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Effect of ion energy on photoresist etching



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JOURNAL OF APPLIED PHYSICS VOLUME 89 NUMBER 10 15 MAY 2001 Effect of ion energy on photoresist etching in an inductively coupled traveling wave driven large area plasma source K Takechia and M A Liebermanb Department of Electrical Engineering and Computer Sciences University of California Berkeley California 94720 Received 17 November 2000 accepted for publication 21 February 2001 We report on the effect of ion energy on photoresist etching in an inductively coupled large area plasma source driven by a 13 56 MHz traveling wave with oxygen gas To control the ion energy at the substrate surface the electrode on which the substrate is placed is independently driven by a capacitively coupled 1 MHz power source The etch rate increases with increasing ion energy for gas pressure ranging from 1 to 100 mTorr Ion induced desorption rate constants etch yields are shown to be proportional to the square root of the ion energy An increase in the ion energy leads to etch uniformity improvement over the processing area of 40 cm 50 cm particularly at a low gas pressure of 5 mTorr A modified photoresist etch kinetics model combined with a spatially varying oxygen discharge model is used to explain these experimental results 2001 American Institute of Physics DOI 10 1063 1 1364648 I INTRODUCTION toresist etch kinetics model combined with the spatiallyvarying oxygen discharge model introducing the functional dependence of etch yields versus ion bombarding energy A set of etch rate data and the corresponding self bias voltages are used to obtain the functional dependence Inductively coupled plasma sources have been studied extensively in the past ten years as candidates for advanced etch and deposition processing tools 1 5 The driving forces for the effort are the recognition of their capabilities to generate high density plasmas under low pressures and control ion neutral fluxes and the ion bombarding energy independently 6 Although inductively coupled plasma sources have many merits the plasma generated is inherently nonuniform due to the antenna standing wave effect especially when the plasma source is scaled to large size comparable to the driving rf wavelength 7 To overcome this difficulty we are investigating an inductively coupled large area plasma source LAPS driven by a 13 56 MHz traveling wave launched along an antenna embedded in the plasma The LAPS is a rectangular metal chamber providing a processing area of 50 cm 40 cm for large size wafers and glass substrates for flat panel displays 7 8 We previously characterized the LAPS for photoresist etching with oxygen gas placing an emphasis on how various gas pressures influence the etch rate and etch uniformity over the processing area No external bias was applied to the substrate holder electrode during the experiments We developed a simplified spatially varying oxygen discharge model and photoresist etch kinetics model in order to explain the experimental results 9 In this study we investigate the effect of ionbombarding energy at the substrate surface on the photoresist etching in the LAPS changing the substrate self bias voltage V bias by a capacitively coupled 1 MHz power source The etch rate and etch uniformity are measured for various selfbias voltages and gas pressures We describe a modified pho II EXPERIMENTS The chamber is a rectangular stainless steel box with interior dimensions of 70 cm in width 60 cm in height and 20 cm in depth pierced horizontally by a planar line array of eight quartz tubes each 2 58 cm outer diameter 1 7 mm thick with center to center spacing of 7 65 cm The interiors of the tubes are at atmospheric pressure The rf excited 13 56 MHz antenna system consists of eight copper rods threaded through the interiors of the quartz tubes For this work we use an antenna pattern consisting of four sets of two adjacent antenna rods each connected in series in a serpentine configuration 9 Typical operating parameters were an oxygen gas pressure of between 1 and 100 mTorr and a 13 56 MHz source power P abs of 1000 W To control the ion energy at the substrate surface the electrode on which the silicon wafers were placed was independently driven by a capacitively coupled 1 MHz power source as shown in Fig 1 Here the impedances of the low pass filter elements L and C are 628 and 4 respectively such that only 0 6 percent of the rf voltage appears across the capacitor and the dc self bias voltage on the electrode is directly measured with a high impedance voltmeter The self bias voltage induced by the 1 MHz power was in the range of between 0 and 120 V For the measurements of photoresist etch rate half of a four inch silicon wafer with 2 m as measured by ellipsometry of hardbaked Novolak positive tone photoresist was clamped at the center and for uniformity measurements at the vertical edges of the processing area 20 cm from the center The etch rate and etch uniformity were measured for a Permanent address Functional Devices Research Laboratories NEC Corporation 1 1 Miyazaki 4 chome Miyamae ku Kawasaki Kanagawa 2168555 Japan b Electronic mail lieber eecs berkeley edu 0021 8979 2001 89 10 5318 4 18 00 5318 2001 American Institute of Physics Downloaded 14 May 2007 to 128 32 47 46 Redistribution subject to AIP license or copyright see http jap aip org jap copyright jsp J Appl Phys Vol 89 No 10 15 May 2001 K Takechi and M A Lieberman 5319 FIG 1 dc self biasing system and probe with L and C sized for 1 MHz operation FIG 3 Etch rate at the center of the substrate holder as a function of gas pressure for various self bias voltages various self bias voltages and gas pressures The thickness of photoresist film removed was divided by the etch time to determine the etch rate We also measured plasma density profiles along a vertical line perpendicular to the antenna rods with a Langmuir probe changing the self bias voltage and identified the achievement of launching a traveling wave using four voltage sensors equally spaced along the antenna coil 8 Ion saturation currents were used to calculate the plasma densities based on the orbital ion motion model for Langmuir probes 6 much whereas at higher pressures increasing the voltage significantly increases the etch rate In the low pressure regime the etch rate is O atom flux limited therefore increasing the self bias voltage has little effect on the etch rate In the high pressure regime the etch surface becomes flooded with O atoms and the etch rate is determined by the energetic ion flux


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