Conventionally, a plasma processing apparatus which employs high-frequency glow discharge of a reactive gas introduced into a processing chamber has been widely used for a semiconductor fabrication process in order to perform a microprocessing on an object to be processed such as a semiconductor wafer. A plasma processing apparatus which employs high-frequency glow discharge, otherwise known as a parallel plate type plasma processing apparatus, includes an upper electrode and lower electrode and performs a plasma processing on an object to be processed mounted on the lower electrode by applying a high-frequency power only to the upper electrode.
However, such a plasma processing apparatus has disadvantages in that it is difficult to control plasma voltage between the two electrodes, and some of electrical discharges will be lost through the wall surface in the processing chamber which is grounded, thereby making plasma non-uniform and unstable. Consequently, the apparatus tend to be unsuitable for microprecessing on a quarter-micron to half-micron scale as demanded in the industry recently.
Therefore, attempts are underway to control the density of plasma generated in a processing chamber by applying high-frequency powers to an upper and lower electrode for microprocessing. FIG. 8 illustrates an example of such a plasma processing apparatus. A processing chamber 100 includes a lower electrode 102 to be mounted thereon an object to be processed W in a processing chamber 101, and an upper electrode 103 facing the lower electrode 102, wherein a first high-frequency power supply 106 applies a first high-frequency power to the upper electrode 103 via a first matching unit 107, and a second high-frequency power supply 108 applies a second high-frequency power to the lower electrode 102 via a second matching unit 109, thereby controlling the density of plasma generated in the processing chamber 101. Thus, a glow discharge is formed between the lower electrode 102 (grounded) and the upper electrode 103 to give rise to a plasma of reactive gas species. Desired etching is performed by making ions in the plasma hit the surface of an object to be processed mounted on the lower electrode 102 through the use of a potential difference between the two electrodes.
With respect to managing the problems of microprocessing, attempts are underway to achieve a high selectivity and high etching-rate without charge-up damages by setting the process condition at low pressure, e.g., 100 mTorr or less, or otherwise using certain gases.
(Reference 1) Japanese Patent Laid-open Application No. H8-162293
(Reference 2) U.S. Pat. No. 5,716,534
However, when using a conventional plasma processing apparatus such as in FIG. 8, it is difficult to control the process as desired during the plasma generation stage because of interferences between high-frequency power signals from the two high-frequency power supplies, or distortions in waveform thereof. At times, an object to be processed is subject to charge-up damages.
Further, depending on the manner in which two high-frequency power supplies provide high-frequency powers, the high frequency power or the matching unit can be overloaded or some plasma can be lost by an activation of a safety circuit therein. This can be caused by, for example, generation of reflected waves from an abrupt impedance change in the plasma processing apparatus.
To solve the above problems, the present inventor developed the methods disclosed in References 1 and 2 in order to reduce damages in an object to be processed. With these methods, a plasma for etching is generated by forming a plasma through applying a high-frequency power to one electrode first, then, a high-frequency power is applied to the other electrode. As a result, the dissociation rate increases and the plasma density becomes high, thereby reducing charge-up damages in the object to be processed.
However, as demands for microprocessing on a smaller scale and for extending useful life of a plasma processing apparatus have been growing, it has become necessary to further minimize damages on an object to be processed and to reduce loads on a high-frequency power supply, matching unit, and the like.