Dan Goodman, Shaun Pewsey, Millipore Corporation, Allen, Texas
Metal etch and low pressure chemical vapor deposition processes are placing additional demands on established vacuum pressure control and construction techniques. These difficult processes often generate significant quantities of solid and condensable byproducts that can deposit inside the entire vacuum system, requiring frequent service of pressure control components. Use of heated throttle valves can, however, improve overall system uptime for these processes.
In many etching and chemical vapor deposition (CVD) wafer fabrication processes, the desired process vacuum is achieved and maintained via downstream pressure control. In this method, a high-accuracy total pressure gauge, such as a capacitance diaphragm gauge (CDG), measures the actual chamber vacuum and then sends either a digital or analog signal to a dedicated pressure controller module. This module compares the actual chamber vacuum with the desired setpoint and adjusts an electronically driven throttle valve mounted in the pumping line between the process chamber and vacuum pumps (Fig. 1).
By opening or closing the throttle valve, the effective pumping speed on the process chamber is precisely balanced with the gas inflow to the process chamber. This allows the desired vacuum level to be maintained to within 0.50% or less of the desired setpoint and with short stabilization times.
Millipore's heated motor-driven valve (above). Valves undergo packaging (left) in Millipore's valve assembly area.
In addition to stringent process performance requirements, many of today's advanced etch and CVD processes place additional burdens on equipment designers and tool operators. A key burden to overcome in some processes is reducing the system downtime that can occur due to buildup of process byproducts throughout the vacuum system. The buildup of solid byproducts has multiple effects on the process system:
- It reduces the conductance of the vacuum pumping line, which slows down the process and reduces the system's throughput.
- It can cause drift in process vacuum gauges, which in turn causes the process to degrade from its original performance.
- It can become a source of particle generation from back-flow of the deposited material into the process chamber.
Some of the more difficult etch and CVD processes can require disassembly and cleaning of vacuum systems as often as every few days; each cleaning costs the tool operator several hours of lost production time.
Metal and polysilicon etch as well as several common low pressure CVD (LPCVD) processes are the worst maintenance-overhead offenders. While process chemistries vary widely between tool manufacturers, current process chemistries can produce a wide range of byproducts, such as aluminum chloride (AlCl3), ammonium chloride (NH4Cl), and silicon nitride (Si3N4) that condense onto the inner surfaces of vacuum pumping lines.
Figure 1. A typical layout of a downstream pressure control arrangement.
Potential throttle valve problems
The throttling valve used for downstream pressure control is particularly vulnerable to such deposition because of its operating mechanism. As the thickness of the deposited layer builds up over time, the internal parts of the valve eventually become coated and are frozen in position. When that happens, the valve is unable to move its throttling elements (plate, gate, ball, etc.) and pressure control is no longer possible. The only remedy is to remove the valve for cleaning or to replace it entirely. In either situation, the process system vacuum piping must be purged, disconnected, and then reconnected and repurged, which causes hours of system downtime.
Figure 2. Throttling valve with external heating jacket for 80?C operation.
Even in processes where a throttle valve does not fail catastrophically, there are frequently other effects on the process from the buildup of byproducts. For example, as the valve's throttling element sweeps across the sublimated buildup inside its own internal surfaces, particles are frequently released back into the process chamber, causing wafer contamination and lost production. The higher particle generation can also cause accelerated mechanical wear inside the throttle valve, which usually leads to increased hysteresis, erratic pressure control, or both.
To help control these types of problems, many etch and CVD process systems are equipped with heated vacuum piping and throttle valves that keep the process byproducts in their vapor phase. The temperature required to stop the byproducts' solidification is dependent upon the process pressure and the byproducts' respective vapor pressure curves, but a general guide is:
- polysilicon and aluminum etch >90°C,
- LPCVD silicon nitride >130°C, and
- TEOS LPCVD >140°C.
Figure 3. a) NW100 valve body with circumferential grooves for heater elements, and b) complete NW100 heated valve with installed heaters.
It is important to note that these temperatures apply to all surfaces within the vacuum system, including the throttling valve control elements, not just the external surfaces.
These heated components protect the vacuum lines from suffering buildup of the condensable solids, metal oxides, and metal salt by-products. Some process tools use fitted jackets to provide heating of the vacuum components, providing effective heating of the vacuum piping in some applications (Fig. 2).
Although heater jackets have proved to work well with conventional vacuum pipework in less demanding applications, heating complex subassemblies such as throttle valves to temperatures in excess of 125°C with external jackets is much less effective. In these higher-temperature applications, the surface area available for an external heater strip is relatively small and thus the heat transfer from the jacket to the valve body and internal parts is not very effective. For example, the external surface area of a valve with the ISO100 flange is only about 150cm2. This small surface area is further compounded by the poor thermal conductivity of stainless steel (16.3W/m.K at 100° C compared with 160W/m.K for aluminum). Finally, various clearances inside the valve and under vacuum cannot transfer heat except by radiation.
The net effect of these design realities is that there can be a substantial difference in valve temperatures when the valve is heated by external jackets. Tests have shown that the throttle plate temperature in a jacket-heated NW100 throttle valve can be up to 35°C cooler than the rest of the valve. This results in a relative "cold trap" inside the valve where condensable materials can build up faster than elsewhere, particularly in difficult applications such as LPCVD of silicon nitride.
There is also a practical limit on heater jackets based on most process tools' electrical systems. Increasing the operating temperature of larger valves that have more stainless steel requires substantially more heat and thus electrical power. This can be shown with:
Qmc = °T
where Q is the heat added in joules; m is the amount of material (316 stainless steel) measured in kilograms; c is the specific heat of the material (stainless steel 500J/kg.K) defined as the number of joules required to raise the temperature of 1g of the material 1°C; and °T is the change in temperature.
For smaller valves with NW50 or NW80 flanges that need to operate around 100°C, a relatively small heater jacket (<300W) can be used, but for larger valves, the power input increases rapidly to compensate for the mass of stainless steel in the valve. For example, a NW200 stainless steel throttling gate valve with heater jackets will need at least 1.2kW of electrical power to maintain an external surface temperature of 150°C; this is substantially more power than most process tools' internal power supplies can deliver.
Throttle valves with embedded heaters
Etch and CVD system designers have been looking for a more efficient method of heating the downstream control valves. For example, the amount of stainless steel to be heated could be reduced or another method to apply heat could be used.
One solution is found on current state-of-the-art throttle valves that use both of the methods mentioned above to improve heating efficiency. These valves have circumferential grooves machined into the valve flange with four embedded heaters mounted in the grooves to provide extremely efficient conductive heating of the flange directly (Fig. 3a and 3b). The heaters have a surface area of about 480cm2, a 3x increase over typical external heating jackets. The embedded heating technique provides a more uniform heat distribution across the valve body and better thermal transfer to the throttle plate, resulting in a temperature differential of <7°C between valve body and throttle plate.
In addition, embedded heaters require far less input power to operate typically 500W vs. ~1 kW for a comparable external heating jacket.
Figure 4. a) An unheated valve taken from a nitride CVD system that failed after two weeks in operation, and b) a replacement heated prototype valve after 26 weeks of continuous use.
Embedded heater throttle valves were developed to address improved reliability and a lower cost of ownership through a reduction in maintenance cycles on etch and CVD systems. During field research, it became clear that unheated valves or inefficiently heated designs (valves with third-party heater jackets and edge heated designs) were typically lasting between one and four weeks in aggressive processes before maintenance was required. In these cases, contamination of the control valve was found to rapidly degrade process pressure control, contribute to elevated particle levels, and significantly increase system downtime. Once removed from the process tool, the valves were found to require extensive cleaning and repair due to the excessive levels of contamination buildup. Figure 4a shows an example where a valve's throttle plate and inner valve surfaces were completely coated with ammonium chloride. This valve was replaced by a heated prototype (see Fig. 4b) that was in service for more than six months without requiring removal and cleaning.
Field trials of throttle valves with embedded heaters have shown significant improvement in the reliability and time between maintenance cycles on a number of contaminating processes (see table).
The introduction of temperature-controlled heated throttle valves can remove a potential weak link in heated vacuum lines, leading to improved system uptime and a reduction in back-diffused particles generated in the pumping line. Further optimization of the complete pressure control system can also help reduce the propagation of chamber particles with soft pump-down and vent-up control, and through smoothing pressure transients in multistep processes.
Dan Goodman received his BSME from Carnegie-Mellon University. He has nearly 20 years of semiconductor marketing and applications engineering experience with companies such as Millipore, Tylan General, and Leybold Vacuum. Goodman is group marketing manager for Millipore, Microelectronics Gas Division, 915 Enterprise Blvd., Allen TX 75013; ph 972/359-4000, fax 972/359-4106, e-mail email@example.com.
Shaun Pewsey received his technical degree from the London Institute of Electronic Systems. He has been working in process and vacuum technology since 1985 with Millipore, Tylan General, and Spectra Tech. Pewsey is senior market manager for Millipore's vacuum products.