Effect of ion energy on photoresist etching in an inductively coupled,traveling wave driven, large area plasma sourceK. Takechia)and M. A. Liebermanb)Department of Electrical Engineering and Computer Sciences, University of California,Berkeley, California 94720共Received 17 November 2000; accepted for publication 21 February 2001兲We report on the effect of ion energy on photoresist etching in an inductively coupled large areaplasma source driven by a 13.56 MHz traveling wave with oxygen gas. To control the ion energyat the substrate surface, the electrode on which the substrate is placed is independently driven by acapacitively coupled 1 MHz power source. The etch rate increases with increasing ion energy forgas pressure ranging from 1 to 100 mTorr. Ion-induced desorption rate constants 共etch yields兲 areshown to be proportional to the square root of the ion energy. An increase in the ion energy leadsto etch-uniformity improvement over the processing area of 40 cm⫻50 cm, particularly at a low gaspressure of 5 mTorr. A modified photoresist etch kinetics model combined with a spatially-varyingoxygen discharge model is used to explain these experimental results. © 2001 American Instituteof Physics. 关DOI: 10.1063/1.1364648兴I. INTRODUCTIONInductively coupled plasma sources have been studiedextensively in the past ten years as candidates for advancedetch and deposition processing tools.1–5The driving forcesfor the effort are the recognition of their capabilities to gen-erate high density plasmas under low pressures and controlion/neutral fluxes and the ion-bombarding energyindependently.6Although inductively coupled plasma sources have manymerits, the plasma generated is inherently nonuniform due tothe antenna standing wave effect, especially when the plasmasource is scaled to large size comparable to the driving rfwavelength.7To overcome this difficulty, we are investigat-ing an inductively coupled, large area plasma source共LAPS兲, driven by a 13.56 MHz traveling wave launchedalong an antenna embedded in the plasma. The LAPS is arectangular metal chamber providing a processing area of 50cm⫻40 cm for large-size wafers and glass substrates for flatpanel displays.7,8We previously characterized the LAPS forphotoresist etching with oxygen gas, placing an emphasis onhow various gas pressures influence the etch rate and etchuniformity over the processing area. No external bias wasapplied to the substrate holder electrode during the experi-ments. We developed a simplified spatially-varying oxygendischarge model and photoresist etch kinetics model in orderto explain the experimental results.9In this study, we investigate the effect of ion-bombarding energy at the substrate surface on the photoresistetching in the LAPS, changing the substrate self-bias voltage(Vbias) by a capacitively coupled 1 MHz power source. Theetch rate and etch uniformity are measured for various self-bias voltages and gas pressures. We describe a modified pho-toresist etch kinetics model combined with the spatially-varying oxygen discharge model, introducing the functionaldependence of etch yields versus ion-bombarding energy. Aset of etch rate data and the corresponding self-bias voltagesare used to obtain the functional dependence.II. EXPERIMENTSThe chamber is a rectangular stainless-steel box withinterior dimensions of 70 cm in width, 60 cm in height, and20 cm in depth, pierced horizontally by a planar line array ofeight quartz tubes each 2.58 cm outer diameter, 1.7 mmthick, with center-to-center spacing of 7.65 cm. The interiorsof the tubes are at atmospheric pressure. The rf-excited共13.56 MHz兲 antenna system consists of eight copper rodsthreaded through the interiors of the quartz tubes. For thiswork, we use an antenna pattern consisting of four sets oftwo adjacent antenna rods each, connected in series in a ser-pentine configuration.9Typical operating parameters were an oxygen gas pres-sure of between 1 and 100 mTorr and a 13.56 MHz sourcepower (Pabs) of 1000 W. To control the ion energy at thesubstrate surface, the electrode on which the silicon waferswere placed was independently driven by a capacitivelycoupled 1 MHz power source, as shown in Fig. 1. Here, theimpedances of the low-pass filter elements L and C are 628⍀ and 4 ⍀, respectively, such that only 0.6 percent of the rfvoltage appears across the capacitor, and the dc self-biasvoltage on the electrode is directly measured with a highimpedance voltmeter. The self-bias voltage induced by the 1MHz power was in the range of between 0 and ⫺120 V.For the measurements of photoresist etch rate, half of afour-inch silicon wafer with 2␮m 共as measured by ellipsom-etry兲 of hardbaked Novolak positive tone photoresist wasclamped at the center and 共for uniformity measurements兲 atthe vertical edges of the processing area 共⫾20 cm from thecenter兲. The etch rate and etch uniformity were measured fora兲Permanent address: Functional Devices Research Laboratories, NEC Cor-poration, 1-1, Miyazaki 4-chome, Miyamae-ku, Kawasaki, Kanagawa 216-8555, Japan.b兲Electronic mail: [email protected] OF APPLIED PHYSICS VOLUME 89, NUMBER 10 15 MAY 200153180021-8979/2001/89(10)/5318/4/$18.00 © 2001 American Institute of PhysicsDownloaded 14 May 2007 to 128.32.47.46. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspvarious self-bias voltages and gas pressures. The thickness ofphotoresist film removed was divided by the etch time todetermine the etch rate. We also measured plasma densityprofiles along a vertical line 共perpendicular to the antennarods兲 with a Langmuir probe changing the self-bias voltage,and identified the achievement of launching a traveling waveusing four voltage sensors equally spaced along the antennacoil.8Ion saturation currents were used to calculate theplasma densities based on the orbital ion motion model forLangmuir probes.6III. RESULTSFigure 2 shows vertical plasma density profiles for vari-ous self-bias voltages at a gas pressure of 20 mTorr. Theprofiles are close to symmetric about the center, and theplasma density increases only slightly with increasing self-bias voltage, which indicates that independent control of theion/radical fluxes 共through the source power兲 and the ion-bombarding energy 共through the substrate electrode power兲is achieved in this system.Figure 3 shows the etch rate at the center of the substrateholder electrode as a function of