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232ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234The best etching occurs at the time when thephotoresist pattern is almost the same as the originoptical mask where the difference between the highergray level f i (x, y) and the lower gray g i (x, y) levelturns out to be the largest: " # qmaxi qia f i ðx; yÞx;ya g i ðx; yÞx;y , (9)where a x;yf i ðx; yÞ means the part of the higher grayin the ith partial scan area, and a x;yg i ðx; yÞ means thepart of the lower gray in the ith partial scan area.A gray level of the photoresist on the plate h(x, y)can be transferred to a thickness value t(h(x, y))from a profile meter. Function t is nonlinear. Hence,the intensity measurement of the photoresist plateis acquired by grabbing the image off the plate,with the calculations performed automatically by acomputer.3. Exposure and development experimentsThe testing substrate was placed on the experimentalstage and a 180 180 pixels testing area wasselected. When the exposure button is pressed, thegray-level statistical value of the selected area wascalculated and the exposure variation is recordedwith a histogram diagram.This experiment must be done in a staticenvironment; hence, the testing substrates withdifferent exposure time should be prepared in adust-free room. When the optimum exposure time isattained, the sharpness of the pattern edge becomeshighest. This is the characteristic of the optimumexposure timing.The development fluid was poured in a cup andput on the experimental table; the testing substratewas added to the cup to initiate the developingprocess. When the development start button ispressed, the gray-level summation of the selectedarea with a grabbing frequency 60 frame/s iscounted and recorded. After a certain process time,the system shows the contrast variation diagram ofthe selected area. The maximum contrast of theimage is the optimum development time.At the optimum developing time, the contrastdifference of the exposed and unexposed areabecomes the largest. Figs. 6 and 7 are the relationshipdiagrams between image contrast and developmenttime, which are grabbed by the high-speedimaging system. It shows that before the optimumpixel ratio10.80.60.40.20017 33 50exposure time (second)Fig. 6. The relationship between the exposure time and thepercentage ratio of low gray-level pixels.pixel ratio10.80.60.40.20017 33 50exposure time (second)Fig. 7. The relationship between the exposure time and thepercentage ratio of high gray-level pixels.development time, the percentage ratio of the lowgray-level pixels decreases very fast in the agitateddevelopment liquid, and the percentage ratio of thehigh gray-level pixels increases very fast, whichmeans that the contrast difference between exposedand unexposed areas is larger. The optimumdevelopment period is about 20–29 s. Meanwhile,the contrast difference is the largest, the percentageratio of the low gray-level pixels is the minimum,and the percentage ratio of the high gray-levelpixels is the maximum. When it passes this checkpoint, the contrast difference is slowed down, whichmeans that the percentage ratio of the low gray-levelpixels is increased and the percentage ratio of thehigh gray-level pixels is decreased, and the developmentfluid begins to etch the entire photoresist,where the histogram changes from two peaksback to a single peak, until the entire photoresistdisappears. Instead of using the normal speedimaging system (30 frame/s), the high-speed imagingsystem (233 frame/s) is the most effective way tofilter out noise of reflected image in the agitateddevelopment liquid.In summary, the optimum development time isabout 20–29 s, where the photoresist is ma-P1225and the development fluid is ma-D330 in this
TransversethicknessTransversethicknessTransversethicknessµm2.5001.5000.5000ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234 233µm2.5001.5000.5000µm2.5001.5000.5000Ra: 1.122 µmRq: 1.090 µm2.197µm0.00 40.00 80.00 µmLongitudinal positionRa: 1.529 µmRq: 1.319 µmAHeight ProfileHeight Profile0.00 40.00 80.00 µmLongitudinal positionRa: 1.122 µmRq: 1.091 µmAA2.954 µmHeight Profile2.287 µm0.00 40.00 80.00 µmLongitudinal positionRp: 1.456 µmRv: -1.108 µmARp: 1.626 µmRv: -1.519 µmARp: 1.488 µmRv: -1.112 µmFig. 8. The AFM thickness analysis for exposure time (A) 17 s, (B) 22 s, (C) and 37 s.Aexperiment. In the exposure experiment, the graylevel of the image of the exposed area is the highestupon reaching the best exposure and no low graylevelpixels mix inside. Hence, to compare the entireselected image, this exposure area has a maximumdifference of gray level. In the development experiment,a single peak histogram will be evolved at thebeginning. After a little while, the histogram willchange to two peaks with a smaller difference ofgray level. Then the higher gray level will be equal to1 and the lower gray level will be equal to 0 in thenormalized histogram. When it occurs, then thatis the best developing time. The analysis of thethickness of photoresist by the system software isshown in Fig. 8, where the thickness difference iscalculated between unexposed and exposed point A.When the system is adjusted, the thickness predictionerror for the on-line inspection is retained towithin 4%. From the testing result, it is obvious thatif the developing process is over or under theoptimum time, then the photoresist is not only thinbut also rough. This is because the incorrectdeveloping process causes incomplete etching andcauses the unevenness problem on the photoresistsurface. In contrast, when the optimum exposuretime is selected, a smooth image and sharp patternedge that is better than the over or under exposedimages is obtained.4. Discussion and conclusionThe target of this research was to develop anautomatic photo etching system and to test theeffectiveness and stability of the system. Theexposure experiments not only used the grabbedimage to compare with the original mask patternbut also used the gray level variation of the selectedarea by the histogram method for finding theoptimum exposure time. When the optimum exposuretime is selected, a sharp image that is betterthan the over or under exposed images is obtained.The number of high gray-level pixels for the selectedarea of the test substrate is smaller, whereas thepercentage ratio is smaller. In contrast, whenthe exposure time is over or under the optimumvalue, the image has bright and dark pixels mixed
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