PROBLEM The ASTRON family of remote plasma sources produce reactive gas species for semiconductor device fabrication applications. Reliable ignition of the plasma in the ASTRON depends strongly on the ambient conditions under which the plasma is being struck. Predictable ignition of the plasma can only be achieved when the ignition sequence for the plasma is appropriate for the process conditions and equipment configuration employed in a particular process. BACKGROUND ASTRON is a family of self contained, lid-mountable remote plasma sources that produce reactive downstream chemistries. These sources create reactive gas streams by the passage of precursor gases such as through MKS Instruments patented low-field toroidal plasma technology. ASTRON sources may be installed in a wide variety of configurations on Si wafer or large-area substrate process chambers. ASTRON sources have proven to be effective and economical in providing the reactive gases needed for CVD chamber clean, FPD chamber clean, solar chamber clean, photoresist bulk ash, oxidation, nitridation, and chemical abatement. The best results in the cleaning and other processes that use ASTRON sources can only be assured when reliable and predictable plasma ignition occurs in the process. There are several equipment and process configuration factors that will influence the predictability of the plasma ignition process. Reliable and accurate sensing of the process condition is critical for repeatable operation. Pressure and temperature within the equipment configuration must be accurately known in order to program the system to achieve repeatable plasma ignition. Any materials that are exposed to the reactive gas must be compatible with the harsh environment, otherwise component failure and subsequent leakage of air into the system can occur. The presence of air in the plasma gas will impact the conditions necessary to achieve ignition. Other contaminants in the gas used for plasma can shift the ignition conditions in the system and negatively affect reliable ignition. In addition, damage to materials caused by overheating in the system can produce leakage with concomitant degradation of ignition performance. ASTRON Remote Plasma Source Ignition Best Practices Typical ASTRON Ignition Sequence Figure 1 shows a typical equipment configuration that might employ an ASTRON remote plasma source. The following steps would constitute a typical ignition sequence for this system: 1) Initially, the Ar plasma gas flow is set to 1-4 slm and the pressure, as measured at the ASTRON plasma block exit, stabilized at 1-4 Torr. Flow and pressure specifications may differ depending on the ASTRON model (Table 1). Furthermore, depending on the gas conductance characteristics of the transport region in Figure 1, a significant pressure differential may exist between ASTRON source and the process chamber. Only careful selection and correlation of the process chamber pressure to the ASTRON source pressure, or the use of pressure readings taken close to the ASTRON source can ensure reliable ignition conditions. Model Ar Flow (slm) Pressure (Torr) ASTRON 2 2.0 ASTRON i 2 2.0 ASTRON e/ex 2 2.0 ASTRON hf 2 2.0 ASTRON hf-s 2 2.0 ASTRON hf+ 3 2.0 ASTRON G6/G7 15 15.0 Table 1 - Recommended Flow and Pressure for ignition on various ASTRON units Note: Pressure is measured at ASTRON unit exit flange 2) Once gas flow and pressure are stabilized, the operator sends a Plasma Enable signal to the ASTRON that initiates a series of automatic actions: a. The system applies power to an ignition assembly to initiate gas breakdown. b. The induced electric voltage in the plasma channel is raised, forming a toroidal plasma. c. The spark plug voltage is then disconnected and the power supply transitioned from ignition state to steady state by lowering the plasma loop voltage and raising plasma current. d. The plasma status (Plasma On and other information) is reported to the control system.
Page 2 BACKGROUND (CONT.) 3) The cleaning gas, typically, is then introduced and smoothly increased to the desired flow and pressure while simultaneously changing or eliminating argon if desired. This application note describes the best practices that can be used to ensure reliable ignition performance for the following ASTRON remote plasma sources: SOLUTION ASTRON (2L) ASTRON i (3L) ASTRON e/ex (4-8 L) ASTRON hf (15 L) ASTRON hf-s (15 L) ASTRON hf+ (22 L) Equipment Best Practice ASTRON G6/G7 (30 L) Properly executed system design can assure ignition predictability and success since there are certain components in an equipment configuration that can strongly impact ignition performance. These components include the transport device, chamber flange, and the gas distribution device (i.e. a baffle, a single-stage showerhead, or a multi-stage showerhead; see Figure 1). Following the guidelines below can help in avoiding common ignition problems and other process issues. 1) Materials Compatibility: Contamination due vacuum leaks or the corrosion of chamber components degrades ignition performance and other process characteristics. All materials in the system, including the transport device, flanges, O-rings, baffle or showerhead, vacuum valves, exhaust lines, as well as the chamber itself, must be compatible with the active gas. For example, stainless steel corrodes and Viton O-rings decompose when exposed to atomic fluorine. Aluminum is the metallic process chamber material of choice for active fluorine environments, and perfluorelastomer O-rings are more resistant than Viton to fluorine attack. Additionally, reactive gas recombination, thermal compatibility, and process contamination must be carefully considered when selecting system materials. To minimize activated gas recombination upstream of the chamber, use proper materials (i.e. aluminum for atomic fluorine), avoid sharp bends and keep the transport device as short as possible. 2) Gas Purity: The ASTRON source requires high purity argon as the working gas for plasma ignition. MKS validates ignition performance using 99.998% (Grade D) Argon and recommends this purity (or better) for the ignition gas; any contamination by electronegative gases (i.e. oxygen due to leakage or process gas residues due to poor purging) will reduce the probability of successful ignition. Therefore, the gas delivery system must be designed to avoid any contamination of the ignition argon by other gases. This can be accomplished, for example, by valving off all other process gases during the ignition cycle. Note that flow controllers ARE NOT shut-off valves - do not rely on them to completely stop any gas flow; particularly for high flow ASTRON units, the use of the flow control component for gas shut-off can allow electronegative process gases such as into the ASTRON chamber. Gas delivery designs must also provide provisions to purge gas delivery lines to eliminate residual air and adsorbed water after the line has been opened to ambient or to remove residual process gas (e.g. ) prior to introducing the argon gas in an ignition cycle. Note that, in the presence of silicon product wafers, the purity of argon is dictated by the needs of the product; this may be significantly higher than that needed for chamber etching/cleaning. Figure 1 - Typical ASTRON remote plasma source setup
Page 3 3) Gas Temperature: Since the activated gas exiting the ASTRON source transports a significant amount of heat and chemical energy (up to 500 W per slm of inlet gas) throughout the equipment gas path, it is good practice to monitor the equipment/gas temperature at a number of different downstream locations during initial equipment design testing. System overheating can damage seals, destroying leak integrity. For example, flange O-rings, may fail and leak if overheated. Active water cooling should be added to any areas at risk of material failures or burn hazards due to high gas temperatures. 4) System Pressure Differentials: Showerheads or baffles can create a significant pressure differential between the gas exit of the ASTRON source and the process chamber, where the pressure is typically measured. Because of the harsh chemical and thermal conditions that exist at the ASTRON exit, it is usually neither convenient nor desirable to monitor the pressure at this point. Instead, modeling or experimentation employing an inert gas at equivalent flows should be used to establish a reliable correlation between the ASTRON pressure and the process chamber pressure. 5) ASTRON Source Contamination: The adsorption of certain gases such as fluorine or CVD precursors on the interior surfaces of the ASTRON source may impede plasma ignition. Typically, electronegative process gases adsorbed in this manner exhibit the largest negative effect. While a purge gas can provide some protection for the ASTRON internal surfaces, in certain cases it may be necessary to install an isolation valve between the ASTRON and process chamber. Processes that use liquid precursors, in particular, benefit from an isolation valve. Since the location for this isolation valve experiences harsh conditions of temperature and reactive gas concentrations, only valves that can be properly cooled and that have components made from compatible materials e.g., fluorine-resistant O-rings should be used. Note 1: To determine pressure drop experimentally, add a second pressure measurement device to the transport device as close to the ASTRON exit as possible (see below). Flow inert gas through the ASTRON and record both chamber and ASTRON exit pressures. To correlate inert gas flow to, assume the latter completely dissociates to F and N atoms, with half the N recombining to N 2. For example, to reproduce the pressure drop associated with 4 slm of, use 15 slm of argon or N 2, to replicate the following condition: Note 2: Chamber Pressure ASTRON Exit Pressure P2 4 12F + 2N + N 2 P1 Gas Inlet Substrate ASTRON Transport Region (Cooling Jacket) Gas Baffle Process Chamber Figure 2 - Schematic for setup to determine the pressure drop between the ASTRON remote plasma source and the process chamber In this application note and in any ASTRON specifications, the pressures cited always refers to the pressure at the ASTRON outlet and does not consider any system-dependent pressure drops.
Page 4 Cleaning Process Best Practice A properly designed cleaning recipe can help to ensure successful plasma ignition. The use of the following guidelines can significantly increase the ignition success rate in an ASTRON plasma source that has been properly installed on a process chamber: Note: Even those steps that may seem unnecessary after limited testing in a controlled lab setting will increase the ignition success rate on a fleet of systems installed in a wide variety of manufacturing facilities. Therefore, always employ all of the steps defined below. Figure 3 - Pressure trace for a typical chamber clean 1) Allow sufficient stabilization time in the ignition step of the process recipe. The actual time needed for stabilization will depend on the characteristics of the gas delivery and process chamber pressure control components of the system. Figure 3 shows a typical trace for pressure stabilization in a process chamber where the pressure can be seen to stabilize over time following a set point change. During the development of the process recipe, monitor the pressure as a function of time following a step change. Fully characterize the dynamic pressure response of your process chamber configuration, and select a stabilization time that results in successful ASTRON ignition 100% of the time. The stabilization time can also be used for purging the system of residual trace gases that might impede ignition. Ensure that the purge flow is sufficient to adequately purge the environment in the time allowed. If using flow and/or pressure feedback control with a stabilization end point make sure the endpoint is not triggered during oscillations associated with overshoot. Attempting to ignite during a dynamic step, for example, while gas flow is ramping from one setpoint to another, may result in ignition delays or faults. Although the latter process will typically work on the original system on which it was developed, it will not be robust over time on a large number of systems. 2) Following pressure stabilization and ignition, the system should transition smoothly to the cleaning process. During this step, large gas bursts that may occur on introduction of the or other cleaning gas must be avoided since this could extinguish the plasma. Such bursts can occur when opening gas line valves if an upstream flow controller is fully open, for example. Plasma may also extinguish if the ignition argon flow ceases before the cleaning gas ramps to a level sufficient to sustain plasma. One effective method for avoiding these issues is the use of a 3-step transition in which each step lasts a few seconds: a. with the ignition argon flowing, introduce the cleaning gas at 1/3 of the target flow; b. turn the argon flow off; ramp the cleaning gas flow to 2/3 of the target value; c. ramp the flow of the cleaning gas to its final target value. 3) It is good practice to transition back to argon and continue running argon plasma for several seconds after completing the chamber clean. This is due to the fact that residues from the electronegative cleaning gas can impede the next ignition and a post-clean argon purge/ plasma step can remove these residues. Use the same pressure/flow condition in the post clean step as that employed during ignition.
Page 5 4) The remote plasma source should be purged during the chamber deposition or etch process. This prevents backstreaming by unwanted process gases from the process chamber into the ASTRON plasma block. Such gases can adsorb on ignition assembly components and impede ignition. To avoid this, flow an inert gas, preferably Ar, during process steps in which the ASTRON is not operating. Gas flow need only be sufficient to maintain the ASTRON plasma block at a higher pressure than that of the process chamber; often no more than 0.5 slm is necessary. If there is a risk that the inert gas might interfere with the process, purge with a carrier gas, such as N 2 or H 2. Contact MKS applications for an assessment of any other purge gases. 5) Examples of cleaning recipes for each model are shown below: Models 1 Pre-Ignition 2 Ignition ASTRON (2L) 3 Transition 1 4 Clean 5 Post Purge Ar (slm) 2 Same as step 1 Same as step 1 Same flow as 1-4 (slm) 0 0 1/2 of Total flow final flow 0 Press (Torr) 2.0 Same as step 1 1-10 1-10 Same as step 1 ASTRON Off On On On On Time (seconds) > 10 s Up to 40 5 As required > 10 s Models 1 Pre-Ignition ASTRON i (3L) and ASTRON e/ex (4-8 L) 2 Ignition 3 Transition 1 4 Clean 5 Post Purge Ar (slm) 2 Same as step 1 Same as step 1 0 1-4 (slm) 0 0 1/2 of Total flow final flow 0 Press (Torr) 2.0 Same as step 1 1-10 1-10 Same as step 1 ASTRON Off On On On On Time (seconds) > 10 s Up to 40 5 As required > 10 s Models ASTRON hf (15 L), ASTRON hf-s (15 L), ASTRON hf+ (22 L) and ASTRON G6/G7 (30 L) 1 Pre-Ignition 2 Ignition 3 Transition 1 4 Transition 2 5 Clean 6 Post Purge Ar (slm) 1-4 Same as step 1 Same as step 1 0 0 Same as step 1 (slm) 0 0 1/5 of Total flow 2/3 of Total flow final flow 0 Press (Torr) 1-4 Same as step 1 1-10 1-10 1-10 Same as step 1 ASTRON Off On On On On On Time (seconds) > 10 s Up to 40 5 5 As required > 10 s
Page 6 Maintenance and Troubleshooting Best Practice Ignition failures or delays may occur because of maintenancerelated factors. Listed below are some best practice maintenance procedures: 1) Following all maintenance procedures, leak-check the gas delivery and vacuum systems to ensure continuing high levels of gas purity within the ASTRON. A leak in the gas delivery line to the chamber will introduce air, water vapor, or other contaminants that can impede ignition. 2) Regularly calibrate all flow control and pressure measurement devices. This helps to reduce system-tosystem variation. 3) Allow the system to reset after a failed ignition attempt. The ASTRON will interrupt and reset if ignition has not occurred within its default time (up to 20 or 40 s, depending on the model). A Reset procedure may take as long as 80 s, during which time the unit will ignore all Plasma On commands. Always confirm that the unit displays Ready status before attempting another ignition. 4) Ignition may sometimes be more difficult under socalled cold-start conditions; that is, following an extended period during which the ASTRON has been idle. Residual vacuum species (or air if the unit has been vented) may adsorb on the ignition assembly during such idle periods. The following practices can help to avoid cold-start ignition delays. a. Maintain a constant low argon purge through the unit during long idles. b. During an extended idle periodically (approximately every 30-60 minutes) ignite the unit and allow it to run at the ignition flow/pressure for ~2 minutes. Some systems include control settings that allow periodic running automatically. c. If an ASTRON does not ignite, purge it with the highest available argon flow for at least 1 minute, then stabilize the argon flow and pressure at the ignition values and attempt ignition. If ignition is still not successful, repeat the high-flow purge and attempt ignition at a different pressure within the specified ignition window (e.g., if ignition is not successful at 4 Torr, attempt ignition at 1 Torr). 5) A hardware malfunction may cause ignition failures. If the procedures above have not solved the problem, replace the unit: it may require repair by a qualified MKS service technician. CONCLUSION Following best practices in system design, process setup and maintenance procedures can improve ASTRON ignition performance and overall reliability. Mounting the ASTRON source close to the process chamber, using properly cooled, chemically compatible materials, and employing a stable, contamination-free gas delivery system are design best practices associated with reliable ignition. Using a robust process sequence, that includes a pre-ignition stabilization step, a smooth transition from ignition to the cleaning process, and a post-process purge step can also help ensure successful ignition in high volume manufacturing. For further information, call your local MKS Sales Engineer or contact the MKS Applications Engineering Group at 800-227-8766. ASTRON is a registered trademark of MKS Instruments, Inc., Andover, MA. Viton is a registered trademark of E.I. Dupont, Wilmington, DE. App. Note 04/11-7/11 2011 MKS Instruments, Inc. All rights reserved. MKS Global Headquarters 2 Tech Drive, Suite 201 Andover, MA 01810 978.645.5500 800.227.8766 (within USA) www.mksinst.com