THIN FILM DEPOSITION PLASMA PROCESSING VACUUM INSTRUMENTATION JULY 2003

Transcription

1 THIN FILM DEPOSITION PLASMA PROCESSING VACUUM INSTRUMENTATION JULY 2003

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3 By monitoring gas species at various locations on a vacuum deposition system, it is possible to highlight process excursions and contaminants so that product quality and yield can be optimized while maximizing process system uptime. Such locations would typically include the point of delivery of bulk process gas and the in situ monitoring of various zones in the deposition process itself. However, the gas analyzer deployed depends upon a number of factors, such as the gas pressure, species of interest, and composition range. Two practical and cost-effective methods for accurate and rapid on-line gas analysis include residual gas analysis and Fourier transform infrared (FTIR) spectrometry. Together these gas analysis technologies enable process engineers to maximize productivity and quickly achieve a return on investment. FTIR technology has been used successfully to monitor impurities in ammonia bulk gas. Trace amounts of water and oxygen that may be present in the incoming ammonia have been shown to reduce device yields of subsequent GaN-based blue and white LEDs. Although there are several purifiers on the market today to remove water and other contaminants from the high-purity NH 3 gas used during the metal organic chemical vapor deposition (MOCVD) process, a need exists to monitor the gas from the purifier. This is done to ensure that the resulting contamination levels are within specifications, which are typically in the low ppb range. In situ analysis of a vacuum deposition process can help one to gain a better control of the process, thereby improving product quality and yield. In large surface area glass coating, for example, monitoring for vacuum quality and the presence of contaminants can 3

4 be vital. Gas composition tracking also is critical during the deposition process itself. Traditionally, for monitoring process pressures approaching 8 mtorr ( mbar), residual gas analyzers (RGAs) requiring expensive and complex differential pumping systems have been the only option. However, with the advent of high-pressure compact quadrupole RGAs, in situ operation at these pressures has become a cost-effective reality. FTIR Theory of Operation FTIR exploits the phenomenon that most gases absorb infrared radiation with a characteristic, molecule-specific absorption spectrum. Spectroscopists often refer to the mid-infrared spectrum (2 to 25 µm) of a molecule as the fingerprint spectrum because the frequencies absorbed are unique to each molecule. When IR radiation traverses a gas sample, the measured infrared spectrum, along with temperature and pressure information, can be used to extract the absolute gas concentrations or partial pressures within the sample. IR measurements performed with high signal-tonoise and excellent photometric accuracy produce a powerful technique for both qualitative and quantitative analysis of gas mixtures, even when the mixture includes similar materials. Although other methods have been employed for infrared spectroscopy, FTIR methods reign supreme in the areas of signal-to-noise and photometric accuracy. At the heart of every FTIR is an interferometer that is employed to modulate the radiation provided by an IR source (usually a ceramic or nichrome thermal glower). The interferometer acts to amplitude modulate the infrared energy at each wavelength (or optical frequency) at a different audio frequency proportional to the optical frequency. For example, an FTIR might modulate energy at 10 microns (1000 cm -1 in frequency units) at 2 khz, and energy at 5 microns (2000 cm -1 ) would then be modulated correspondingly at 4 khz. To perform a gas phase measurement, the modulated infrared beam is passed through a gas sample, usually contained in a cell equipped with a pressure transducer, a temperature sensor, heaters, and infrared transparent windows. In the gas cell the infrared beam is attenuated by infrared absorption from the gas sample. After passing through the cell, the infrared radiation is focused onto an infrared detector and converted to a digital signal with a high-fidelity A to D converter. This raw time domain signal, referred to as an interferogram, contains the spectral information in the signal encoded as amplitude modulations, with each wavelength modulated at a different frequency (Figure 1, Left). A fast Fourier transform (FFT) is then used to recover the beam amplitude spectrum in units of optical wavelength or frequency. This transformed spectrum is called the single beam spectrum (Figure 1, Center). The single beam represents the product of the source intensity, instrument transmission, gas absorptions, and detector responsivity, as a function of Figure 1. Raw interferogram (infrared intensity verses time), single-beam spectrum (infrared intensity verses frequency), and absorbance spectrum (absorbance of IR light verses frequency). wavelength or frequency. Absorbance spectra are then normally computed by taking the Log 10 of a background spectrum to the sample spectrum ratio (Figure 1, Right). The background spectrum is essentially a single beam collected while the cell is purged with an infrared transparent gas such as nitrogen. The absorbance domain is normally used because the measured absorbance is roughly linear with sample concentration. Data Analysis Most FTIR systems employ a derivative of the classical Michelson interferometer, an instrument whose roots can be traced back to the late 19 Century. Modulation in a Michelson interferometer occurs by dividing a light beam into two approximately equal halves at an optical surface called a beamsplitter. This beamsplitting affect is similar to looking through a window at night, the observer can see reflected as well as transmitted light. The two approximately equal halves of light propagate to two mirrors and are reflected back to the beamsplitter. If the two beams travel exactly the same distance in their respective optical paths within the interferometer, constructive interference would occur for all wavelengths (frequencies) at the beamsplitter, generating a large signal amplitude (called the zero path difference centerburst ). In an FTIR one or both mirrors that reflect the IR light back to the beamsplitter are scanned (moved) such that at any given time the two paths in the interferometer are of different lengths. As the mirror(s) scan, different wavelengths go in and out of phase depending on the distance difference between the two mirrors. Because a mirror will scan through a complete interference fringe at five-micron wavelengths twice as quickly as it will at ten-micron wavelengths, one can see how a Michelson interferometer imparts the characteristic frequency proportional amplitude modulation to an infrared signal. To be more specific, consider, for example, if a quasi single wavelength or frequency from say, a laser, were passed through a Michelson interferometer. The 4 vtcmag@snet.net July 2003 Vacuum Technology & Coating

5 Figure 2. Absorbance spectrum of high-purity ammonia. The ammonia calibration spectrum is shown in red. resulting signal would then be a pure cosine wave at some frequency in the Hz to MHz region depending on the speed of the moving mirror and the original frequency of the light. The equation for the imparted amplitude modulation frequency for a Michelson interferometer is: F f = 2 v v f where F f is the imparted amplitude modulation frequency in Hz, v is the speed of the moving mirror in cm/sec, and v f is the IR frequency in wavenumbers (cm -1 ). So, at 5000 cm -1 (2 µm wavelength) the modulated frequency at 1 cm/sec mirror speed would be 10 khz and at 500 cm -1 (20 µm wavelength), and the modulated frequency would be 1 khz. If one delves a little deeper into the theory of Fourier transforms, one also finds that the distance that the mirror moves determines the spectral resolution of the spectrometer. The equation for resolution on a Michelson interferometer is 1.0 cm, the resolution of the spectrum will be 0.5 cm -1. Gas concentrations are generally extracted from the absorbance spectrum using regression methods. Though there are many forms of regression analysis, they all, in their simplest form, generate a synthetic spectrum to fit the measured spectrum. When applied to IR gas analysis, the fit is generated by adding previously stored spectra of the component gases with appropriate weighting coefficients to best fit the measured absorbance spectrum. The weighting coefficient determines the level or concentration of each component in the sample. Using this approach, along with the uniqueness inherent in each molecular infrared spectrum, tens if not hundreds of components can be analyzed simultaneously from a single infrared spectrum. An ammonia sample absorbance spectrum is shown in Figure 2. This spectrum is slightly different from traditional absorption spectra in that the sample is processed at two resolutions. These two resolutions are compared to one another [Abs = Log (Low Res Spec/ High Res Spec)] and the resultant spectrum appears be similar to a differential spectrum. (This type of spectrum is useful when baseline stability is paramount.) In Figure 2, the measured, or sample spectrum is shown in white, and the stored ammonia calibration spectrum is shown in red. Note that all the complicated spectral features visible in this figure are due to ammonia. The impurities listed on the right side are present and measurable, but their concentrations are so low that the spectral features due to the impurities are too small to be seen in the spectrum with visual inspection. As mentioned above, all the calibration spectra are added together at their appropriate concentrations to match the sample spectrum. If a high-quality match is generated when the two data sets are subtracted from one another, all that should remain is noise, as seen in Figure 3. The fragmented spectra shown in Figure 3 are the residual spectra computed from the regression. The regions where data are R = 1 / 2d where R is the resolution of the collected data set d is the distance the mirror has moved in cm For example, if during a scan the mirror travels a maximum displacement of Figure 3. Spectral residuals for the analysis of CO, CO 2,CH 4,NH 3, and H 2 O in ammonia. The axes are absorbance verses frequency. Vacuum Technology & Coating July

6 Photo 1. The MultiGas Purity Analyzer, an FTIR-based analyzer for real-time impurity analysis in bulk gases. shown are limited to the analysis regions for each material. A low residual across the entire spectrum indicates that all the component gases are have been accurately measured because the component calibrations do a good job of accounting for all the observed absorptions in the spectrum. When the match deteriorates, this generally means that a new gas is present in the spectrum with no corresponding component calibration, or that the instrument is malfunctioning or running with improper settings. Thus, monitoring these residuals visually or mathematically provides the users a simple method for checking for the presence of unexpected gases or instrument faults. The residual spectrum also is useful in that it can provide a visual appreciation of the detection limit. In cases where there is a strongly overlapping material, as is often the case with gases with many spectral features spread over a wide wavelength range such as ammonia, the high concentration material will ultimately control the lowest possible level of detection for overlapping impurities. As this detection limit is reached the uncompensated features of ammonia will start to appear in the residual spectrum above the noise level. The H 2 O residual spectrum in Figure 3 contains no strong ammonia features, which suggests that a lower detection limit could be obtained with a more sensitive detector or longer path length gas cell. However, in the case of CH 4 there appear to be features greater than the noise level, which suggests that the instrument is nearing a detection limit that is controlled by the ammonia interference. FTIR Instrumentation for Improving Yields in LED Manufacturing Analytical instrumentation has developed to where ppb and lower impurity levels can be detected in bulk pad and process gases. Historically analyzers have been developed that monitor one or two impurities in these gases in real-time. The calibrations, if supplied with these systems, drift over time requiring continual calibration and certification. Recent advances in FTIR instrumentation have eliminated these problems. An example of such an Figure 4. Plot of moisture concentration in vapor phase ammonia when the cylinder is near empty. (Figure supplied courtesy of BOC Edwards Electronic Materials.) FTIR-based analyzer that includes multiple impurity calibrations and the methods for analyzing these components simultaneously and in real-time is the MKS MultiGas Purity Analyzer (Photo 1). This integrated analyzer has certified calibrations that remain constant to within a few percent of the reading. The Purity analyzer is composed of a high-resolution rugged process FTIR spectrometer and a high optical throughput gas sampling cell with construction features designed to ensure low background contamination levels within the analyzer. Application-specific analysis software and calibrations for the impurities of interest are supplied with the analyzer. The FTIR spectrometer has a spectral resolution of 0.5 to 128 cm -1 and a measurement range from ppb to percent levels. The gas cell has a small 200-mL volume that produces very fast time response to impurity changes, and the gas cell mirrors have proprietary aberrationcorrecting prescriptions that optimize sensitivity and optical throughput. Sophis-ticated data collection and analysis software acquires and processes spectra while computing the individual impurity concentrations simultaneously. In addition, the software performs automatic corrections for gas pressure and temperature variations, which are directly measured by the analyzer. The Purity analyzer has proven a particularly valuable tool for the production of high-purity ammonia. High-purity bulk ammonia (NH 3 ) gas is commonly used in the production of GaN high brightness blue and white LEDs. The NH 3 delivered to the fab can have a wide range of impurity grades from ppm to ppb or lower levels. If the gas is supplied with very high purity certifications, singledigit ppb or lower impurity levels, minimal additional purification is generally necessary. High-purity gases also allow a greater overall utilization of the cylinder. Of course, gases supplied with higher certifications tend to have higher costs per liter. NH 3 can be economically supplied with higher impurities, providing purifiers are added to the gas train. When lower grade NH 3 is used, however, there is less utilization of the cylinder due to rising impurity levels as the remaining 6 vtcmag@snet.net July 2003 Vacuum Technology & Coating

7 ammonia level drops. Thus, although lower quality gas is less expensive, there is an increased fab cost for on-site purification and quality assurance. The issues of purity grades and purification costs need to be weighed to determine the best solution for the fab. Regardless of whether high- or lowpurity NH 3 grades are utilized, purifiers are generally required upstream of critical processes, such as GaN fabrication, to ensure the product yield. Two purifier styles are used commonly: point of use purifiers for low-flow applications (~1 to 100 slpm) and fab wide bulk purifiers for high flows (>1000 slpm). Several purifier technologies are commercially available today to remove water and other contaminants from the NH 3 gas. These purifier technologies are mostly based on metal oxide, metal, or zeolite materials. Ammonia purity can be a critical process parameter; it has been proven that trace amounts of water and other oxygen containing contaminants in the NH 3 gas during MOCVD process adversely affect the yield of the corresponding blue and white LEDs, sometimes by as much as 90%! Also, it is known that the impurity levels in ammonia vary with time or bottle consumption, as shown in Figure 4, and that the removal efficiency of purifiers varies with age. As impurity specifications for ammonia become tighter, there is a rising need to determine and certify that the impurities are meeting the production specification before the gas enters the process. Tests were conducted at several major bulk gas suppliers to monitor trace levels of H 2 O, CO, CO 2,CH 4, and N 2 O in 100% NH 3 gas, in production environments. Figure 5 demonstrates the degree to Figure 5. Absorbance spectrum of high-purity ammonia (white sample) with water present. The H 2 O calibration is in yellow, and the NH 3 calibration spectrum is in blue. which the H 2 O and NH 3 spectra overlap and the ability of FTIR to differentiate their spectra. Table 1 summarizes the detection levels that were demonstrated. The MultiGas Purity Analyzer has demonstrated through numerous tests at fabs and bulk gases suppliers to be a very valuable tool in obtaining purity information on process ammonia. Its detection limits have been proven to be significantly below even the most critical MOCVD process requirements, providing significant benefit to LED process yields. The analyzer provides production cost reduction as well by allowing the factory to monitor purifier performance continuously and in real-time, which allows purifiers to be switched, regenerated, or replaced only when required, saving significant time and money. Process risk is reduced further through the ability to observe gas purity excursions as they occur, preventing major contamination spikes from damaging either expensive reactors or purifiers. The analyzer itself is well suited for the production environment, with permanent stable calibrations and requiring only an occasional gas cell cleaning, as compared to extensive maintenance and recalibration needed on more traditional Table 1. Typical detection limits achieved by FTIR in real-time production environments. Contaminant H 2 O CO CO 2 CH 4 N 2 O Typical Detection Limits 20 ppb 5 ppb 5 ppb 50 ppb 5 ppb Figure 6. Schematic of quadrupole residual gas analyzer (RGA). Vacuum Technology & Coating July

8 Photo 2. MKS HPQ-2S high-pressure RGA with integrated Baratron capacitance manometer. analyzers. Designed for continuous, realtime high-purity monitoring, the MultiGas Purity Analyzer has had a significant impact on the quality, yield, and manufacturing costs during the production of GaN based LEDs. Quadrupole Residual Gas Analyzer: Theory of Operation A complementary method to FTIR gas analysis is quadrupole mass spectrometry. RGAs analyze gases by ionizing the molecules and atoms, separating the ions by mass/charge (m/e) ratio, and displaying a mass spectrum (plot of m/e versus peaks intensity). All the gas components and fragments are analyzed within a single scan. Where identities of the major components are known, specific mass peaks can be monitored and calibration coefficients applied to obtain the relative composition for each species present. A quadrupole mass spectrometer consists of an ion source, a mass filter, and an ion detector, as illustrated in Figure 6. Various methods of ionization can be used, but electron-impact ionization is most common. During ionization, a steady stream of positively charged ions is created for subsequent separation by the quadrupole mass filter. The ionization process starts by resistively heating a filament that emits electrons. These electrons are then drawn toward a positively charged ion cage, where a few of the electrons collide with atoms or molecules, causing the removal of one or more electrons. This creates positive ions, which are then drawn toward the lower potential of a focus plate and injected into the quadrupole mass filter. The quadrupole mass filter separates ions by mass/charge ratio. The filter consists of four rods, with the opposite rod pairs interconnected, with one rod-pair connected to a positive DC voltage upon which a sinusoidal RF voltage is superimposed. The other rod pair is connected to a negative DC voltage upon which a sinusoidal RF voltage is superimposed, 180 out of phase with the RF voltage on the first set of rods. The net result is that the rod sets acts as high- and low-bandpass filters. By simultaneously varying the amplitude of the DC and RF voltages, an entire mass spectrum can be rapidly scanned. The final step in quadrupole mass spectrometry is to detect the ions after separation. A robust Faraday cup detector is used to measure the ion current directly. With IC-based electronics circuitry, it is possible to determine quickly and efficiently the partial pressures of gases in a vacuum system in the range from 8 mtorr ( mbar) to Torr ( mbar). Advances in High-Pressure Process Monitoring By reducing the geometry of mass filter designs compared to those traditionally used in commercial RGAs, it is possible to operate certain high-pressure RGAs at the pressures encountered during many vacuum deposition processes. With performance consistent with traditional RGA designs, process monitoring at pressures up to 8 mtorr ( mbar) can be achieved without any expensive and complex support-pumping Figure 7. Large area glass coater multi-sensor RGA installation schematic. 8 vtcmag@snet.net July 2003 Vacuum Technology & Coating

9 package, providing a higher level of reliability at a substantially lower cost. The absence of differential pumping, plus associated control electronics, inlet hardware, and cabling results in a sensor package that is compact and consumes very little space around the vacuum deposition system. An example of this type of RGA instrumentation is shown in Photo 2. Access to total pressure information from the integrated total pressure gauge provides a reference for sensitivity correction to ensure a linear response up to 8mTorr ( mbar). In addition, special gas-specific algorithms, which compensate for sensitivity variations resulting from ion-molecule interactions that occur at higher pressures, are required to address the specific characteristics of species including water, oxygen, nitrogen, methane, helium, and hydrogen in argon. These sensitivity and gas-specific calibration factors are stored within the RGA control unit and applied automatically. From a maintenance perspective, the analyzer incorporates two filaments in the ion source, where a second "back-up" filament enables continued operation in the event of a filament failure. The userserviceable analyzer designs allow the ion source to be removed and dismantled for cleaning. This reduces the costs and labor associated with maintenance of the RGA sensors. High-Pressure RGA Instrumentation for Improving Yields in a Large Area Glass Coating Application Figure 8. Remote data monitoring and access through dedicated html interface representing the various zones of the vacuum deposition system. A large architectural glass manufacturing facility needed to monitor and control the process gas levels in multiple zones of a large area glass coater because variations in the process gas mixtures could adversely affect the film quality as well as the optical properties of the film stack. Of particular interest was maintaining the process gas conditions (i.e., argon-to-oxygen/nitrogen ratio for reactive sputtering). In addition, there was a desire to verify acceptable base vacuum conditions prior to starting the process, to monitor the contaminants outgassing from the glass substrate and to check vacuum conditions after preventative maintenance interventions. Owing to the physical size of the coating plant and the location of the control room, it was required that the RGA sensors be located up to 660 feet (200 meters) from the server. Furthermore, the live information and stored data had to be accessible over the factory network and key personnel notified immediately in the event of alarm conditions due to process excursions. With base vacuum levels in the range of mtorr ( mbar) and typical process pressures of 3.75 mtorr ( mbar), it was clear that that an RGA that could monitor the higher pressure would be required. However, the costs associated with differentially pumped RGA systems were deemed prohibitive. It was decided that the most cost-effective method would be to integrate multiple high-pressure RGA sensors into the large area coating system, with each sensor monitoring a specific zone in the process. In terms of the software platform and RGA interface required to operate and respond to the multiple RGA sensors, several key requirements were identified: The software had to be fully net work compatible with the ability to view RGA sensor information (and information from other tool sen sors) from any workstation on the network. The RGA interface had to be compatible with operation over large distances. Control and operation of the RGA sensors had to be fully automated with built-in ability to acknowledge different phases of the process and optimize the performance of the sensor during each phase (pump down, baseline, deposition). Automatic calibration would be required to eliminate any sensor drift. Alarm conditions (process excursions) had to be simple to interpret and concise with simple messages explaining the nature of the excursion along with recommended solution. Process Eye 2000 was selected for this purpose because it met these criteria and offered the flexibility to integrate with other software applications. A recipe-based platform, Process Eye 2000 allows a series of recipes to be configured for the optimized monitoring of each phase of the process or tool stage with Vacuum Technology & Coating July

10 Ability to monitor and display data from other critical non-rga parameters (temperature, flow rate, etc.) in relevant units Ability to transfer data to the tool controller software or to the SPC/APC systems Cost savings through use of existing computers and network systems Reduced tool downtime through the ability to check baseline conditions after maintenance to avoid situations where pump downs would not reach base pressure Figure 9. Customized alarm limits and settings, combined with visual depictions of the large area coater, enable quick and effective notification of out-of-specification conditions. associated intelligent alarms. Recipes can be triggered to begin as a result of a change in chamber conditions (e.g., the introduction of a process gas to trigger the process monitoring recipe) or a system hardware event (e.g., the closing of a valve). The distributed architecture of the software platform allowed the multiple RGA sensor heads to be positioned at each of the multiple deposition zones and allowed the use of existing hardware on the coater. A schematic of the installation is given in Figure 7. Once the system is installed, the process engineers were easily able to monitor the process parameters of interest through a dedicated html-based user interface (created within Process Eye 2000), as shown in Figure 8. With user adjustable alarm limits and customized alarm messages as shown in Figure 9, the process engineers were quickly notified if a critical parameter went out of specification before yields were negatively impacted. The sensor control platform also was capable of tracking and alarming on data from other, non-rga sensors (e.g., temperature, pressure, gas flow, power etc.) and displaying data in units relevant to the sensor in question. The process engineers at the glass coating facility experienced many benefits from the in situ high-pressure RGAs on the large area coating systems including: Improved film and film stack yield associated with tighter process control and early warning of deviations from normal processing conditions Real-time monitoring and control of parameters of interest, such as total pressure, nitrogen level, water level, and argon-to-oxygen percentage, without time consuming data conversion Ability to measure at their process pressure without the use of expensive and complex differentially pumped RGA systems Visual indication of out-of-spec conditions tied directly to a specific zone and with the custom messages they desired Convenient, remote viewing of data through a web-based html user interface Automated sensor calibration sequences to ensure optimum sensitivity Summary By using gas analyzers it is possible to increase yields for deposition processes such as those associated with GaN LEDs and those used in large area glass coaters. Specifics of the application dictate which type of gas analyzer (FTIR or quadrupole mass spectrometry) is best suited to a particular application. When the appropriate sensor is chosen, incorporating a realtime, on-line gas analyzer can be a practical and cost-effective method for providing the necessary insight to improve yield and productivity. Peter Rosenthal is the general manager for the On-Line Products Group of MKS Instruments. Peter has 10 years of experience developing advanced infrared technology for applications in gas and thin film measurements, with an emphasis on providing instrumentation for semiconductor and related industrial process control. He earned his doctorate in Applied Physics from Stanford University in 1991 and served as a post doctoral fellow at the National Institute of Standards and Technology from 1991 through vtcmag@snet.net July 2003 Vacuum Technology & Coating