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  Research - Plasma Etching and Micromachining

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 above, 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 attracts ions from the plasma toward the coil, which then can sputter 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 above. 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.

Deep Plasma Etching of Polymethylmethacrylate (PMMA) to a depth of 80 microns

Deep Plasma Etching of Polymethylmethacrylate (PMMA) to a depth of 120 microns with a 50% overetech

For further information, 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).

Anisotropic Etching of Cesium Iodide

Thick films of cesium iodide are used to convert x-ray images into visible light.  The visible light is then detected by a CCD camera, resulting in a digital x-ray (see the figure below).  The thickness of the CsI film causes some scattering of the light, and this scattering reduces the quality of the x-ray image.  With the support of the NIH and RMD, Inc. (Waltham, MA), we have developed a method of micromachining the CsI film into closely packed pixels.  The light generated inside an individual pixel is trapped by a reflective coating around the pixel and coupled to the CCD through a fiber-optic plate.  The resulting image has approximately twice the MTF as the unpixelated CsI plate.

 

The scanning electron microscope image below shows an array of pixels micromachined into a 50 micron thick CsI film.  The fiber optic base plate is exposed in the lower part of the left photo, where three pixels are missing (each box in the pictures below is one pixel).

     

How is CsI etched using plasma?  The actual reactions are somewhat complex, but we know that the etching rate is temperature dependent and it follows the Arrhenius relation with an activation energy of 0.13 eV.  We also have discovered that CsI etches readily in CF4 plasmas, but does not chemically etch in XeF2 plasmas.  Therefore, fluorine is not an active etchant and we hypothesize that a simplified etching reaction is CF3(g) + CsI(s) --> CF3I(g) + Cs.  This is chemical reaction is followed by the thermal evaporation of the excess Cs atoms, which have a very high vapor pressure:  Cs(s) + temperature --> Cs(g).

The anisotropy of the etch process (i.e., the vertical profile of the pixels) is a result of ion enhanced etching and sidewall passivation.  EDS studies show that the sidewalls of the pixels consist of a layer of carbon, fluorine, and cesium.  This layer protects the sidewalls from etching by CF3 radicals.  Experimental data also show that the etching process requires ion bombardment with a threshold energy on the order of 20 eV. This ion bombardment serves to remove the passivation layer from horizontal CsI surfaces and/or enhances the desorption of CFxIy species from the CsI surface.

 
 
Related Publications
 
  • "Physical mechanisms for the anisotropic plasma etching of cesium iodide," Xiaoji Yang and Jeffrey A. Hopwwod, Journal of Applied Physics, Vol 96(9), 4800-4806 (2004).
  • "Plasma Etching of Cesium Iodide," X. Yang, J. Hopwood, S. Tipnis, V. Nagarkar, and V. Gaysinskiy, Journal of Vacuum Science and Technology A, 20(1), 132-137 (2002)