Characterization of a new electron cyclotron resonance source working with permanent magnets

Abstract

Most electron cyclotron resonance (ECR) sources available today are working with electromagnets. A new source, developed for etch applications, is supplied with permanent magnets. This leads to strong short-range magnetic fields and the ECR condition is fulfilled on a ring near the chamber wall. Having the maximum electric field strength of the 2.45 GHz along this ring in the microwave radiator leads to efficient plasma production. The principle source characteristics are demonstrated with oxygen plasmas. At low pressure (some 10−4—some 10−3 mbar) and moderate microwave power (100–500 W) the plasma burns as a ring plasma. Increasing pressure and/or microwave power over the above mentioned values leads to a ‘‘bulk’’-type plasma. The change from ‘‘ring’’-type to bulk-type plasma occurs discontinuously and is connected (a) with a discontinuity in the run of the reflected power and (b) with a sudden change in the relative ion flux densities of different ionic fragments measured by mass spectrometry. Absolute ion densities up to 5×1010 cm−3 have been found by Langmuir probe measurements in a bulk plasma at a pressure of 1×10−3 mbar. Anisotropic etching of polysilicon over SiO2 was performed in the ring mode with pure Cl2 as feed gas and applying a radio-frequency (rf) extraction to the ECR plasma. Low damage etching was achieved due to the low direct-current (dc)-bias voltage of 150–200 V with very good anisotropy. The poly-Si etchrate was 85 nm/min with a selectivity of 18:1 to SiO2 and a uniformity of ±7.5% over 200 mm wafers.