WISCO International (Bird

The Bird Model 4421 is a precision RF power meter that, while originally intended for general purpose laboratory applications where high accuracy is required, has found wide acceptance in semiconductor processing applications. The product is configured as a system, consisting of the Model 4421 digital display, and a 4020 series power sensor, selected for the application based upon maximum power and operating frequency.

Advantages

+/- 3% Accuracy � This is accomplished through the use of an automatic frequency compensation scheme, where the error contributions due to directional coupler frequency response characteristics are eliminated.

Wide Dynamic Range � The instrument will meet the full accuracy specification over a 35 dB dynamic range.

Excellent Measurement Repeatability � Typically <0.1%

Digital Display � Along with automatic VSWR calculation

Computer Interface � Choice of RS-232 or IEEE 488 interface through the use of plug-in cards

Application

The 4421 system is used extensively to measure the RF Generator output power in Dielectric Etch wafer processing systems. This semiconductor processing method uses RF power to excite a plasma, which is then used to selectively etch silicon to form semiconductor circuits. Typical RF power levels in these systems range from 500 to 2000 watts.
The system configuration is as outlined below. The RF generator is connected to a reactive matching network, to match the 50 ohm, fundamentally resistive output of the RF generator to the reactive impedance of the processing chamber.
During the etch cycle, the impedance of the chamber will change from a primarily capacitive reactance before the plasma is excited, to an inductive reactance after plasma ignition. The level of RF power applied to the process, as well as the total time that RF power is applied, is directly related to the etch depth. When these two parameters (power applied and duration) are arrived at for the particular semiconductor device to be fabricated, they must be very tightly controlled, as the process yields will be determined by the degree to which they are held. For this reason, the 4421 is ideal for the application, with excellent accuracy and repeatability.

A typical procedure for the use of the 4421 in the above application would be as follows:

Determine appropriate RF power level � This is done both analytically, and empirically, and is tailored to the device to be fabricated.
Measure the RF generator output � This step is done using the 4421, with the output terminated into a precise 50 ohm resistive impedance.
Transfer this power measurement to the RF generator � The RF generator will have independent output control, and the precise 4421 measurement is used to calibrate the generator output monitor.
Record the operating power level � This then becomes the process operating power. Typically, the RF generator calibration is checked periodically, or whenever a new device type is to be fabricated. The calibration interval varies by manufacturer, and device type.