Evolution of High Capacity
Planar Electrodes
In the period from the late 1973’s through to the early 1980’s, several manufacturers attempted to design and bring to market high capacity planar electrode plasma etching systems. Most of these systems failed to provide the required process uniformity, process repeatability and process temperature control, all of which are critical parameters for any plasma etching system. Several of these defective system designs were marketed before the industry realized the problems and, whilst some of the more fortunate users were able to return the systems to the manufacturers for at least a partial refund, most users were stuck with their defective systems. As can be imagined, these early systems created some negative feeling in the industry as to the viability of plasma as a manufacturing process and the management of some larger companies stated that they would never implement the plasma process again.
The reason these systems failed to meet expectations was obvious to Plasma Etch. Early plasma system research centered around semiconductor wafer level manufacturing. In order to develop high capacity multiple planar electrode plasma systems, manufacturers simply utilized this existing wafer level plasma system technology and scaled it up for increased system capacity. Vacuum chamber and electrode design, vacuum pumping capacity, vacuum level, gas flow, gas ratio, R.F. power, etc. were all scaled up. They found out that the scale up was not a linear function and along the way there were unforeseen processing surprises that do not arise in wafer level plasma processing technology. Even more importantly they failed to take into account the effect of process temperature and the interaction of the plasma with the vacuum chamber walls.
Plasma Etch is one of the few companies which have been successful in implementing this process. Others tended to approach the issue theoretically and failed to consider the uniqueness of the application when existing plasma technology failed to supply the answers. Plasma Etch believes that this is because the same scientists who had typically been involved in developing the earlier plasma technology for wafer level process applied the same theories to the new process. When it did not behave as expected, they lacked the flexibility to analyze the application and implement new process techniques. Plasma Etch, too, took the existing plasma technology as a guide but, importantly, did not treat it like a Bible. The company approached the plasma process from an applications point of view and simply drew on existing plasma theory when appropriate.
Plasma Etch entered the market in 1980, shipped its first system in 1984 and now in 1997 is the world’s leading supplier of high capacity planar electrode plasma etching systems, supplying systems ranging from benchtop R&D systems to fully automated inline and robotics equipment.
The company’s R&D Manager explains why they have been so successful. “When we started designing our systems we were aiming for the following: chemical stability, process consistency, environmental compatibility, no chemical residues, low operating costs and the elimination of safety hazards and chemical analysis costs. These were all problems inherent in the chemical process and it was clear to us that the plasma process was the only viable alternative.”
Having reached this point, the company set some further goals for its process:
- It would use exclusive primary plasma
- Process temperature should remain constant throughout the plasma sequence
- There should be no ramping of temperature during the process
- Throughput rate would be predictable and repeatable thanks to temperature stability
- Throughput could be accelerated by raising the process temperature
- The temperature stabilization cycle which limits throughput should be eliminated
- Introduce a start and stop system operation which required no time consuming temperature re-stabilization cycle
- Temperature control should be independent of the plasma process
- The process should use temperature control techniques, use a proven and highly reliable design using a recirculating heat transfer fluid
- Heating elements should not be exposed to the plasma process
“We aimed to satisfy all of these goals in order to overcome the limitations of the plasma processes which were then available” says the company’s R&D Manager. They were demanding goals and the company, having looked hard for innovative ways in which to meet them, decided that the answer lay in special Multiple Planar electrodes. These electrodes had to be configured in such a way as to generate only primary plasma. They also had to eliminate plasma reactions with the vacuum chamber walls, reactions which severely impact processing uniformity. The electrodes additionally had to be such that the process is unaffected by the orientation of the materials being processed.
The result of these design goals consists of multiple electrode pairs mounted in a rectangular aluminum vacuum chamber. The electrodes are horizontally oriented, in a planar configuration. They are alternately powered (R.F. Hot and R.F. Gnd) and a separate plasma field is generated between each pair of electrodes. The plasma is confined between each electrode pair, thereby generating a uniform, high energy primary plasma field. Electrostatic shielding further enhances the uniformity of the plasma field. The materials to be processed are loaded directly onto the electrode surfaces and are exposed to a uniform plasma field, thereby guaranteeing uniform surface treatment, irrespective of location or orientation
(see Fig.1). In order to understand how effective the Plasma Etch system is, it is also helpful to understand how most other systems work. They usually use multiple unpowered electrodes mounted in a vacuum chamber (see Fig.2) with an external powered electrode (RF Hot) and the vacuum chamber wall acting as the R.F. Ground electrode (RF and Ground). The plasma is generated external to the unpowered electrode assembly and migrates between the elements, plasma field intensity decreasing as a function of the distance from the plasma source. This design precludes the use of electrostatic shielding and this results in further distortion of the plasma field. The combined effect of gradient and distortion within the plasma field results in significant non-uniformity on the process as materials to be treated are loaded directly onto the electrode surfaces, and effectiveness of the surface treatment ends up being highly dependent on location and orientation.
Process temperature control
Temperature is the dominant parameter in the plasma process and has a marked effect on process uniformity and repeatability.
- To obtain maximum process uniformity and repeatability the process must be controlled independently of the plasma process, regulated throughout the entire plasma sequence and must be uniform over the entire electrode surface
- Temperature also influences processing rates considerably – the higher the process temperature, the faster the processing rate
- It is essential that temperature is tightly controlled when processing temperature sensitive devices as uncontrolled process temperature can cause distortion, delamination, discoloration and chemically modify the properties of such devices
In most other systems the only heat source is the plasma reaction and temperatures are wither uncontrolled or are controlled by modifying the plasma process, where the only way to reduce process temperature is to reduce RF power, which in turn slows throughput. Repeatability is also an issue with systems which lack a constant process temperature.
The temperature curve achieved during the Plasma Etch process remains steady (see Fig. 3) in comparison with most other systems thanks to the company’s unique electrode temperature control system: the electrodes onto which process material is loaded have their temperature maintained by continuously recirculating heat transfer fluid through them (see Fig. 4). Process temperature is independent of the plasmas process and programmable in the range of 0°F to 300°F (-180 C to 1490 C) +/-0.5°F. As the material being processed allows, an elevated process temperature can be selected for maximum process throughput. This system was patented by the company in 1987.
Electrostatic shielding
The process generates high energy plasma fields at the edges of the electrodes, resulting in higher etch rates at the electrode edges, with a gradual reduction towards the center of the electrode. This can be viewed as a target or bull’s eye effect, which results in non-uniform processing and is caused by plasma reacting with the vacuum chamber walls. TO obtain uniform etch rates, the high energy edge effects must be minimized or eliminated. Plasma Etch has solved these problems by electrostatically shielding all of the internal surfaces of the vacuum chamber using a unique feature which was patented in 1992 (see Fig. 5). This means that the plasma activity, and therefore etch profile, is uniform across the electrode surface. In other systems, etch profile has been improved by distancing the electrodes from the vacuum chamber walls, however, it does not completely eliminate the problem and increases the internal volume of the vacuum chamber so pump down time for the vacuum chamber is longer.
RF Power
RF power is a critical parameter in the plasma process, with RF frequency and power levels directly affecting the processing rates. For maximum and predictable processing rates, the RF source must:
- Utilize high frequency as opposed to low frequency power
- Supply high enough watt density to ensure full etch potential
- Supply constant power over the entire plasma sequence
- Be independent of all other plasma parameters
Plasma Etch uses high frequency RF power which is completely independent of the plasma process (13.56 Mhz). The cost of implementing higher frequencies is greater because an active matching network is required, however processing results are better. At higher frequencies, plasma is much more reactive and is isotropic (it etches uniformly in all directions), which is particularly important when processing three dimensional objects.
Lower frequency power (40 to 100 Khz) is cheaper to implement but throughput tends to be lower. In order to overcome this, higher RF power levels (watt densities) can be applied, but this can compound the problem of temperature control, as it introduces more heat into the system making it more difficult to control.
Gas distribution also has a direct impact on process etching efficiency. Plasma Etch machine have multiple gas ports to ensure uniformity. Given the importance of temperature to the correct functioning of the plasma process, there can be as many as three separate phases within the plasma process. In the first, the temperature is raised to the necessary processing level, the second is the plasma process and the third is devoted to removal of the polymer (ash) which has been deposited during the etching process. The Plasma Etch system is dedicated entirely to the plasma process as processing temperature is constant and there is no polymer deposition.
When the system is on standby waiting for materials to be loaded, there is no delay in restarting the plasma process from standby. Introducing preheated materials eliminates wait time completely and as loading is quite quick and simple, requiring no racking or fixturing, materials can be preheated and loaded directly from the oven. This means that cycle time is significantly reduced (see Fig. 6).
Nitrogen purging is automatic and functions wherever the vacuum pump is operational. This prevents contamination of the vacuum pump oil with hydrofluoric acid (HF) that attacks the vacuum pump and severely limits its service life, when using CF4 as a process gas.
In a market which has had a difficult start, Plasma Etch is dedicated to development of the plasma process. With an eye to the changing needs of the PCB market, the company has a number of highly innovative patents to its name which benefit its customer base greatly and have led it to its leading position in this specialized market, with more that 150 systems now installed worldwide.
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