Plasma processing method and apparatus with control of rf bias

Details Plasma processing method and apparatus with control of rf bias Plasma processing method and apparatus with control of rf bias

1999-03-31

2001-07-24

Shingleton, Michael B. (Department: 2817)

Electric lamp and discharge devices: systems

Discharge device load with fluent material supply to the...

Plasma generating

C315S111710, C118S7230IR, C118S7230ER, C118S7230AN

Reexamination Certificate

active

06265831

ABSTRACT:

FIELD OF INVENTION
The present invention relates generally to vacuum plasma processors for processing workpieces on a workpiece holder and more particularly to a method of and apparatus for controlling an a.c. source coupled by a matching network to an electrode of the workpiece holder so a tendency for a discontinuity in the amount of power reflected back to the source is overcome by varying the source output power.
Another aspect of the invention relates to a method of and apparatus for determining an impedance component of a plasma in a plasma processor and more particularly to a method of and apparatus for determining the impedance component in response to thickness of a sheath between a solid surface of a vacuum chamber of the processor and the plasma in the chamber.
BACKGROUND ART
Vacuum processors for processing a workpiece (i.e., etching materials from or depositing materials onto the workpiece) typically include first and second ports respectively connected to a vacuum pump and one or more sources of ionizable gas. The gas is excited to a plasma in the chamber by an electric source including a reactance responsive to a first a.c. source, typically an r.f. or microwave source. A first matching network is usually connected between the first a.c. source and the reactance for exciting the plasma. If the source is an r.f. source, the reactance is either a coil for supplying magnetic and electric fields to the chamber interior via a dielectric window or a parallel plate capacitive arrangement for supplying an electrostatic field to the chamber interior.
The workpiece, which is typically a semiconductor wafer or a dielectric sheet or a metal plate, is clamped in place on a workpiece holder, i.e., chuck, that frequently includes an electrode covered by a dielectric. D.C. voltage is typically applied to the electrode to provide an electrostatic clamping force to hold the workpiece in situ on the holder. The workpiece is usually cooled by applying a coolant agent, such as helium, to a recess in the chuck. To accelerate ions in the plasma to the workpiece, a second a.c. source is connected to the electrode by way of a matching network. Each matching network includes a pair of variable reactances having values that are varied by motors, typically step motors.
Sensors for electric parameters associated with the plasma, as coupled to the excitation reactance and as coupled to the chuck electrode, derive signals which assist in controlling the values of the variable reactances. Pressure and flow rate transducers respectively in the chamber and in a line supplying gas to the second port derive signals which assist in controlling the vacuum pressure in the chamber and the flow rate of gas flowing into the chamber through the second port.
A controller including a microprocessor and a memory system including a hard drive, random access memory (RAM) and a read only memory (ROM), responds to the signals derived by the transducers and signals from an operator input console to produce signals for controlling the variable reactances, output power of the two a.c. sources, the vacuum pressure in the chamber and the flow rate of gases supplied to the chamber through the second port. The memory system stores several recipes, each in the form of signals representing various parameters controlling the deposition and etching of the workpieces for differing situations. The parameters of each recipe are, inter alia, gas species to be supplied to the chamber, flow rates of the species, vacuum pressure in the chamber and output powers of the two a.c. sources. Each recipe can include other parameters, such as a time for carrying out each recipe and/or the thickness of the layer being deposited. The controller responds to the parameters of the recipe to control valves for the flow of the gases into the chamber, the chamber pressure, as well as the output power of the first and second a.c. sources. During processing, the controller controls the reactances of the first and second matching networks so that there is an efficient transfer of power between the first and second a.c. sources and the loads they drive so the impedances seen looking into the output terminals of the first and second sources are substantially equal to the impedances the first and second sources respectively see by looking from their output terminals into cables connected to the first and second matching networks.
During processing, anomalies occur in the chamber pressure and gas flow rates. The anomalies affect the plasma impedance as coupled to the excitation reactance and the r.f. bias electrode. The controller responds to the plasma impedance changes resulting from these anomalies to change the reactances of the first and second matching networks in an attempt always to minimize the power reflected back to the output terminals of the first and second a.c. sources. Ideally, the reflected power is zero; in actuality the minimum power reflected back to the a.c. source driving the r.f. bias electrode is about two percent. The power absorbed by the loads driven by the first and second a.c. sources is maximized and the reflected power coupled back to the a.c. sources is minimized when the load impedance the source drives equals the complex conjugate of the impedance seen looking into the output terminals of the a.c. source. (The complex conjugate of (A+jB) is (A−jB), so that a load having an output represented by the real and imaginary components A and B, having a magnitude of {square root over (A
2
+L +B
2
+L )} and a phase angle of &thgr;=arctan
A
B
,
is matched when the load it drives has a magnitude of {square root over (A
2
+L +B
2
+L )} and a phase angle of −&thgr;.
The sensing arrangement monitors how well the matching network is “tuned” to the load, i.e., the degree of match between the impedance seen looking into the output terminals of the r.f. source and the impedance seen looking into input terminals of a cable that the source drives and which in turn drives the matching network. The sensing arrangement can measure many different parameters, e.g., the impedance seen looking into the cable or the fraction of power reflected back to the a.c. generator output terminals. The controller responds to the sensing arrangement to continuously adjust the reactances of the matching network according to a feedback theory. Many different tuning theories have been developed for matching a plasma to an a.c. source, such as disclosed in the commonly assigned, U.S. Pat. Nos. 5,689,215 and 5,793,162.
In the past, the assumption has been that the measured quantity, either the impedance seen looking into the cable input terminals or the power reflected back to the output terminals of the a.c. source is a continuous function of the values of the variable reactances of the matching network. In other words, the prior assumption has been that a small change in either of the variable reactances results in a small change in the impedance seen looking into the cable or the reflected power. The assumption is true if the plasma load impedance is constant. However, the plasma load impedance is not constant, but instead is a function of a.c. power delivered to the plasma.
We have observed that small changes in the value(s) of at least one of the variable reactances of the matching network coupling power from the second source to the r.f. bias electrode can, under some circumstances, result in a step or discontinuous change in the impedance seen looking into the cable input terminals or the power reflected back to the second a.c. source output terminals. We believe the step or discontinuous change occurs because the plasma load impedance coupled to the r.f. bias electrode is not constant. Instead, the a.c. power the r.f. bias electrode delivers to the plasma apparently depends on the plasma load impedance, which in turn depends on the amount of a.c. power the r.f. bias electrode delivers to the plasma. The inter-dependency of the a.c. power the r.f. bias electrode delivers to the plasma and the plasma load impedance affec

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