THIN FILM GROWTH by PECVD

Transcription

1 THIN FILM GROWTH by PECVD This manual consists of four sections: section 1 and 2 provide a brief introduction to chemical vapour deposition and plasma technique respectively, section 3 gives the detailed procedure of the experiments for thin film growth by plasma-enhanced CVD (PECVD), section 4 introduces several commonly used characterization techniques of the grown thin films. 1. ABOUT CVD Chemical vapor deposition (CVD) is a technique of modifying properties of solid (substrate) surfaces by depositing a layer or layers of materials of interest on the substrate through a vapor phase chemical reaction at elevated temperatures. CVD can offer many advantages, such as excellent conformal coverage of complex structures, simpler deposition equipment, low temperature film fabrication and the ease of large scale deposition. It has rapidly developed in recent years as a method to prepare thin films for a variety of applications in the solid-state-electronics and coating industries. Typical materials deposited by CVD, and applications of the CVD techniques are summarized in Table 1 and 2, respectively. Table 1 Material Metals Compounds Ceramics Typical Materials Deposited by CVD Example Al, As, Be, Bi, Co, Cr, Cu, Fe, Ge, Hf, Ar, Mo, Nb, Ni, Os, Pb, Pt, Rc, Rh, Ru, Sb, Si, Sn, Ta, Th, Ti, U, V, W, Zr, also carbon and boron II-VI and III-V compounds, borides, carbides, nitrides, and silicides of transition metals, as well as sulfides, phosphides, aluminides, etc. Al 2 0 3, AlN, B 2 O 3, BN, SiC, Si 3 N 4, UO 2, Y 2 0 3, ZrO 2, etc. Page 1 of 15

2 Table 2 Applications of the CVD Technique Tribological coatings Wear-resistant coatings Emissive coatings High-temperature coatings for oxidation resistance Coatings for fibre composites Photovoltaic films Decorative Films Superconducting Films Dielectric insulating films Free-standing structural shapes Optical/reflective films Powders and whiskers Three fundamental steps are involved in a CVD process (see Figure 1), i.e. the transportation of the component elements in the vapour phase (or often in a carrier gas), the chemical reaction which takes place in a gaseous medium surrounding the substrate at elevated temperatures, and the removal of volatile reaction-by-products. Therefore a modern CVD equipment generally contains the following units: gas or volatile liquid/solid sources, a gas distribution and mixing system, a reaction chamber (often, with a vacuum pumping system), a system for providing the energy for the reaction and for heating the substrates; and a neutralization (disposal/scrubber) system for the exhaust gases. Main flow of reactant gases Gaseous by-products Boundary layer Interface (negligible thickness) Substrate 1. Diffusion in of reactants through boundary layer 2. Absorption of reactants on substrate 3. Chemical reaction takes place 4. Desorption of absorbed species 5. Diffusion out of by-products Figure 1 Sequence of events during deposition Page 2 of 15

3 The chemical reactions between various constituents occur normally in the vapour phase over the heated substrate, and the film is deposited on the surface. In some cases, however, a film can also be formed from a reaction between the substrate surface and one or more of the constituents of the vapor phase. Therefore gas flow rate, gas composition, system pressure, deposition temperature, and chamber geometry are all important process variables by which thin film deposition is controlled. CVD reactor designs can be classified into the atmospheric-pressure and low-pressure CVD categories. Atmospheric-pressure CVD (APCVD) has disadvantages including the need for a large stream of carrier gas, large apparatus size, and high levels of contamination. Low-pressure ( torr) CVD (LPCVD) has undergone significant development in the past years. It allows removal of carrier gas and uses only small amounts of reactive gases at low partial pressures. In conventional CVD, the reaction is thermally activated. In what is called a hot-wall reactor the whole reactor reaches the process temperature, the substrate residing in an isothermal environment produced by uniform furnace heating. In a coldwall reactor the heated area is limited to the substrates or the substrate holder. There have been recently developed different variations of conventional CVD method in order to lower the reaction temperatures and to enhance the deposition efficiency. These include metalorganic CVD (MOCVD), laserinduced (LCVD), photo-enhanced CVD (photo-cvd), plasma-enhanced CVD (PECVD), and microwave electron cyclotron resonance CVD (ECRCVD). In the MOCVD process metalorganic compounds, which are sufficiently volatile and have low decomposition temperature to desired materials, are used as CVD precursors so that the reactive molecules can be transported in the gas phase in high enough concentrations to allow reasonable film deposition rates and low deposition temperatures. As for LCVD, photo-cvd and PECVD the activation of the chemical reaction and thus the low deposition temperature is achieved by using a laser (for LCVD), UV light (for photo-cvd), or plasma (for PECVD) source. LCVD can be divided into two broad categories, pyrolysis and photolysis, depending on the mechanism that initiates the chemical reaction. In the former case the ambient gases react with the locally heated substrate, while in latter photons of sufficient energy are absorbed mainly to activate gaseous reactant atoms or molecules. Photo CVD Page 3 of 15

4 relies on high energy photons for selective absorption. The commonly used sources are low pressure Hg discharge lamps or excimer lasers. PECVD is yet another versatile technique for depositing a variety of materials for microelectronic, photovoltaic, and many other applications. In the present experiment a PECVD system manufactured by PlasmaQuest is employed to grow films such as Si 3 N 4 or SiO 2 on Si(100), (111), KBr pellet, or other substrates. 2. ABOUT PLASMA A plasma is a complex gaseous state of matter comprising free radicals, electrons, photons, ions, and various neutral species at many different energy levels. Natural plasmas exist mainly as stars of our universe. Lightning is among few examples of natural plasmas on the earth. Manmade plasmas can be produced in thermonuclear reaction, electric arc, combustion flames, low pressure gas discharge and so on. In PECVD the plasma is generated and controlled by ionizing a gas with an rf electromagnetic field of sufficient power, although dc and microwave fields have also been used. The primary role of the plasma is to promote chemical reactions. While the bulk gas is at room temperature (~ ev), the temperatures (the kinetic energy) of free electrons in the ionized gas can be times higher, thus producing an unusual, chemically reactive environment at ambient temperatures, the average electron energies (1-20 ev) in the plasma being sufficient to ionize and dissociate most type of gas molecules. Although electrons are the ionizing source, collisions involving excited species can lead to the formation of free radicals and can assist the ionization process. Once ionized, excited gas species react with surface of materials placed in the gas glow discharge, resulting in dramatic modifications to the surface and enhancing the deposition efficiency. In addition high energy UV photons, resulting from combination and relaxation of dissociated species, may be absorbed by the substrate, creating even more active sites. The colour of the glow discharge depends on the plasma chemistry, and its intensity depends on the processing variables. Note that the plasma process modifies only several molecular layers, thus the appearance and bulk properties are usually unaffected. Page 4 of 15

5 In order to ensure the high quality and the reproducibility of a given plasma process many parameters must be controlled with care, such as the pressure and the flow rate of the reagent gas or gas mixture, the discharge power density, the substrate temperature and the electric potential of the workpiece and so on. The film deposition rate depends on the rf energy, the mole fraction of the reactants, the total pressure, the substrate temperature, etc. Plasma equipment usually consists of six functions: vacuum system, power supply, matching network, power monitor, reactor chamber, and controller. Low-pressure plasma is generally produced in the pressure range from 0.1 to 10 torr, with a continuous gas flow into the reactor. The vacuum system must be able to maintain this pressure/flow regime. The plasma excitation power generally ranges from 50 to 5000 W (0 to 500 W for our PlasmaQuest system). The electromagnetic energy can be input by different coupling methods (inductive or capacitive), electrode configuration (a coil or two plates), and frequency (DC, AC, RF to MF). DC plasmas are not advantageous. Most plasma reactors use AC electrical power supplies, operating at audio-, radio-, or microwave frequencies. Commercial plasma systems usually operate in certain fixed frequencies, i.e. the ISM frequencies specific for industrial, scientific, medical applications other than telecommunications. These include low frequency (LF, khz), radio frequency (RF, or MHZ), and microwave (MW, 915 MHZ) frequencies. Although there has been a gradual shift from RF to MW in recent years, RF plasma is widely used because of easy, reliable operation as well as sufficient activity. For RF plasmas a matching network is necessary to match the impedance of the plasma to that at the generator output. Most plasma systems are designed for batch operation, which involves loading a batch of samples, evacuation, plasma processing, purging to atmospheric pressure, and the removal of the samples. 3. EXPERIMENTAL PROCEDURES The goal of the experiment is to deposit Si 3 N 4 (or SiO 2, or other materials of interest) thin films on both Si(100) and KBr substrates with PlasmaQuest, and then characterize the films grown with Fourier transform infrared spectroscopy (FTIR) or other analysis methods. Page 5 of 15

6 3.1 Preparation of the substrates Si chips cut from a Si(100) or (111) wafer are dipped in a concentrated HF solution for 5 mins, then rinsed with deionized/distilled water, and dried with a soft tissue paper. This pre-cleaning is designed to remove the native silicon oxide as well as contaminations from the substrate surface to facilitate the subsequent film growth. KBr pellets are prepared from pure KBr powder, which is wellgrounded, spread on a die, and pressed at < 8 tonne/m 2 pressure to give a self-supporting disk. The thickness of the pellets is around 1 mm, but not strictly important, in order to prevent the pellets from breaking into pieces during the process. 3.2 CVD reaction chamber with water cooling system The reaction chamber, which is made of welded stainless steel, is constructed of an RF biased gas showerhead and a non-biased resistively-heatable chuck, on which substrates are loaded (Figure 2). Figure 2 Schematic diagram of the PECVD system Page 6 of 15

7 Water cooling is provided for the chuck to chamber transition flange, turbo pump, and the RF matching network, to prevent overheating of the transition seal at ultra high chuck temperatures, and RF generator system. To start the experiment the chiller controller must first be turned on (the switch is located on the chiller, NESLAB RTE-111) so that the green light indicator of the deionizer (located near the chiller and the roughing pump) is on. Chiller reading should be set at 20 C (using "set point adjust" fine and coarse knobs). If the chilled water flow is not properly set, the RF power supply, RF matching system, chuck heater, or turbopump will be disabled. 3.3 Process vacuum system The process vacuum system consists of a turbomolecular pump backed by a mechanical pump (see Figure 2). This pump package provides the reaction chamber with sufficient pumping capacity to maintain high gas flows and low process pressure required for advanced processing. The process chamber should remain under vacuum at all times except during maintenance or sample loading. Turn on the roughing and backing pump switches (located on the wall of the lab) and be sure they are operating. 3.4 Electronics Rack Switches of the power supplies for the whole machine, computer, RF generator and automatic matching network are located in the front of the electronics rack with the main frame of the machine. To start the machine, turn on these switches one at a time, from the bottom to upper panel. 3.5 System computer In our PlasmaQuest system all the processes are automatically controlled by a microprocessor that can be programmed to remember Page 7 of 15

8 recipes, to determine all the machine parameters and the sequence of the processing steps. The processing information including the plasma power is displayed on a screen, so that the operator is constantly informed. Command entry is through the light pen or keyboard. ID and password are required for starting the computer, and they (PQ/752762) can be found from the label attached to the keyboard. Once the system software is booted up, it will be in the Status mode, and five icons, including Service, Edit, Run and Configuration, will be displayed for selection in all software modes. Using the mouse, enter into Service Mode and select System Reset (in the right column of the Service Mode screen) to automatically place the reactor in its idle condition by pumping down the process chamber to process base pressure setpoint (<3x10-5 torr). The chamber will be first pumped by the roughing pump through the roughing valve. At the pressure below 40 mtorr the throttle valve will be opened to speed up the pumping rate. When the process chamber reaches crossover set point (~ 20 mtorr), the roughing valves close. Turn on the turbo pump switch in the front of electronic rack, and the chamber is pumped by it. 3.6 Unloading/loading In order to load/unload samples the process chamber has to be vented with N 2. The valve to the N 2 cylinder should be opened now. By activating Service Mode, followed by selecting the System ATM and selecting Yes, the valves connected to the pumps will be closed and N 2 allowed into the chamber. The yellow clock on the computer screen will disappear when the chamber reaches atmospheric pressure. Once vented, turn clockwise the key switch in front of the chamber to open it. Turn the key back to its neutral position after the chamber wall (with the shower head) has been lifted to open. Page 8 of 15

9 Sample unloading/loading can be carried out now. The sample/substrate is put on the chuck. Close the chamber by turning the Key anticlockwise and pressing the break release button next to the Key to ease the chamber wall back to the original position. When the chamber is in the closed position, the machine will be automatically self-tested until the pumping speed of the turbo pump, displayed on panel 3 of the electronic rack, is ~ 830. To pump down the system use light pen to apply system Reset, as already stated in section 3.5. The chamber is then pumped down. Alternatively, pumping can be performed by using the mouse to activate the roughing, throttle (at < 40 mtorr), and hi vac valve (< 20 mtorr) in sequence. 3.7 Process control The process chamber is equipped with a Balzers full-range gauge, (which is used for atmospheric, crossover and base pressure monitoring, and will be separated from the chamber when the gases start to flow into the chamber) and one MKS Baratron absolute pressure transducers, (for process pressure monitoring between 20 mtorr and atmospheric pressure). Process gases enter the reactor chamber through an upstream gas manifold that controls gas flow through the showerhead. Once the chamber pressure is low enough (< 3x10-5 Torr), open the valves of the gas cylinders to be used, and the deposition can be started by activating Run Mode, which allows to execute a given process, whose recipe is stored in memory and can be picked up easily. Upon entry, the computer displays the last process recipe executed. To select an existing recipe (such as SiN and so on) to run, select the Recipe icon at the right side of the screen, and a file Page 9 of 15

10 selection box will be displayed. The user can adjust all parameters by activating Edit Mode through numeric input. The attached is the recipes for the deposition of Si 3 N 4 (see Appendix 1). The process procedures/parameters will be automatically shown on the computer screen. Ask demonstrator for more details. The chemical reaction involved is SiH 4 (g) + O 2 (g) = SiO 2 (s) + 2H 2 (g) for SiO 2 and 3SiH 4 (g)+ 4NH 3 (g)= Si 3 N 4 (s) + 12H 2 (g) for Si 3 N 4. After deposition allow the chuck to cool down (which may take many hours) and follow the unloading procedure in Section 3.6 to remove the sample out for characterization. 3.8 Safety Caution needs to be taken at all times during reactor operation. Some of the basic safety guidelines are listed in the attached paper (see Appendix 2). One Emergency Power Off (EPO) switch is located on the front of the system. Push it to turn off the machine in emergency cases. 3.9 Summary of operation 1. prepare substrates 2. open chamber 3. load substrates 4. close chamber 5. open roughing valve 6. after the chamber pressure lower than crossover pressure, close roughing valve and open high vacuum valve Page 10 of 15

11 7. edit recipe 8. after the chamber pressure lower than base pressure, open the gas cylinder valves, run recipe 9. turn off turbo-pump after the recipe finishes, and close gas cylinder valves 10. after cooling to the room temperature, take out the samples. Note if SiH 4 is used in deposition, N 2 purge the SiH 4 gas line is needed before opening the chamber. 4. CHARACTERIZATION of the GROWN FILMS Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Atomic Force Microscope (AFM), X-ray photoelectron spectroscopy (XPS) and other analytic methods can be employed to characterize the film grown by PECVD. AFM and SEM are used to observe the surface morphology, and XPS is used for analysis of surface composition and bonding structure of the grown thin film. Here only FTIR is briefly introduced. The goal of the basic infrared experiment is to determine changes in the intensity of a beam of infrared radiation as a function of wavelength or frequency ( µm or cm -1 ) after it interacts with the sample. If the IR is absorbed by the sample, it may be converted into vibrational energy of molecules (pairs or groups of bonded atoms) in the sample. As the vibrational energy is characteristic of molecules, FTIR can identify specific functional groups, providing chemical bonding information about the sample. A complete assignment of all the absorption bands is possible in favourable cases (e.g. see Figure 3 for the IR spectrum of Si 3 N 4 ). Many reference books are available for the interpretation of IR spectra. These include Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th edition, by Nakamoto and Spectrometric identification of organic compounds, 5th edition, by Silverstein, Bassler and Morrill. Page 11 of 15

12 WAVELENGTH [µm] (a) 1620 Å Si-O 1090 cm -1 TRANSMISSION [a.u.] N-H(S) 3340 cm -1 (b) 3 µm CO 2 (AIR) N-H(B) 1200 cm -1 Si-N(A.S) ~850 cm -1 SUB. Si-N(S) WAVENUMBER [cm -1 ] Figure 3 Infrared transmission for (a) thin (162.0 nm) and (b) thick (3000 nm) silicon nitride films pyrolitic-deposited on silicon substrates with NH 3 /SiH 4 =200. In this experiment the sample using KBr pellets as the substrate can be tested using transmission FTIR, in which the absorption bands are obtained by passing infrared beam through the sample since KBr is transparent to IR. For the films deposited on Si chips transmission FTIR can also be used, in which Si wafer should be recorded firstly as background. The alternative way is using specular or diffuse reflection absorption techniques, where IR beams reflected will be recorded to give IR spectra. REFERENCES PlasmaQuest Series H Operation Manual Joy George, Preparation of thin films C.E. Morosanu, Thin films by chemical vapour deposition, D. Satas, Coatings technology handbook, LJY Page 12 of 15

13 APPENDIX 1 PROCESS DEMO System: 157 Type: Si 3 N 4 Demo: Results: SiO 2 Demo: Results: Series II NH 3 = 18 sccm; 5% SiH 4 in Ar = 130 sccm; Process Pressure = 700 mtorr; RF Power = 40 Watts; Temperature = 350 C; Time duration = 150 sec. Center = 2.08 r.i.; nm; Avg. = ± 1.7 nm; Uniformity 1.5% based on 9 measurements; Rate = 43.5 nm/min. Sample repeatable from wafer to wafer. O 2 = 12 sccm; 5% SiH 4 in Ar = 130 sccm; Process Pressure = 580 mtorr; RF Power = 40 Watts; Temperature = 350 C; Time duration = 60 sec. Center = 1.46 r.i.; nm; Avg. = ± 5.2 nm; Uniformity 4.9% based on 9 measurements; Rate = nm/min. Sample repeatable from wafer to wafer. Page 13 of 15

14 APPENDIX 2 SAFETY GUIDELINES Followings are some of the basic safety guidelines for working with the reactor: 1. Operation of this reactor may involve use of highly toxic and hazardous gases which can be lethal in case of an accident or if not handled properly. It is strongly recommended that a minimum of two people be present when hazardous or toxic gas is being used in the reactor. Respirator masks should be available in case an accident occurs. 2. When connecting or disconnecting gas lines it is strongly recommended that a minimum of two people be present when hazardous or toxic gas is plumbed to the machine. Respirator masks should be worn when working on hazardous or toxic gas lines. It is good practice to wear a respirator mask when working on gas lines of any type. 3. High pressure compressed gas cylinders should be stored in vented gas cabinets or in a contained gas storage area and securely fastened with belts or chains to the wall. 4. Toxic or hazardous gases should be shut off at the cylinder when not in use (overnight, over the weekend) or in the event of fire, earthquake, or other emergency. 5. Automatic cut off valves, N 2 purged interlock systems, and gas detectors are recommended for toxic and hazardous gases. 6. High voltages are present in the system, and caution should be taken when electrical components or wires are to be worked on. Turn off the power circuit breaker to the system component that is to be serviced. 7. Potential danger from high-voltage sources has been minimized as much as possible in the design of the reactor. Voltages have been labelled according to their degree of hazard, however the operator needs to exercise care when working near the following high voltage components: * Turbomolecular pumps & controllers * RF generators * RF network and controllers Page 14 of 15

15 * Main power distribution enclosure * Internal power distribution box * Remote power distribution box * Transformer trailers, for systems requiring 50 Hz operation. 8. Chamber cleanup may expose reaction by-products which can be extremely hazardous. The degree of hazard will depend on the process used in the reactor. The operator should wear safety glasses, acid gas respirator, rubber gloves, and lab coat with long sleeves to minimize exposure to hazardous residue by-products while cleaning the reactor chamber. 9. The heated chuck can reach high temperatures and should be allowed time to cool before working on or near the chuck. 10. Mechanical pump oil may contain acidic or toxic residues. Wear rubber gloves, safety glasses, and an acid gas respirator when changing pump oils. 11. Safety precautions and warnings in the component manuals, located in volume 2 of this manual, should also be read and understood by the user. Page 15 of 15