can produce large area ( ~ 10 cm diameter) contaminant free films ~i. As the name suggests, the process involves the formation of a plasma by microwave excitation of a reactant gas mix resulting in growth upon a heated substrate. Typical conditions for growth are : (i) a pressure of 10-100 mbar; (ii) a substrate temperature of 500-900°C ;
(iii) a gas mix of 0.1-2.0% carbon-containing gas in hydrogen. At present, the exact process by which diamond growth occurs, and the role of the various process parameters, in such systems is still a matter of some controversy ~2. There is, therefore, a need for a deposition system having independent adjustment of deposition parameters, to produce ultra-high purity, large area, single-crystal or polycrystalline diamond films for optics and electronics.
2. Typical microwave growth systems At the time of the design of the Heriot-Watt system there were two main types of reactor used for the microwave plasma CVD growth of diamond : (i) the simple tube reactor ~3 and (ii) the bell jar reactor t4. Such systems, however, have inherent limitations which restrict their use either in industrial applications or in attempting to understand the role of the various parameters. The fact that the substrate sits within the plasma results in a high surface temperature and makes it impossible to decouple the effect of the plasma and substrate heating. Furthermore, it is often difficult to characterize the plasma and at the same time probe the substrate surface since the reactor design does not allow independent, unobstructed views of both the substrate and the plasma. The plasma also etches material from the reactor walls which contaminates the film tS. In addition, many reactors have graphite heaters or sample holders which are exposed to the reactive plasma species. This often results in extra material being etched from the graphite and being deposited as diamond, giving spuriously high growth rates when based only on the carbon balance in the reactant gas. Further limitations include small 499
M G Jubber et al: Design of a UHV reactor
sample size, poor base vacuum and the necessity of exposing the growth chamber to air in order to load a sample. The last two limitations are a problem since nitrogen is an n-type dopant for diamond, and oxygen and water vapour have been shown to influence the growth even in small quantities ~6. The small sample size used in some reactors can produce misleading results, since it does not reveal the different morphologies, range of crystallite sizes, and variation in growth rate which can be obtained when the substrate is larger than the diameter of the plasma.
additional monitoring or analysis equipment at a later date. The typical growth conditions and the slow growth rate (<~ 10 /~m h ~ L) reported to date call for requirements 8, 9 and 10, whilst requirements 10 and 11 allow for the possibility for upgrading to a commercial scale. Design requirement 10 prompted us to make the system computer-controlled. Naturally, the system should be easy to operate, and should incorporate all of the safety features for handling hydrogen.
4. Heriot-Watt growth system 3. Heriot-Watt design requirements The design of commercial diamond deposition reactors is often inflexible, whereas research reactors must allow for independent control of many parameters over a wide range. In addition, facility must be provided for the addition of monitoring and analysis equipment. Based on the above considerations and the range of growth conditions reported in the literature, we decided on the following design requirements : 1. Separable plasma and substrate. 2. In-situ monitoring and analysis capability.
3. 4. 5. 6. 7. 8. 9. 10. 11.
Separate growth chamber and sample loading chamber. Plasma not in contact with chamber walls. Base pressure ~<10- s mbar. Independent control of deposition parameters. Ability to add further chambers. Operating pressures from 1-100 mbar. Substrate temperatures up to 1000':C. Continuous unattended running. Capacity for I00 mm diameter samples.
Requirements 1 and 2 allow independent analysis of both the plasma by techniques such as optical emission spectroscopy and the growing surface via Raman spectroscopy, ellipsometry, and FT-IR, whilst requirements 3, 4 and 5 prevent contamination of the films by material from the chamber walls or the atmosphere. Point 6 allows us to independently investigate the effect of each of the process variables and point 7 gives us the option of adding
The deposition system comprises two main sections : the growth chamber and a loadlock for sample entry/removal. Since nitrogen is an impurity in diamond, the role of the loadlock is to prevent atmospheric contamination of the growth chamber. A schematic diagram of the entire deposition system is shown in Figure 1.
5. Growth chamber A 380 mm (15 in.) diameter stainless steel sphere (Leisk Engineering/VSW) forms the basic growth chamber. It has 16 ports : two 150 mm internal diameter Kodial glass windows for viewing, two 63 mm diameter windows at 70° to the vertical for in-situ ellipsometry, another two 63 mm diameter windows at 70° to the vertical for laser reflectance thickness monitoring, two 35 mm ports at 45 '~ for gas inlet lines, one 200 mm diameter port for the platen and heater, one 200 mm diameter port for the microwave applicator, four 35 mm diameter ports for vacuum gauges, one 150 mm diameter pumping port and one 150 mm diameter port with a gate valve, providing access to the loadlock. The deposition chamber behaves as a stirred-flow reactor and has a total volume of approximately 40 1. All flanges on the chamber are Conflat type UHV flanges, fitted with copper gaskets. There are two states in which the growth chamber can operate : during sample transfer and between experiments it operates in a base vacuum state of < 10 -s mbar, during deposition various gases flow through the chamber at pressures in the 1 100 mbar range.
KEY loadlock pumping system transfer a r m
°© @ Q
Figure l. The Heriot-Watt designed UHV reactor deposition system. 50O
spherical multiport growth chamber heated platen throttle valves roots pumping system diffusionpump system microwaveair cavity dummyload lkW microwave source pneumaticgate valves bypass line
M G Jubber et al: Design of a UHV reactor
5.1. Base vacuum state. To attain the base vacuum, the chamber is equipped with a liquid nitrogen trapped Edwards E04 diffusion pump (with a pumping speed of 6001 s- ~), backed by an Edwards EDM12A rotary pump (with a pumping speed of 14.6 m 3 h - 1). The diffusion pump is isolated from the chamber by a computercontrolled pneumatically operated gate valve. All pumps in the system are fitted with nitrogen purge to their ballast valves and/or exhaust lines to dilute any potentially explosive gases to below their explosion limits before they come into contact with air. A Leisk UHV trigger Penning gauge monitors the chamber in the 10 4--10 tt mbar range, with an Edwards PRH10 Pirani gauge monitoring the diffusion pump backing pressure.
MICROWAVES
WATER IN --'-~ I ~
_ WATER ---~OUT
ifoooooooooooooool :0%%%0000°;°0
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5.2. Process state. Prior to deposition, the diffusion pump is isolated and the process gases are then pumped through a nitrogen purged Edwards EH250 Roots Blower and Edwards E2M40 rotary pump set (combined pump speed of 240 m 3 h - l). The nitrogen flow is set to reduce the maximum possible hydrogen flow from the exhaust to below the 4% explosion limit in air. Typical operating pressures within the chamber are in the range 10-100 mbar. The chamber pressure is measured by two MKS Baratron Type 221A gauges which cover the lYJ0 and 0--1000 mbar ranges. 6. Substrate heater Substrates are heated on a molybdenum support on top of the platen. The heating element is a serpentine patterned graphite resistor powered by an 8 kW low voltage power supply. The heater is fully encapsulated in stainless steel and molybdenum which provides vacuum integrity between the heater and the chamber and ensures that the carbon element is not in contact with either the plasma or the gases in the chamber. The inside of the heater can be argon purged or evacuated to prevent oxidation and corrosion of the heated regions, Water cooling is provided to the two electrodes, the bottom flange and to the stainless steel casing of the platen. Control of the heater is achieved by setting the percentage of transformer output power. This can be set to a particular value, ramped from one value to another or cycled between two values. A thermocouple located within the black body section of the heater measures the temperature, however, it should be noted that this does not represent the substrate surface temperature. Using an Ircon model W-15C20 pyrometer operating in the 0.9-1.08 /~m range, the surface temperature across the silicon substrate surface was measured, without the plasma on, along with the thermocouple reading as a function of heater input power. Over the range of useful deposition temperatures (500-800°C surface temperature) the two were found to be linearly proportional artd hence the surface temperature could be calculated from the expression : Ts = 0.569Tt + 120°C, where 7", is the surface temperature and T~ is the thermocouple reading. In addition, the pyrometer showed that the surface temperature uniformity was good to within 5% over the 4 in. diameter substrate. By replacing a stainless steel section of the platen mount with a ceramic break, it is possible to apply a dc bias to the substrate. "1. Microwave system
The microwave system comprises a 1 kW magnetron producing 2.45 GHz radiation from an RFA switch mode power supply, a
SUBSTRATE
Figure 2. The Heriot-Watt microwave applicator design.
circulator, a dummy load, motor driven tuning stubs and an antenna which feeds the microwaves from the rectangular waveguide into a circular microwave applicator. The design and construction of the applicator was carried out at Heriot-Watt University. The applicator, as shown in Figure 2, is fitted with a water-cooled quartz window which is sealed with Viton O-rings to maintain the vacuum integrity between the reaction chamber and the rest of the microwave system. A perforated cylindrical stainless steel extension is fitted below the window, extending down towards the substrate. This encourages a stable plasma ball to form just above the substrate surface, but still allows for optical monitoring of the plasma. It is also possible to obtain a stable plasma ball in the centre of the chamber even with the heater and platen completely removed and the lower port blanked off. For safety reasons, the microwave waveguides are flushed with nitrogen so that, in the event of failure of the quartz window, nitrogen would flood into the hydrogen filled chamber rather than air, and the windows are screened with mesh to prevent microwave leakage. 8. Loadlock Samples are introduced into the reaction chamber via a loadlock. An extended six-way cross forms the main section of the loadlock, as shown in Figure 1. It has four 150 mm internal diameter ports, one supporting an x - y translation stage with a linear motion transfer arm and one providing access to the main chamber. The other two are currently blanked offallowing for future expansion. There are two 200 mm diameter flanges, the top one provides sample access and has a Viton rubber seal, the bottom one is fitted with a bellows-sealed linear motion system for moving samples up and down. In addition, the six-way cross is also fitted with two 63 mm diameter viewports to ease sample loading and transfer. The top flange is fitted with a 63 mm diameter Viton Oring sealed fused quartz window which transmits down to 160 nm. This allows in-situ cold bakeout of the substrates using a Wotan XBO150W/4 xenon lamp with a Suprasil envelope emitting down to 160 nm. One pumping line leads from the loadlock to a smaller six-way cross. This cross is fitted with four 35 mm diameter ports to which two gauges and an inlet for the argon used to vent the chamber are connected. The loadlock is pumped to a base pressure of ~ 10 .6 mbar by a Balzers TPU050 turbo501
M G Jubber et al: Design of a UHV reactor
molecular pump backed by an Edwards E2M2 rotary pump (with pumping speeds of 50 1 s- ~ and 2.8 m 3 h - ~, respectively) via a computer-controlled pneumatically operated gate valve. The pumps are fitted with a nitrogen purge to the exhaust lines, so that if the main chamber valves or pumps fail, the reactant gases can be pumped away safely through the loadlock. Pressure monitoring in the loadlock is achieved by an Edwards PRH10 Pirani and CP25K Penning combination. The gauges cover the 103_10 3 mbar and I0-2-10 -7 ranges, respectively. An argon vent line is fitted to minimize the amount of oxygen, nitrogen and water getting into the system. Samples up to 100 mm in diameter are placed on circular molybdenum holders. Four holders can be placed in a cassette mechanism within the loadlock, thus minimizing the number of times the loadlock has to be opened. Samples are transferred to the growth chamber by a magnetically coupled transfer arm fitted with a fork which locates in slots in the molybdenum holders. Future expansion of the system by the addition of analysis chambers is provided for by a number of spare ports on the loadlock. Computer control allows automated venting and pumpdown sequences. 9. Computer system
The computer program involved is very complex, some 5000 lines long, providing a mouse-driven, screen-based control interface. It is written in HP BASIC for ease of understanding and then compiled for speed. In brief the program performs the following main functions : Remote operation of all valves and most pumping groups, Control of gas flows via mass flow controllers. Remote control of heated platen. Elaborate checks to prevent operator error. Pressure monitoring to check for leaks. Mass spectrometer operation and data collection. On-screen display of system status. Automatic shutdown in case of system failure. The program is also capable of automatic start and stop. 9.1. Hardware. The control computer is a 20 MHz Hewlett Packard series 9000, model 340, which is connected to two main input/output units which allow control and monitoring of most of the system's components. Firstly it is connected via a RS232 link to a Type 815 Eurotherm controller which reads and sets the percentage power, the setpoint givefi to the heater and, thus, from a precalibrated relationship, the platen temperature. In order to minimize thermal shock the setpoint is normally reached via a timed ramp function. The control program combines this ramp function with the computer's internal clock to set the platen to heat up or cool down at any time. This function is normally used to warm the platen in the early morning so that it has achieved required working temperature at the beginning of the working day, saving approximately 2 h. Secondly, it is connected via an IEEE-488 link to two Microlink units which control and monitor most of the other system components. A Microlink unit comprises a main-frame containing the circuitry necessary for complete IEEE-488 bus operation, and a number of modules which transfer data between the bus and the computer. Each module is addressed separately through a secondary address, i.e. to send data or commands to a particular module, the computer sends out an extended (two byte) listed address where the first byte is the primary address of the Microlink interface and the 502
second byte is the secondary address of the module within the interface. The subsequent list gives the particular Microlink modules used, a brief description of their general function and how they fit into our particular system. Details of how each unit interacts with other parts of the system are given later. H D R 4 module : Each module provides four heavy-duty relays which are used to control mains power to the pumping system, the microwave power supply and, combined with electropneumatic valves, the gate valves and gas flow control valves. Q12DA module: Each module provides four 12-bit digital to analogue converters which are set to 0-10 V output range. These are used to set the individual gas flows a.nd to set the required mass for the quadrupole mass analyser. A12D module: 12-bit analogue to digital converter. PGA16 module: Accepts 16 differential analogue inputs which are sent to the AI2D for conversion to digital form. In our system this monitors the various gas flows, the loadlock and main chamber pressures, the diffusion pump backing pressure and the partial pressure of the set mass in the quadrupole mass analyser. AL8 module : Provides a way of handling multiple alarm inputs which are assumed to be logic level inputs (TTL, 5 V CMOS or switch closure). These generate SRQ (service request) interrupts to signal the computer that an external event has taken place. The alarm states are determined by reading the logic state of the inputs. In this mode, the unit can give information on the state of its various inputs, for example, it can determine the current position of the gate valves. Whilst in the alarm mode, it is used to generate an alarm when one or more of the system parameters changes unexpectedly.
As stated previously, some of these units interact with other components of the system mainly by the reading or setting of various voltages. In the following sections we will discuss how this is done in more detail. 10. Gas flow control
Process gases are admitted to the chamber through a distributor ring via mass flow controllers and computer-controlled pneumatic valves. The gases used and their maximum flow rates are shown in Table 1. The mass flow controllers are of two types : (a) MKS model 2259B mass flowmeters for CO (0-10 standard cubic centimetres per minute (seem)), CH4 (0-10 sccm) and H2 (0-1000 seem) ; and (b) Brooks model 5850TR flowmeters for Ar (0-100 sccm, 0-10 standard I per min (slm)), and 02 (0-10 sccm). In each case there is an absolute on/off valve fitted immediately downstream of the flow controller. These flow controllers are connected to two corresponding units which act as power supplies, readout and control units: (a) an MKS 247C 4-channel readout unit; and (b) a Brooks model 5876 dual channel unit. For both units the flow rate can be read/controlled remotely, since they output a 0-5 V signal which is linearly related to each flowmeter range and accept a corresponding 0-5 V signal for the required flow rate. Thus by combining these units with the PGAI6 and Q12DA modules it is possible to control the gas flows by computer. 11. Pressure control
One of the design requirements was for independent control of the flow rates and the chamber pressure. To achieve this, the output from the Baratron gauges is used to control an MKS 252-A throttle valve controller connected to an MKS Type 253-A
M G J u b b e r et al." Design of a UHV reactor
Table 1. Gases used in the deposition system.
Gas H2 CH 4
CO Ar Ar 02
Max flow (sccm)
Purity
Major impurities (ppm) ~7 Ar CH 4 CO CO2
H2
H20
N2
02
1000 10 l0 100 10,000 10
99.999% 99.995% 99.997% 99.998% 99.998% 99.5%
. . -2 --3000
0.5 <1 <1 <1 0.35
2 3 2 <2 <2 5
3 5 8 <6 <6 150
1 0.3 I 12 2 -<2 <1 <2 < l -<1
. -1 ----
. --0.5
. -<1 <1 <1
sealing butterfly throttling valve on the chamber pump line. The Edwards gauges are powered and read by an Edwards 1105 controller, the trigger Penning by a Leisk VC50 U H V Penning controller and the Baratrons by an M K S P D R - C - 2 C readout unit. Although all the controllers give out a 0-10 V signal for each pressure gauge connected to it, the relationship between this signal and the actual pressure is different in each case. The P D R C-2C gives a signal for each Baratron which is linearly related to its range. The Leisk VC50 controller also gives a linear signal, however, this is for a particular pressure decade, e.g. 1-10 x 10 - 7 mbar, or for a logarithmic measurement of the pressure over the entire 10 4-10 ~0 m b a r range. The case for the Edwards controller is more complex since there is no simple relationship between the output voltage and the pressure. The manufacturer does, however, supply a graph which shows the complex relationship for each different pressure gauge. F r o m such graphs, lookup tables were compiled giving the pressure at various voltages. Using such tables and assuming a linear relationship between consecutive look-up values allows the calculation of the pressure reading from each gauge for any given output voltage. Thus, by using the PGA16 module to monitor these voltages, and knowing which pressure range the VC50 controller is set to, all the pressure readings can be measured.
12. Safety features Many safety features have been incorporated into the system, both at the hardware level and in the computer software. In the event of mains power failure, the computer and Microlink modules are reset, and power is lost from all electrically operated valves which are arranged so as to shut off all gas flows and close all gate valves. Power is lost from the heater, so the substrate cools down safely and at the same time the plasma is extinguished. When power returns, the computer program does not automatically restart, leaving the Microlink modules inactive, so no valves open and no gases flow, and the heater stays off. An interlock ensures that the microwave power supply remains off. In the event of cooling water failure, a flow switch interlock alerts the computer which shuts off the gas flows, closes the gate valves and turns off the heater before alerting the operator with a fault warning message on the screen. In exactly the same manner, an airflow switch in the fume cupboard where the pump exhaust lines are vented and a flow switch in the nitrogen purge gas inlet line will cause a controlled shutdown if either the fume cupboard goes off or the purge gas runs out. As mentioned previously, the reactant gases (mainly H2) are diluted to < 4 % with nitrogen before being vented into the fume cupboard. The maximum flow of H2 in our system is 1 slm,
Hydrocarbons
so even without nitrogen dilution, the hydrogen concentration rapidly falls below the explosion limit of 4% in air away from the end of the exhaust line, since the flow of air through the fume cupboard is 2100 1 per rain (a M S A TC-3 thermal conductivity gas analyser monitors the hydrogen concentration in the laboratory). To allow continuous running, the purge gas must flow continuously and is provided from a bank of 15 cylinders located outside the building. During sample transfer, there is a small risk that power failure may occur and the large gate valve will lose on the transfer arm : to prevent such accidents, each gate valve is fitted with an isolation valve on its pneumatic inlet line so that it cannot be moved accidentally. Errors of judgement may occur when operating such a complex system. For example, the operator may decide to open the gate valve separating the loadlock and the main chamber when deposition is taking place and while the loadlock is vented. This would result in instant oxidation of the substrate and heater and the possible leakage of both microwaves and flammable/toxic gases into the laboratory. To prevent such errors, the computer software has an extensive system of checks which prevent any unusual operations such as turning on/off pumps in the wrong sequence, flowing gases whilst the main chamber is being pumped by the diffusion pump and opening or closing valves at the wrong time during the process. In addition, the software monitors the system pressures and gate valve positions. If a window were to fail and the chamber pressure rose to over twice its set value then the system would shut down. Similarly, if a gate valve moved unexpectedly, then the system would also shut down. The loadlock-chamber gate valve can only be moved when the transfer arm is fully retracted, to prevent accidental closure of the gate valve on the arm or the sample. During unattended overnight running, the system also monitors the mass spectrum of the gases in the main chamber. A crack in one of the viewports would allow air into the system, but the throttle valve would open to maintain constant pressure, so a pressure rise may not be detected if it occurred slowly enough for the throttle valve to react, however an increase in the oxygen, nitrogen and water peaks in the spectrum would alert the computer and a controlled shutdown would take place.
13. In-situ monitoring A variety of in-situ monitoring techniques have been incorporated into the system design. A quadrupole mass spectrometer monitors the stable species formed in the growth chamber and provides residual gas analysis when the chamber is evacuated. Excited species within the plasma are monitored by optical emis503
M G Jubber et al:
Design of a UHV reactor
sion spectroscopy. Diffuse and specular laser reflectance monitoring provide in-situ growth rate measurement. Ellipsometry is used to study the initial stages of growth. 13.1. Mass spectrometer. The computer program, as well as controlling the system, has the capability to run a VSW vacuum analyst quadrupole mass spectrometer which is used in our deposition system as a residual gas analyser or to monitor the stable gas species in the growth chamber. The exact configuration of the gas sampling is shown in Figure 3. The long path from the plasma region to the sampling region results in the measurement of only stable gas species. In order to reduce the pressure of the gas being sampled from the 1% 100 mbar range to a suitable level for the mass spectrometer, the gas is pumped through a needle valve by a Balzers TPU050 turbomolecular pump backed by a Balzers Duo 1.5A rotary pump (with pumping speeds of 50 1 s and 1.8 m 3 h [). The mass spectrometer consists of a 1-100 anau quadrupole head with a corresponding VSW vacuum analyst controller. This controller is fitted with facilities for remote programming of the mass scale via an input signal of 0 l0 V (corresponding to 0-100 ainu) and for remote reading of the partial pressure via an output signal of 0 1 0 V full scale. These input/ output signals are linked to the computer by combining such signals with the PGA16 module for pressure reading and with the Q12DA module to set the required mass value. The program has been designed such that it can perform a mass scan over an operator specified range, scan a series of masses with time or do both. Figure 4 shows a mass spectrum of the growth chamber during a growth run using a methane/hydrogen gas mix, the major peaks are due to hydrogen and methane, but products such as acetylene and ethylene can also be seen. 13.2, Optical emission spectrometer. Optical monitoring of the plasma is carried out using a Monolight 6800 series Optical Spectrum Analyser. This analyser comprises a scanning grating monochromator, a PMT detector and a system controller interfaced to a computer. The grating and PMT combination is optimized for emissions in the 200-900 nm range and the system has a resolution of 0.7 nm. A single scan takes 80 ms and typically 4000 scans are averaged to reduce the noise level. The light from the plasma is collected through either the large Kodial glass window or through a quartz window fitted to one of the 70' ports, and is focused onto the end of a quartz fibre bundle which transmits the light to the specrometer. The major peaks seen in the spectra are shown in Table 2.
/~
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6
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0
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.
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.
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20
30
40
50 m/e
60
70
80
90
100
Figure 4. A mass spectrum of the growth chamber. A typical spectrum of a plasma containing H2 and CH4 showing the peaks tabulated above is shown in Figure 5. A full paper on the emission spectra observed from various plasma conditions is in preparation. It also discusses the derivation of plasma temperatures from these spectra. 13.3, Reflectance monitor. Growth rates at the centre of the substrate have been monitored by measuring the period of interference fringes produced by a laser beam reflected at 70' to the substrate normal. The oscillation period has been converted to a film thickness change by assuming a constant refractive index of 2.42 for diamond. A 7 mW HeNe laser beam at 633 nm is reflected from the sample onto two silicon photodiode detectors. One detector monitors the specular beam intensity, whilst the second detector monitors the diffuse scattered light. This in-situ method of measuring growth rates allows several values of deposition parameters to be examined during a single growth run. 13.4. Ellipsometer. Ellipsometry is used to investigate the initial stages of growth. The polarizer and analyser from a Gaertner model L117 Production Ellipsometer are bolted onto two of the 7 0 ports. Using this setup, values of Psi and Delta can be determined and from these the thickness and refractive index of the growing layer can be calculated.
14. Experimental procedure Before any deposition can take place, a substrate must be prepared and cleaned. In order to start growing a deposit it is often necessary to pre-nucleate a sample by scratching it with diamond powder. Substrates such as (111) or (100) silicon wafers, molybdenum, quartz, aluminium and steel are given 5 rain of scratching with 0.75-1.5 #m synthetic diamond dust. Any residual Table 2. Peak assignments and their corresponding wavelengthsIs Peak assignment
Wavelength (nm)
Molecular hydrogen CH H. HI~ C2 H,_ (Fulcher Bands) H~ Ar
240 430 434 486 516 560-630 656 751
M G Jubber et al: Design of a UHV reactor 500
[ I
400 !
5 300 v E c
200
100
0 u.-
300
200
400
500
600
Wavelength
(nm)
700
800
900
H~
and the throttle valve reacts to maintain the required chamber pressure. When the pressure, temperature and flow have stabilized, the microwave supply is switched on and a hydrogen plasma is produced. Typically, 500-1000 W of microwave power go into the chamber with less than 10 W being reflected. This plasma is left on for at least 30 rain in order to clean the substrate. After this period, the reactant gas flows are set, typically 0.5 1.0% CH4 in H2 and deposition begins. At the end of deposition, the hydrogen flow is left on while all other gas flows are stopped. This ensures that no thermal growth of non-diamond carbon occurs. Using the mass spectrometer to monitor the gas species in the chamber, the plasma is continued until the reactant gas concentrations reach the background level, the plasma is then extinguished. Once the chamber has been flushed with hydrogen the heater is set to cool down slowly. The sample is then removed through the loadlock and analysed.
2,000
15. Results H~
1,500
The chamber design has proved to be very flexible and a wide range of deposition conditions have been investigated. A Scanning Electron Microscope (SEM) image of a polycrystalline diamond film is shown in Figure 6. This highly faceted surface and
1,ooo c o~
E
H~
off due to chamber wiftdo'~ cut
500
\ 200
300
Ar ~l~-platen
400
500 600 W a v e l e n g t h (nrn)
700
800
900
Figure 5. An optical emission spectrum of a methane/hydrogen plasma.
dust is then blown offby dry nitrogen, the sample is then cleaned successively in acetone, methanol, very dilute Decon solution and de-ionized water in an ultrasonic bath. Finally, the sample is dried by blowing with nitrogen. An alternative mechanical prenucleation treatment has recently been filed for a patent ~9, and non-mechanical treatments using growth with dc bias on the substrate are suitable for some materials. The stability of pre-nucleated surfaces produced by the scratching technique has been investigated. It has been found that an 11 week delay between scratching a sample and depositing on it has no effect on the nucleation process. After preparing the sample, it is placed into the argon purged loadlock which is pumped down to its base pressure of ~ 1 × 10- 6 mbar before being isolated from the pumps. The main loadlock-chamber gate valve is opened and the sample transferred into the evacuated growth chamber. Once the transfer arm is retracted, the loadlock is again pumped to its base pressure. The main chamber is then pumped down to ~ 1 x 10-8 mbar. The heater cavity is flushed with argon to ensure that there will be no oxidation or corrosion during heating. Cooling water is supplied to the required areas : the microwave transmitting window (2.4 1 min '), the heater electrodes, the heater flanges and the heater jacket (a total of 30 I m i n r ) . In order to prevent thermal shock to the chamber windows, the substrate is always heated up gradually under hydrogen at approximately the same pressure as will be used in the subsequent growth run. The heater is set to ramp to the required power, typically to 45% of its maximum power in steps of 0.5% per minute. Once the required temperature is reached, the Roots and rotary pumps are switched on, the purge gas flows are set and the diffusion pump inlet valve is closed. A flow of hydrogen is set
Figure 6. A scanning electron micrograph of a polycrystalline diamond film, deposited by microwave plasma CVD. 505
M G Jubber et al: Design of a UHV reactor
16. Conclusion
14,000 12,000 10,000 8000 6000 4000 2000
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,
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Figure 7. The Raman spectrum of a diamond film showing the diamond peak at 1333 cm t.
D i a m o n d films have been successfully grown over the entire surface of 100 mm diameter substrates using this custom designed system. A m o n g the substrates coated are silicon, molybdenum, aluminium, stainless steel and quartz. The isolated plasma ball and U H V chamber produce highly pure films as determined by L I M A and CL analyses. A detailed series of experiments has been carried out in order to understand the role of the various process parameters, e.g. gas flow rate and gas composition, substrate temperature, microwave power and plasma conditions. Deposit quality has also been correlated with results of mass spectroscopy, optical emission spectroscopy, specular and diffuse reflectance, and ellipsometry. The customized system is more suitable for scientific study of the diamond C V D process than presently available small commercial reactors whilst producing films of area and quality suitable for application testing.
Acknowledgements the predominant (100) orientation are characteristic of diamond films grown within a narrow range of determined conditions. The diamond nature is confirmed by the presence of the characteristic 1333 cm -~ peak in the R a m a n spectrum shown in Figure 7 and the presence of the diamond 2-phonon peaks in the F T I R spectrum in Figure 8. G r o w t h rates as high as 2 #m h t have been obtained at the substrate centre. The film thickness variation from the centre to the edge of the substrate is consistent with a simple cosine flux distribution of reactant species coming from the plasma. Laser Ionization Mass Analysis (LIMA) has been carried out and the films found to contain only carbon and hydrogen ~~. The samples have been subjected to a variety of analytical techniques including X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), cathodoluminescence (CL), and X-ray diffraction, the results of which have been published elsewhere 11,20
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506
We wish to thank the U K Science and Engineering Research Council ( D K M ) and The Royal Society for research grants supporting this work. M G J is supported by an M O D contract. We also thank Prof G D Pitt of Renishaw Transducer Systems Ltd for the Raman analysis.
References P Bachmann, Phys World, 4, 4, 32 36 (1991). 2y Liou, A lnspektor, R Weimer, D Knight and R Messier, J Mater Res, 5, 2305 2312 (1990). 3R S Yalamanchi and K S Harshavardhan, J Appl Phys, 68, 5941 5943 (1990). a E Kondoh, T Ohta, T Mitomo and K Ohtsuka, J Appl Phys, 72, 705 711 (1992). K Kurihara, K Sasaki, M Kawarada and N Koshino, Appl Phys Lett, 52, 437 438 (1988). 6K A Snail, C L Void, J A Freitas and L M Hanssen, Diamond Depositions: Sci Technol, 2, I (1991). Vp Bachmann and R Messier, Chem Engn9 News, May, 24~39 (1989). Reported in Diamond Depositions: Sci Technol, 2(2), 15 (1991). 'JM W Geis, In Diamond, StTicon Carbide and Related Wide Bandgap Semiconductors (Edited by J T Glass, R Messier and N Fujimori), Materials Res Soc Syrup Proc, 162, pp 15 22. Materials Research Society, Pittsburgh, Pennsylvania, USA (1990). 0G-It M Ma, Y H Lee and J T Glass, J Mater Res, 5, 2367 2377 (1990). ~ M G Jubber, J I B Wilson, 1 C Drummond, P John and D K Milne, DIAMOND 1992, Heidelberg, September 1992. Diamond and Related Materials, 2, 2 4, 402-406 (1993). I~S J Harris and L R Martin, JMater Res, 5, 2313 2319 (1990). ~3y Saito, K Sato, H Tanaka and H Miyadera, J Mater Sci, 24, 293 297 (1989). Hy Liou, A lnspektor, R Weimer and R Messier. Appl Phys Lett, 55, 631 633 (1989). ~ J A Mucha, D L Flamm and D E Ibbotson, J Appl Phys, 65, 34483452 (1989). ~6C-P Chang, D L Flamm, D E lbbotson and J A Mucha, J Appl Phys, 63, 1744 1748 (1988). ~7UCAR Specialty Gases Product Specifications lbr H2, CH4 and CO. BOC Special Gases Catalogue for Ar and O,. JSB Marcus, M Mermoux, F Vinet, A Campargue and M Chenevier. SurJaee Coatings Technol, 47, 608 617 (1991). "~P John, 1 C Drummond and J I B Wilson, Diamond Film Deposition, UK Patent Application 9211107.9, filed 26th May 1992. ;°A Cook, A G Fitzgerald, B E Storey, J I B Wilson, P John, M G Jubber, D Milne, I Drummond, J A Savage and S Haq, Diamond and Related Materials, 1,478-585 (1992).
Pergamon
0042-207X/9457.00+ .00
Design of a U H V reactor for m i c r o w a v e plasma deposition of d i a m o n d films M G Jubber
and J I B Wilson,
Department of Physics, Heriot-Watt University, Riccarton,
Edinburgh EH14 4AS, UK and I C Drummond,
P J o h n a n d D K M i l n e , Department of Chemistry, Heriot-Watt University, Riccarton,
Edinburgh EH14 4AS, UK
A UHV compatible deposition system has been constructed for the growth of diamond films by 2.45 GHz microwave plasma chemical vapour deposition (CVD). The system comprises a Ioadlock and a spherical deposition chamber where the heated 100 mm diameter substrate is exposed to a reactive plasma environment. The design provides ports for in-situ monitoring by ellipsometry, optical emission spectrometry, mass spectrometry, and laser reflectometry and allows for the later addition of analysis chambers such as XPS. Computer control is provided for all major components and operations, including pumps, valves, gas flows, pressure and temperature adjustment. The system has four pumping groups, two for the main growth chamber providing base vacuum and for pumping the process gases, one for evacuating the Ioadlock and the fourth for the mass spectrometer. Microwaves enter the chamber via an antenna-based microwave applicator with a water-cooled quartz window. A key feature of this design is the ability to have a free standing ball plasma which touches neither the chamber walls nor the substrate.
I. Introduction In recent years the growth of diamond films at low pressures and temperatures has received increasing attention. Such films have a wide range of potential industrial applications based on diamond's unique combination of properties ~. A wide variety of growth techniques have been used to deposit diamond, including microwave CVD systems 2, oxyacetylene torches 3, filament systems 4 and plasma torches 5. The oxyacetylene torch methods have been shown to produce reasonable quality diamond at high growth rates 6, but suffer from small sample sizes, high surface temperatures and poor uniformity. A further problem with such systems is contamination of the film with nozzle material 7. Of the CVD systems, the two most commonly used techniques are the hot filament method and the microwave plasma CVD method. The hot filament method has been used to produce complex 3D shapes such as diamond tubes 8 and also to grow doped epitaxial films for use as Schottky diodes or prototype thyristors 9. However, in common with the torch methods, it suffers from inhomogeneous growth over large areas and for metal filaments invariably produces deposits contaminated with the filament material ~0. In terms of deposit purity, the microwave plasma CVD method is superior, since it can produce large area ( ~ 10 cm diameter) contaminant free films ~i. As the name suggests, the process involves the formation of a plasma by microwave excitation of a reactant gas mix resulting in growth upon a heated substrate. Typical conditions for growth are : (i) a pressure of 10-100 mbar; (ii) a substrate temperature of 500-900°C ;
(iii) a gas mix of 0.1-2.0% carbon-containing gas in hydrogen. At present, the exact process by which diamond growth occurs, and the role of the various process parameters, in such systems is still a matter of some controversy ~2. There is, therefore, a need for a deposition system having independent adjustment of deposition parameters, to produce ultra-high purity, large area, single-crystal or polycrystalline diamond films for optics and electronics.
2. Typical microwave growth systems At the time of the design of the Heriot-Watt system there were two main types of reactor used for the microwave plasma CVD growth of diamond : (i) the simple tube reactor ~3 and (ii) the bell jar reactor t4. Such systems, however, have inherent limitations which restrict their use either in industrial applications or in attempting to understand the role of the various parameters. The fact that the substrate sits within the plasma results in a high surface temperature and makes it impossible to decouple the effect of the plasma and substrate heating. Furthermore, it is often difficult to characterize the plasma and at the same time probe the substrate surface since the reactor design does not allow independent, unobstructed views of both the substrate and the plasma. The plasma also etches material from the reactor walls which contaminates the film tS. In addition, many reactors have graphite heaters or sample holders which are exposed to the reactive plasma species. This often results in extra material being etched from the graphite and being deposited as diamond, giving spuriously high growth rates when based only on the carbon balance in the reactant gas. Further limitations include small 499
M G Jubber et al: Design of a UHV reactor
sample size, poor base vacuum and the necessity of exposing the growth chamber to air in order to load a sample. The last two limitations are a problem since nitrogen is an n-type dopant for diamond, and oxygen and water vapour have been shown to influence the growth even in small quantities ~6. The small sample size used in some reactors can produce misleading results, since it does not reveal the different morphologies, range of crystallite sizes, and variation in growth rate which can be obtained when the substrate is larger than the diameter of the plasma.
additional monitoring or analysis equipment at a later date. The typical growth conditions and the slow growth rate (<~ 10 /~m h ~ L) reported to date call for requirements 8, 9 and 10, whilst requirements 10 and 11 allow for the possibility for upgrading to a commercial scale. Design requirement 10 prompted us to make the system computer-controlled. Naturally, the system should be easy to operate, and should incorporate all of the safety features for handling hydrogen.
4. Heriot-Watt growth system 3. Heriot-Watt design requirements The design of commercial diamond deposition reactors is often inflexible, whereas research reactors must allow for independent control of many parameters over a wide range. In addition, facility must be provided for the addition of monitoring and analysis equipment. Based on the above considerations and the range of growth conditions reported in the literature, we decided on the following design requirements : 1. Separable plasma and substrate. 2. In-situ monitoring and analysis capability.
3. 4. 5. 6. 7. 8. 9. 10. 11.
Separate growth chamber and sample loading chamber. Plasma not in contact with chamber walls. Base pressure ~<10- s mbar. Independent control of deposition parameters. Ability to add further chambers. Operating pressures from 1-100 mbar. Substrate temperatures up to 1000':C. Continuous unattended running. Capacity for I00 mm diameter samples.
Requirements 1 and 2 allow independent analysis of both the plasma by techniques such as optical emission spectroscopy and the growing surface via Raman spectroscopy, ellipsometry, and FT-IR, whilst requirements 3, 4 and 5 prevent contamination of the films by material from the chamber walls or the atmosphere. Point 6 allows us to independently investigate the effect of each of the process variables and point 7 gives us the option of adding
The deposition system comprises two main sections : the growth chamber and a loadlock for sample entry/removal. Since nitrogen is an impurity in diamond, the role of the loadlock is to prevent atmospheric contamination of the growth chamber. A schematic diagram of the entire deposition system is shown in Figure 1.
5. Growth chamber A 380 mm (15 in.) diameter stainless steel sphere (Leisk Engineering/VSW) forms the basic growth chamber. It has 16 ports : two 150 mm internal diameter Kodial glass windows for viewing, two 63 mm diameter windows at 70° to the vertical for in-situ ellipsometry, another two 63 mm diameter windows at 70° to the vertical for laser reflectance thickness monitoring, two 35 mm ports at 45 '~ for gas inlet lines, one 200 mm diameter port for the platen and heater, one 200 mm diameter port for the microwave applicator, four 35 mm diameter ports for vacuum gauges, one 150 mm diameter pumping port and one 150 mm diameter port with a gate valve, providing access to the loadlock. The deposition chamber behaves as a stirred-flow reactor and has a total volume of approximately 40 1. All flanges on the chamber are Conflat type UHV flanges, fitted with copper gaskets. There are two states in which the growth chamber can operate : during sample transfer and between experiments it operates in a base vacuum state of < 10 -s mbar, during deposition various gases flow through the chamber at pressures in the 1 100 mbar range.
KEY loadlock pumping system transfer a r m
°© @ Q
Figure l. The Heriot-Watt designed UHV reactor deposition system. 50O
spherical multiport growth chamber heated platen throttle valves roots pumping system diffusionpump system microwaveair cavity dummyload lkW microwave source pneumaticgate valves bypass line
M G Jubber et al: Design of a UHV reactor
5.1. Base vacuum state. To attain the base vacuum, the chamber is equipped with a liquid nitrogen trapped Edwards E04 diffusion pump (with a pumping speed of 6001 s- ~), backed by an Edwards EDM12A rotary pump (with a pumping speed of 14.6 m 3 h - 1). The diffusion pump is isolated from the chamber by a computercontrolled pneumatically operated gate valve. All pumps in the system are fitted with nitrogen purge to their ballast valves and/or exhaust lines to dilute any potentially explosive gases to below their explosion limits before they come into contact with air. A Leisk UHV trigger Penning gauge monitors the chamber in the 10 4--10 tt mbar range, with an Edwards PRH10 Pirani gauge monitoring the diffusion pump backing pressure.
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5.2. Process state. Prior to deposition, the diffusion pump is isolated and the process gases are then pumped through a nitrogen purged Edwards EH250 Roots Blower and Edwards E2M40 rotary pump set (combined pump speed of 240 m 3 h - l). The nitrogen flow is set to reduce the maximum possible hydrogen flow from the exhaust to below the 4% explosion limit in air. Typical operating pressures within the chamber are in the range 10-100 mbar. The chamber pressure is measured by two MKS Baratron Type 221A gauges which cover the lYJ0 and 0--1000 mbar ranges. 6. Substrate heater Substrates are heated on a molybdenum support on top of the platen. The heating element is a serpentine patterned graphite resistor powered by an 8 kW low voltage power supply. The heater is fully encapsulated in stainless steel and molybdenum which provides vacuum integrity between the heater and the chamber and ensures that the carbon element is not in contact with either the plasma or the gases in the chamber. The inside of the heater can be argon purged or evacuated to prevent oxidation and corrosion of the heated regions, Water cooling is provided to the two electrodes, the bottom flange and to the stainless steel casing of the platen. Control of the heater is achieved by setting the percentage of transformer output power. This can be set to a particular value, ramped from one value to another or cycled between two values. A thermocouple located within the black body section of the heater measures the temperature, however, it should be noted that this does not represent the substrate surface temperature. Using an Ircon model W-15C20 pyrometer operating in the 0.9-1.08 /~m range, the surface temperature across the silicon substrate surface was measured, without the plasma on, along with the thermocouple reading as a function of heater input power. Over the range of useful deposition temperatures (500-800°C surface temperature) the two were found to be linearly proportional artd hence the surface temperature could be calculated from the expression : Ts = 0.569Tt + 120°C, where 7", is the surface temperature and T~ is the thermocouple reading. In addition, the pyrometer showed that the surface temperature uniformity was good to within 5% over the 4 in. diameter substrate. By replacing a stainless steel section of the platen mount with a ceramic break, it is possible to apply a dc bias to the substrate. "1. Microwave system
The microwave system comprises a 1 kW magnetron producing 2.45 GHz radiation from an RFA switch mode power supply, a
SUBSTRATE
Figure 2. The Heriot-Watt microwave applicator design.
circulator, a dummy load, motor driven tuning stubs and an antenna which feeds the microwaves from the rectangular waveguide into a circular microwave applicator. The design and construction of the applicator was carried out at Heriot-Watt University. The applicator, as shown in Figure 2, is fitted with a water-cooled quartz window which is sealed with Viton O-rings to maintain the vacuum integrity between the reaction chamber and the rest of the microwave system. A perforated cylindrical stainless steel extension is fitted below the window, extending down towards the substrate. This encourages a stable plasma ball to form just above the substrate surface, but still allows for optical monitoring of the plasma. It is also possible to obtain a stable plasma ball in the centre of the chamber even with the heater and platen completely removed and the lower port blanked off. For safety reasons, the microwave waveguides are flushed with nitrogen so that, in the event of failure of the quartz window, nitrogen would flood into the hydrogen filled chamber rather than air, and the windows are screened with mesh to prevent microwave leakage. 8. Loadlock Samples are introduced into the reaction chamber via a loadlock. An extended six-way cross forms the main section of the loadlock, as shown in Figure 1. It has four 150 mm internal diameter ports, one supporting an x - y translation stage with a linear motion transfer arm and one providing access to the main chamber. The other two are currently blanked offallowing for future expansion. There are two 200 mm diameter flanges, the top one provides sample access and has a Viton rubber seal, the bottom one is fitted with a bellows-sealed linear motion system for moving samples up and down. In addition, the six-way cross is also fitted with two 63 mm diameter viewports to ease sample loading and transfer. The top flange is fitted with a 63 mm diameter Viton Oring sealed fused quartz window which transmits down to 160 nm. This allows in-situ cold bakeout of the substrates using a Wotan XBO150W/4 xenon lamp with a Suprasil envelope emitting down to 160 nm. One pumping line leads from the loadlock to a smaller six-way cross. This cross is fitted with four 35 mm diameter ports to which two gauges and an inlet for the argon used to vent the chamber are connected. The loadlock is pumped to a base pressure of ~ 10 .6 mbar by a Balzers TPU050 turbo501
M G Jubber et al: Design of a UHV reactor
molecular pump backed by an Edwards E2M2 rotary pump (with pumping speeds of 50 1 s- ~ and 2.8 m 3 h - ~, respectively) via a computer-controlled pneumatically operated gate valve. The pumps are fitted with a nitrogen purge to the exhaust lines, so that if the main chamber valves or pumps fail, the reactant gases can be pumped away safely through the loadlock. Pressure monitoring in the loadlock is achieved by an Edwards PRH10 Pirani and CP25K Penning combination. The gauges cover the 103_10 3 mbar and I0-2-10 -7 ranges, respectively. An argon vent line is fitted to minimize the amount of oxygen, nitrogen and water getting into the system. Samples up to 100 mm in diameter are placed on circular molybdenum holders. Four holders can be placed in a cassette mechanism within the loadlock, thus minimizing the number of times the loadlock has to be opened. Samples are transferred to the growth chamber by a magnetically coupled transfer arm fitted with a fork which locates in slots in the molybdenum holders. Future expansion of the system by the addition of analysis chambers is provided for by a number of spare ports on the loadlock. Computer control allows automated venting and pumpdown sequences. 9. Computer system
The computer program involved is very complex, some 5000 lines long, providing a mouse-driven, screen-based control interface. It is written in HP BASIC for ease of understanding and then compiled for speed. In brief the program performs the following main functions : Remote operation of all valves and most pumping groups, Control of gas flows via mass flow controllers. Remote control of heated platen. Elaborate checks to prevent operator error. Pressure monitoring to check for leaks. Mass spectrometer operation and data collection. On-screen display of system status. Automatic shutdown in case of system failure. The program is also capable of automatic start and stop. 9.1. Hardware. The control computer is a 20 MHz Hewlett Packard series 9000, model 340, which is connected to two main input/output units which allow control and monitoring of most of the system's components. Firstly it is connected via a RS232 link to a Type 815 Eurotherm controller which reads and sets the percentage power, the setpoint givefi to the heater and, thus, from a precalibrated relationship, the platen temperature. In order to minimize thermal shock the setpoint is normally reached via a timed ramp function. The control program combines this ramp function with the computer's internal clock to set the platen to heat up or cool down at any time. This function is normally used to warm the platen in the early morning so that it has achieved required working temperature at the beginning of the working day, saving approximately 2 h. Secondly, it is connected via an IEEE-488 link to two Microlink units which control and monitor most of the other system components. A Microlink unit comprises a main-frame containing the circuitry necessary for complete IEEE-488 bus operation, and a number of modules which transfer data between the bus and the computer. Each module is addressed separately through a secondary address, i.e. to send data or commands to a particular module, the computer sends out an extended (two byte) listed address where the first byte is the primary address of the Microlink interface and the 502
second byte is the secondary address of the module within the interface. The subsequent list gives the particular Microlink modules used, a brief description of their general function and how they fit into our particular system. Details of how each unit interacts with other parts of the system are given later. H D R 4 module : Each module provides four heavy-duty relays which are used to control mains power to the pumping system, the microwave power supply and, combined with electropneumatic valves, the gate valves and gas flow control valves. Q12DA module: Each module provides four 12-bit digital to analogue converters which are set to 0-10 V output range. These are used to set the individual gas flows a.nd to set the required mass for the quadrupole mass analyser. A12D module: 12-bit analogue to digital converter. PGA16 module: Accepts 16 differential analogue inputs which are sent to the AI2D for conversion to digital form. In our system this monitors the various gas flows, the loadlock and main chamber pressures, the diffusion pump backing pressure and the partial pressure of the set mass in the quadrupole mass analyser. AL8 module : Provides a way of handling multiple alarm inputs which are assumed to be logic level inputs (TTL, 5 V CMOS or switch closure). These generate SRQ (service request) interrupts to signal the computer that an external event has taken place. The alarm states are determined by reading the logic state of the inputs. In this mode, the unit can give information on the state of its various inputs, for example, it can determine the current position of the gate valves. Whilst in the alarm mode, it is used to generate an alarm when one or more of the system parameters changes unexpectedly.
As stated previously, some of these units interact with other components of the system mainly by the reading or setting of various voltages. In the following sections we will discuss how this is done in more detail. 10. Gas flow control
Process gases are admitted to the chamber through a distributor ring via mass flow controllers and computer-controlled pneumatic valves. The gases used and their maximum flow rates are shown in Table 1. The mass flow controllers are of two types : (a) MKS model 2259B mass flowmeters for CO (0-10 standard cubic centimetres per minute (seem)), CH4 (0-10 sccm) and H2 (0-1000 seem) ; and (b) Brooks model 5850TR flowmeters for Ar (0-100 sccm, 0-10 standard I per min (slm)), and 02 (0-10 sccm). In each case there is an absolute on/off valve fitted immediately downstream of the flow controller. These flow controllers are connected to two corresponding units which act as power supplies, readout and control units: (a) an MKS 247C 4-channel readout unit; and (b) a Brooks model 5876 dual channel unit. For both units the flow rate can be read/controlled remotely, since they output a 0-5 V signal which is linearly related to each flowmeter range and accept a corresponding 0-5 V signal for the required flow rate. Thus by combining these units with the PGAI6 and Q12DA modules it is possible to control the gas flows by computer. 11. Pressure control
One of the design requirements was for independent control of the flow rates and the chamber pressure. To achieve this, the output from the Baratron gauges is used to control an MKS 252-A throttle valve controller connected to an MKS Type 253-A
M G J u b b e r et al." Design of a UHV reactor
Table 1. Gases used in the deposition system.
Gas H2 CH 4
CO Ar Ar 02
Max flow (sccm)
Purity
Major impurities (ppm) ~7 Ar CH 4 CO CO2
H2
H20
N2
02
1000 10 l0 100 10,000 10
99.999% 99.995% 99.997% 99.998% 99.998% 99.5%
. . -2 --3000
0.5 <1 <1 <1 0.35
2 3 2 <2 <2 5
3 5 8 <6 <6 150
1 0.3 I 12 2 -<2 <1 <2 < l -<1
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sealing butterfly throttling valve on the chamber pump line. The Edwards gauges are powered and read by an Edwards 1105 controller, the trigger Penning by a Leisk VC50 U H V Penning controller and the Baratrons by an M K S P D R - C - 2 C readout unit. Although all the controllers give out a 0-10 V signal for each pressure gauge connected to it, the relationship between this signal and the actual pressure is different in each case. The P D R C-2C gives a signal for each Baratron which is linearly related to its range. The Leisk VC50 controller also gives a linear signal, however, this is for a particular pressure decade, e.g. 1-10 x 10 - 7 mbar, or for a logarithmic measurement of the pressure over the entire 10 4-10 ~0 m b a r range. The case for the Edwards controller is more complex since there is no simple relationship between the output voltage and the pressure. The manufacturer does, however, supply a graph which shows the complex relationship for each different pressure gauge. F r o m such graphs, lookup tables were compiled giving the pressure at various voltages. Using such tables and assuming a linear relationship between consecutive look-up values allows the calculation of the pressure reading from each gauge for any given output voltage. Thus, by using the PGA16 module to monitor these voltages, and knowing which pressure range the VC50 controller is set to, all the pressure readings can be measured.
12. Safety features Many safety features have been incorporated into the system, both at the hardware level and in the computer software. In the event of mains power failure, the computer and Microlink modules are reset, and power is lost from all electrically operated valves which are arranged so as to shut off all gas flows and close all gate valves. Power is lost from the heater, so the substrate cools down safely and at the same time the plasma is extinguished. When power returns, the computer program does not automatically restart, leaving the Microlink modules inactive, so no valves open and no gases flow, and the heater stays off. An interlock ensures that the microwave power supply remains off. In the event of cooling water failure, a flow switch interlock alerts the computer which shuts off the gas flows, closes the gate valves and turns off the heater before alerting the operator with a fault warning message on the screen. In exactly the same manner, an airflow switch in the fume cupboard where the pump exhaust lines are vented and a flow switch in the nitrogen purge gas inlet line will cause a controlled shutdown if either the fume cupboard goes off or the purge gas runs out. As mentioned previously, the reactant gases (mainly H2) are diluted to < 4 % with nitrogen before being vented into the fume cupboard. The maximum flow of H2 in our system is 1 slm,
Hydrocarbons
so even without nitrogen dilution, the hydrogen concentration rapidly falls below the explosion limit of 4% in air away from the end of the exhaust line, since the flow of air through the fume cupboard is 2100 1 per rain (a M S A TC-3 thermal conductivity gas analyser monitors the hydrogen concentration in the laboratory). To allow continuous running, the purge gas must flow continuously and is provided from a bank of 15 cylinders located outside the building. During sample transfer, there is a small risk that power failure may occur and the large gate valve will lose on the transfer arm : to prevent such accidents, each gate valve is fitted with an isolation valve on its pneumatic inlet line so that it cannot be moved accidentally. Errors of judgement may occur when operating such a complex system. For example, the operator may decide to open the gate valve separating the loadlock and the main chamber when deposition is taking place and while the loadlock is vented. This would result in instant oxidation of the substrate and heater and the possible leakage of both microwaves and flammable/toxic gases into the laboratory. To prevent such errors, the computer software has an extensive system of checks which prevent any unusual operations such as turning on/off pumps in the wrong sequence, flowing gases whilst the main chamber is being pumped by the diffusion pump and opening or closing valves at the wrong time during the process. In addition, the software monitors the system pressures and gate valve positions. If a window were to fail and the chamber pressure rose to over twice its set value then the system would shut down. Similarly, if a gate valve moved unexpectedly, then the system would also shut down. The loadlock-chamber gate valve can only be moved when the transfer arm is fully retracted, to prevent accidental closure of the gate valve on the arm or the sample. During unattended overnight running, the system also monitors the mass spectrum of the gases in the main chamber. A crack in one of the viewports would allow air into the system, but the throttle valve would open to maintain constant pressure, so a pressure rise may not be detected if it occurred slowly enough for the throttle valve to react, however an increase in the oxygen, nitrogen and water peaks in the spectrum would alert the computer and a controlled shutdown would take place.
13. In-situ monitoring A variety of in-situ monitoring techniques have been incorporated into the system design. A quadrupole mass spectrometer monitors the stable species formed in the growth chamber and provides residual gas analysis when the chamber is evacuated. Excited species within the plasma are monitored by optical emis503
M G Jubber et al:
Design of a UHV reactor
sion spectroscopy. Diffuse and specular laser reflectance monitoring provide in-situ growth rate measurement. Ellipsometry is used to study the initial stages of growth. 13.1. Mass spectrometer. The computer program, as well as controlling the system, has the capability to run a VSW vacuum analyst quadrupole mass spectrometer which is used in our deposition system as a residual gas analyser or to monitor the stable gas species in the growth chamber. The exact configuration of the gas sampling is shown in Figure 3. The long path from the plasma region to the sampling region results in the measurement of only stable gas species. In order to reduce the pressure of the gas being sampled from the 1% 100 mbar range to a suitable level for the mass spectrometer, the gas is pumped through a needle valve by a Balzers TPU050 turbomolecular pump backed by a Balzers Duo 1.5A rotary pump (with pumping speeds of 50 1 s and 1.8 m 3 h [). The mass spectrometer consists of a 1-100 anau quadrupole head with a corresponding VSW vacuum analyst controller. This controller is fitted with facilities for remote programming of the mass scale via an input signal of 0 l0 V (corresponding to 0-100 ainu) and for remote reading of the partial pressure via an output signal of 0 1 0 V full scale. These input/ output signals are linked to the computer by combining such signals with the PGA16 module for pressure reading and with the Q12DA module to set the required mass value. The program has been designed such that it can perform a mass scan over an operator specified range, scan a series of masses with time or do both. Figure 4 shows a mass spectrum of the growth chamber during a growth run using a methane/hydrogen gas mix, the major peaks are due to hydrogen and methane, but products such as acetylene and ethylene can also be seen. 13.2, Optical emission spectrometer. Optical monitoring of the plasma is carried out using a Monolight 6800 series Optical Spectrum Analyser. This analyser comprises a scanning grating monochromator, a PMT detector and a system controller interfaced to a computer. The grating and PMT combination is optimized for emissions in the 200-900 nm range and the system has a resolution of 0.7 nm. A single scan takes 80 ms and typically 4000 scans are averaged to reduce the noise level. The light from the plasma is collected through either the large Kodial glass window or through a quartz window fitted to one of the 70' ports, and is focused onto the end of a quartz fibre bundle which transmits the light to the specrometer. The major peaks seen in the spectra are shown in Table 2.
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Figure 4. A mass spectrum of the growth chamber. A typical spectrum of a plasma containing H2 and CH4 showing the peaks tabulated above is shown in Figure 5. A full paper on the emission spectra observed from various plasma conditions is in preparation. It also discusses the derivation of plasma temperatures from these spectra. 13.3, Reflectance monitor. Growth rates at the centre of the substrate have been monitored by measuring the period of interference fringes produced by a laser beam reflected at 70' to the substrate normal. The oscillation period has been converted to a film thickness change by assuming a constant refractive index of 2.42 for diamond. A 7 mW HeNe laser beam at 633 nm is reflected from the sample onto two silicon photodiode detectors. One detector monitors the specular beam intensity, whilst the second detector monitors the diffuse scattered light. This in-situ method of measuring growth rates allows several values of deposition parameters to be examined during a single growth run. 13.4. Ellipsometer. Ellipsometry is used to investigate the initial stages of growth. The polarizer and analyser from a Gaertner model L117 Production Ellipsometer are bolted onto two of the 7 0 ports. Using this setup, values of Psi and Delta can be determined and from these the thickness and refractive index of the growing layer can be calculated.
14. Experimental procedure Before any deposition can take place, a substrate must be prepared and cleaned. In order to start growing a deposit it is often necessary to pre-nucleate a sample by scratching it with diamond powder. Substrates such as (111) or (100) silicon wafers, molybdenum, quartz, aluminium and steel are given 5 rain of scratching with 0.75-1.5 #m synthetic diamond dust. Any residual Table 2. Peak assignments and their corresponding wavelengthsIs Peak assignment
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and the throttle valve reacts to maintain the required chamber pressure. When the pressure, temperature and flow have stabilized, the microwave supply is switched on and a hydrogen plasma is produced. Typically, 500-1000 W of microwave power go into the chamber with less than 10 W being reflected. This plasma is left on for at least 30 rain in order to clean the substrate. After this period, the reactant gas flows are set, typically 0.5 1.0% CH4 in H2 and deposition begins. At the end of deposition, the hydrogen flow is left on while all other gas flows are stopped. This ensures that no thermal growth of non-diamond carbon occurs. Using the mass spectrometer to monitor the gas species in the chamber, the plasma is continued until the reactant gas concentrations reach the background level, the plasma is then extinguished. Once the chamber has been flushed with hydrogen the heater is set to cool down slowly. The sample is then removed through the loadlock and analysed.
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dust is then blown offby dry nitrogen, the sample is then cleaned successively in acetone, methanol, very dilute Decon solution and de-ionized water in an ultrasonic bath. Finally, the sample is dried by blowing with nitrogen. An alternative mechanical prenucleation treatment has recently been filed for a patent ~9, and non-mechanical treatments using growth with dc bias on the substrate are suitable for some materials. The stability of pre-nucleated surfaces produced by the scratching technique has been investigated. It has been found that an 11 week delay between scratching a sample and depositing on it has no effect on the nucleation process. After preparing the sample, it is placed into the argon purged loadlock which is pumped down to its base pressure of ~ 1 × 10- 6 mbar before being isolated from the pumps. The main loadlock-chamber gate valve is opened and the sample transferred into the evacuated growth chamber. Once the transfer arm is retracted, the loadlock is again pumped to its base pressure. The main chamber is then pumped down to ~ 1 x 10-8 mbar. The heater cavity is flushed with argon to ensure that there will be no oxidation or corrosion during heating. Cooling water is supplied to the required areas : the microwave transmitting window (2.4 1 min '), the heater electrodes, the heater flanges and the heater jacket (a total of 30 I m i n r ) . In order to prevent thermal shock to the chamber windows, the substrate is always heated up gradually under hydrogen at approximately the same pressure as will be used in the subsequent growth run. The heater is set to ramp to the required power, typically to 45% of its maximum power in steps of 0.5% per minute. Once the required temperature is reached, the Roots and rotary pumps are switched on, the purge gas flows are set and the diffusion pump inlet valve is closed. A flow of hydrogen is set
Figure 6. A scanning electron micrograph of a polycrystalline diamond film, deposited by microwave plasma CVD. 505
M G Jubber et al: Design of a UHV reactor
16. Conclusion
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D i a m o n d films have been successfully grown over the entire surface of 100 mm diameter substrates using this custom designed system. A m o n g the substrates coated are silicon, molybdenum, aluminium, stainless steel and quartz. The isolated plasma ball and U H V chamber produce highly pure films as determined by L I M A and CL analyses. A detailed series of experiments has been carried out in order to understand the role of the various process parameters, e.g. gas flow rate and gas composition, substrate temperature, microwave power and plasma conditions. Deposit quality has also been correlated with results of mass spectroscopy, optical emission spectroscopy, specular and diffuse reflectance, and ellipsometry. The customized system is more suitable for scientific study of the diamond C V D process than presently available small commercial reactors whilst producing films of area and quality suitable for application testing.
Acknowledgements the predominant (100) orientation are characteristic of diamond films grown within a narrow range of determined conditions. The diamond nature is confirmed by the presence of the characteristic 1333 cm -~ peak in the R a m a n spectrum shown in Figure 7 and the presence of the diamond 2-phonon peaks in the F T I R spectrum in Figure 8. G r o w t h rates as high as 2 #m h t have been obtained at the substrate centre. The film thickness variation from the centre to the edge of the substrate is consistent with a simple cosine flux distribution of reactant species coming from the plasma. Laser Ionization Mass Analysis (LIMA) has been carried out and the films found to contain only carbon and hydrogen ~~. The samples have been subjected to a variety of analytical techniques including X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), cathodoluminescence (CL), and X-ray diffraction, the results of which have been published elsewhere 11,20
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We wish to thank the U K Science and Engineering Research Council ( D K M ) and The Royal Society for research grants supporting this work. M G J is supported by an M O D contract. We also thank Prof G D Pitt of Renishaw Transducer Systems Ltd for the Raman analysis.
References P Bachmann, Phys World, 4, 4, 32 36 (1991). 2y Liou, A lnspektor, R Weimer, D Knight and R Messier, J Mater Res, 5, 2305 2312 (1990). 3R S Yalamanchi and K S Harshavardhan, J Appl Phys, 68, 5941 5943 (1990). a E Kondoh, T Ohta, T Mitomo and K Ohtsuka, J Appl Phys, 72, 705 711 (1992). K Kurihara, K Sasaki, M Kawarada and N Koshino, Appl Phys Lett, 52, 437 438 (1988). 6K A Snail, C L Void, J A Freitas and L M Hanssen, Diamond Depositions: Sci Technol, 2, I (1991). Vp Bachmann and R Messier, Chem Engn9 News, May, 24~39 (1989). Reported in Diamond Depositions: Sci Technol, 2(2), 15 (1991). 'JM W Geis, In Diamond, StTicon Carbide and Related Wide Bandgap Semiconductors (Edited by J T Glass, R Messier and N Fujimori), Materials Res Soc Syrup Proc, 162, pp 15 22. Materials Research Society, Pittsburgh, Pennsylvania, USA (1990). 0G-It M Ma, Y H Lee and J T Glass, J Mater Res, 5, 2367 2377 (1990). ~ M G Jubber, J I B Wilson, 1 C Drummond, P John and D K Milne, DIAMOND 1992, Heidelberg, September 1992. Diamond and Related Materials, 2, 2 4, 402-406 (1993). I~S J Harris and L R Martin, JMater Res, 5, 2313 2319 (1990). ~3y Saito, K Sato, H Tanaka and H Miyadera, J Mater Sci, 24, 293 297 (1989). Hy Liou, A lnspektor, R Weimer and R Messier. Appl Phys Lett, 55, 631 633 (1989). ~ J A Mucha, D L Flamm and D E Ibbotson, J Appl Phys, 65, 34483452 (1989). ~6C-P Chang, D L Flamm, D E lbbotson and J A Mucha, J Appl Phys, 63, 1744 1748 (1988). ~7UCAR Specialty Gases Product Specifications lbr H2, CH4 and CO. BOC Special Gases Catalogue for Ar and O,. JSB Marcus, M Mermoux, F Vinet, A Campargue and M Chenevier. SurJaee Coatings Technol, 47, 608 617 (1991). "~P John, 1 C Drummond and J I B Wilson, Diamond Film Deposition, UK Patent Application 9211107.9, filed 26th May 1992. ;°A Cook, A G Fitzgerald, B E Storey, J I B Wilson, P John, M G Jubber, D Milne, I Drummond, J A Savage and S Haq, Diamond and Related Materials, 1,478-585 (1992).