The demands of etching small features in integrated circuits has driven the development of high density plasma sources. One of the most successful methods of creating an intense plasma is radio frequency induction. The plasma is sustained by an rf current (5-50 A) carried by an inductive element, usually an air-core coil. This current induces an electric field in the plasma region which is primarily tangential to the plasma boundary. The tangential component of the rf induction electric field may then penetrate the plasma region 1-2 cm and generate an intense plasma. Most significantly, the plasma is sustained without creating a high voltage sheath at the plasma-wall boundary. This improves the purity of the process and greatly increases the ion generation efficiency.

In the ICP design shown to the left, we have developed a coil which significantly reduces the sputtering of contamination from the quartz window. The inductive element in an ICP typically has an rf voltage in excess of 1 kV during operation. This high voltage may attract ions from the plasma toward the coil. These ions may then sputter atoms from the window. To prevent window erosion and subsequent process contamination, we have used a low-aspect ratio helical coil with the lower turn of the coil being electrically grounded. Strong electric fields originating on the upper turn of the coil are terminated on the lower turn, rather than extending into the plasma region. Sputtering of the window is significantly reduced and etching of polymer materials is documented to be residue-free.

Uniformity of etching in the ICP is shown in the figure below. These etching tests were performed on 200 mm wafers that were spin-coated with polymer. The etch uniformity at low pressure (2.5 and 5 mTorr) is measured at better than 3% (standard deviation/average). At 20 mTorr, however, the etch rate is considerably higher in the center of the wafer than at the edge. The rf bias was not used in this experiment. With the addition of a 30w bias to the wafer, the etch rate exceeds 1.5 um/minute. (View a scanning electron micrograph here (~1MEG.)

For further information, you may refer to the article "Inductively coupled plasma for polymer etching of 200 mm wafers," by N. Forgotson, V. Khemka, and J. Hopwood, J. Vac. Sci. Technol. B 14, 732 (1996).

Electron Devices Group Home Page

Prof. Hopwood's Home Page