Plasma Etch Cleaning Technology

Plasma Etch Overview

As is often the case with manufacturing processes, the key to successful design and implementation of plasma etching systems is control of several factors. Specifically, optimum etch rate and throughput can only be achieved by careful control of all plasma parameters and system designs, including process temperature, electrostatic shielding, R.F. power, gas distribution, vacuum, electrode configuration, etc... Lack of proper control of any of these interactive parameters and system designs will have a detrimental effect on system performance.

Plasma Etch Process Temperature Control

GENERAL
Process temperature is the single most important parameter in the plasma process. Process temperature has primary control over etch rate and has a secondary effect on etch uniformity.
The temperature at which the process operates has a major influence on processing rates. The higher the process temperature, the faster the processing rate.
Process temperature control is mandatory when processing temperature sensitive devices. Uncontrolled process temperatures can cause distortion, delamination, and discoloration, and can chemically modify the properties of temperature sensitive devices.

DESCRIPTION
• Process temperature is regulated by the patented electrode temperature control system. Electrode temperatures are maintained by continuously recirculating a heat transfer fluid through the electrodes. (See diagram below.)
• The boards are loaded directly onto the temperature controlled electrode surfaces.
• Process temperature is independent of the plasma process and programmable in the range of O°F to 300°F ± 0.5°F.
• An elevated process temperature is selected which is compatible with the type of board material (epoxy, polyimide, acrylic/kapton, Teflon, etc.) being processed, while maintaining maximum processing throughput.
• The temperature control system is a patented feature, unique to the Plasma Etch product line.

 
ADVANTAGES
• Process temperature is constant throughout the plasma sequence. No ramping of temperature occurs during the plasma process. (See graph below.)
• Etch rates are predictable and repeatable, due to steady state process temperature.
• Etch rates are accelerated by using elevated process temperatures.
• No throughput limiting temperature stabilization sequencing ("dummy plasma cycling") is required.
• Start and stop system operation requires no throughput limiting temperature stabilization cycle.
• Temperature control is independent of the plasma process.
• Temperature control techniques use proven and highly reliable design (recirculating heat transfer fluid). No temperature control elements are exposed to the plasma process.
• Any process temperature from 175°F to 300°F may be selected for compatibility with all board materials. Optionally, the temperature range may be extended from O°F to 300°F.

CURVE I (Plasma Etch Temperature Profile)
1. Points A and B are the respective starting and the ending points of the plasma process sequence.
2. Processing temperature is essentially constant throughout the plasma sequence. Any temperature within the control range may be selected and is automatically maintained.
3. Constant process temperature ensures plasma process repeatability.

CURVE II (Typical Competition Temperature Profile)
1. Points C and D are the respective starting and the ending points of the plasma process sequence.
2. The starting temperature at point C is undefined, as it depends on residual system heat from the previous process cycle.
3. The slope of tile temperature profile from point C to point D is dependent on the thermal loading effect of the materials (mass of the load) being processed and the plasma processing parameters (primarily R.F. power level).
4. The ending temperature at point D is a variable, as it is dependent on items 2 and 3 above.
5. The repeatability of the plasma process is severely impaired by this lack of a constant processing temperature.

Plasma Etch Electrostatic Shielding

GENERAL
The plasma process generates high energy plasma fields at the edges of the electrodes, which causes accelerated etching at these edges. These localized high energy plasma fields are caused by the plasma reacting with the vacuum chamber walls.
The overall effect is higher etch rates at the electrode edges, with a gradual reduction in the etch rate as it approaches the center of the electrode. This can be viewed as a target or bull's eye effect, which results in severe process non-uniformity.
If unchecked, this non-uniformity will cause parts positioned at the periphery of the electrode to etch more severely than those positioned at the center of the electrode. To obtain uniform etch rates the high energy edge effects must be minimized or eliminated.

DESCRIPTION
In Plasma Etch systems, all vacuum chamber internal surfaces are electrostatically shielded to eliminate plasma reactions with the chamber walls. (See diagram below.) These electrostatic shields are a patented feature, unique to the Plasma Etch product line.

ADVANTAGES
• Plasma activity is uniform across the electrode surface, thereby creating a uniform etch profile.
• Results are controllable and repeatable.
• Surface treatment of individual parts is independent of location and orientation on the electrodes.
• Some plasma equipment manufacturers attempt to minimize the bull's eye effect by distancing the electrodes from the vacuum chamber walls. This somewhat improves the uniformity of etch, but does not eliminate the problem. It has the added negative effect of increasing the internal volume of the vacuum chamber and thereby increases the pumpdown time of the vacuum chamber.

High Frequency R.F. Power

GENERAL
The use of high frequency R.F. power is instrumental in creating a high efficiency plasma. High frequency plasmas produce greater process uniformity and improved throughput.

DESCRIPTION
• The physics of a high frequency R.F. plasma is very different from a low frequency R.F. plasma. The high frequency generates a much more reactive plasma.
• The high frequency plasma tends to be isotropic (etches uniformly in all directions).
• Nominal R.F. power levels (watt densities) are required to generate an optimum etch rate.
• R.F. power is held constant throughout the plasma process, and is independent of all other plasma parameters.
• High frequency RF power requires an active matching network along with greater care in initial system design. Though these factors increase up-front costs, the system yields superior processing results.
• Plasma Etch systems use proven commercially available R.F. generators. We have elected to use a proven "bullet proof" technology, which uses a high power tube as the final amplifier.

ADVANTAGES
• The more reactive plasma generated by high frequency has a faster etch rate.
• The isotropic nature of high frequency plasma allows for the processing of three dimensional objects.
• Competitors' systems may use much higher R.F. power levels (watt densities) in an attempt to overcome the restricted etch rates associated with low frequency R.F. power. The higher power levels also compound the problem of temperature control, as they introduce more heat into the system.
• Constant RF power produces maximum and predictable etch rates.
• The "bullet proof" R.F. generators used in Plasma Etch systems have proven very reliable.

Nitrogen Purging

GENERAL
The purpose of nitrogen purging is to prevent contamination of the vacuum pump oil. Without nitrogen purging, the vacuum pump oil will quickly contaminate with hydrofluoric acid (HF), when using CF4 as a process gas. This acid will attack the vacuum pump and severely limit its service life.

DESCRIPTION
• Nitrogen purging is automatic and functional any time the vacuum pump is operational.
• The purging operation is transparent to the operator and has no impact on normal processing.

ADVANTAGE
The reliability and service life of the vacuum pump are extended.

High Capacity Vacuum System

GENERAL
System designs must support high gas flow rates at low operating pressures to meet the requirements of the desmear/etchback process.

DESCRIPTION
Plasma Etch Desmear/Etchback Systems utilize a two stage vacuum system consisting of a backing pump in combination with a blower.

ADVANTAGES
• High gas flow rates and low chamber pressure result in faster process times and optimum etchback characteristics.
• Process uniformity and processing throughput are enhanced.