Chern-Sheng Lin, Shi-Xiang Chan, Yun-Long Lay, Shiaw-Wu Chen ...

ARTICLE IN PRESSMaterials Science in Semiconductor Processing 10 (2007) 227–234Automatic inspection in photoresist development processingwith a partial area-imaging deviceChern-Sheng Lin a, , Shi-Xiang Chan a , Yun-Long Lay b ,Shiaw-Wu Chen a , Hsing-Cheng Chang aa Department of Automatic Control Engineering, Feng Chia University, Taichung, Taiwan, ROCb Department of Electronic Engineering, National Chin-Yi University of Technology, Taiwan, ROCAvailable online 10 March 2008AbstractThis study further researches the relationship between the exposure and development time using an automatic inspectionsystem, and the relationship between the development time and photoresist depth. This progressive scan camera with 233frames per second in partial scanning mode was used to obtain the photoresist development processing parameters. In thisreal-time imaging system, we calculate the image contrast for the exposed area and non-exposed area to find the optimumdevelopment time. When the database is adjusted, the thickness prediction error for the on-line inspection is retained towithin 4%. The image-based measurement application with unsophisticated and economical equipment is confirmed in thephoto etching processing.r 2008 Elsevier Ltd. All rights reserved.Keywords: Automatic inspection system; Photoresist depth; Photo etching processing1. IntroductionIn micro-fabrication, the key techniques forupgrading production depend on the ability ofphotolithography technology that dominates notonly the characteristics of the coated photoresist butalso the exposure and development processes [1–3].Too short an exposure time makes the final imageincomplete. On the other hand, prolonged exposuregenerates some unexpected patterns on the image.Hence, the exposure time deeply affects photoquality. How to control the exposure and developmenttime is very important in the photolithographyprocedure. Most studies on the photoresist etching Corresponding author.E-mail address: lincs@fcu.edu.tw (C.-S. Lin).characteristics were carried out by an APM, wherethe photoresist has been coated on the surface of aglass or semiconductor. Although those experimentsmeasured the thickness of the photoresist statically,the information within the development procedurecould not be measured and corrected during thetesting time [4]. The prolonged development timemight possibly cause the whole film to fail. Hence,how to measure the characteristics of a photoresistin the development stage is an important subject.In this paper, a novel algorithm has been proposedby using a high-speed imaging system [5] to measureand compare the partial area image during processing.Our imaging hardware included Corona-IIframe grabber and CV-M40 CCD camera with aresolution of 659 494 pixels. This progressive scancamera provides up to 233 frames per second in1369-8001/$ - see front matter r 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.mssp.2008.01.003

228ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234partial scanning mode. All the CCD functions can becontrolled manually or automatically using a computerthrough the RS-232C interface. The CV-M40CCD camera grabs the image of the target on theexperimental platform and passes it to a Corona-IIframe grabber for digitizing the image. The combinationof partial scanning and double-speed readoutwith progressive scan sensors was suitable for theinspection and measurement tasks in real-timedevelopment analysis.Among the parameters that effect photolithography,the most important are exposure and developmenttime, which effect the coating photoresistcharacteristics. During development, the contrast ofthe image will change with time [6]. The high-speedmicro photo measurement system has a CV-M40CCD camera with the frame-delay readout modethat reset the unsynchronization mode using anexternal trigger signal and eight different shutterspeeds set to control the exposure time. Thecontrolling speed for video readout is triggered byan external signal with different pulse lengths thatinclude three modes called normal readout, binningreadout, and partial scan. The timing diagramof the partial scan mode is shown in Fig. 1. Inorder to obtain high resolution of intensity distributionof each pixel accurately and effectively, thesetting control bias for the light intensity is in theimage-processing procedures. We divided the biasadjustment into four levels so that the total scalewill be 256 for brightness with 2 mm 2 mm visualfield.2. Imaging system designDuring the development processing operation,the designed chip or glass substrate is kept under theCCD camera, and the aperture as well as the focusis adjusted to obtain a sharp image. The images aregrabbed by the computer through the interfaces andthen analyzed. To confirm the distribution ofintensity from reflection in advance, we calculatethe intensity of reflection and transmission ray [7] bywave theory as shown in Fig. 2.We could get the s vector of reflectance by thewave theory:r ? ¼ n 1 cos y 1 n 2 cos y 2, (1)n 1 cos y 1 þ n 2 cos y 2where y 1 and y 2 are the incident and refractiveangle. n 1 and n 2 are the refractive indices of themedium.The p vector of oscillation reflex rate:r P ¼þ n 2 cos y 1 n 1 cos y 2. (2)n 2 cos y 1 þ n 1 cos y 2The s vector of transmission:2n 1 cos y 1t ? ¼. (3)n 1 cos y 1 þ n 2 cos y 2The p vector of transmission:2n 1 cos y 1t P ¼. (4)n 2 cos y 1 þ n 1 cos y 2The light source we used is not polarized. Theenergy from p and s vectors of light source is half ofExt.Trig0.1ms~40ms1HVIDEO OUTWEN(Positive)WEN(Negative)Effective readout line240 line120 line60 line30 lineExposuretimeA45H66H80H87HB240H120H60H30H1Hv.transferarefreezedC13H17H19H20HAValid periodB7HWaiting forExt.TriggerNote1:Sync or HD is not mentionedNote2:1H=31.777£gsNote3:WEN polarity can be chosen at JP22 of 1/F boardCFig. 1. The timing diagram of a frame-delay readout in the partial scan mode.

230ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234smoothness of the image and sharpness of thepattern edge, the best exposure processing time canbe found. If the test substrates are over or underexposed, the image will not be sharp enough. Thisshows that the image has bright and dark pixelsmixed inside and the summation of the low graylevelpixels will be larger, also the percentage ratiowill be higher for the selected area. On the otherhand, the number of high gray-level pixels for theselected area of the test substrate as well as thepercentage ratio will be smaller. A properly exposedsubstrate shows a sharp image better than those ofthe over or under exposed images.Fig. 4 shows images grabbed in real time by thehigh-speed imaging system. From the images, it iseasy to find that the longer the development time theclearer the image of the photoresist pattern. Thecontrast of the exposed portion is higher than that ofthe un-exposed portion for the selected area. However,when the optimum developing time passes, thedevelopment fluid begins to etch the entire photoresist,which destroys the exposed and unexposedportions and causes the contrast to degrade. Hence,as the optimum development comes, the substrate istaken out from the development fluid and preparedto do the next manufacturing process.Fig. 5 consists of histograms which show a singlepeak that occurs when the developing time is notlong enough. The exposed and unexposed areas arenot clear, hence the contrast of the image is notobserved. After a little while, the pattern of theexposed area shows up, and the histogram willchange from single peak to double peaks. The moretime the passes, the better the image contrast with asmaller percentage ratio of low gray-level pixels andlarger high gray-level pixels [6].Fig. 4. The 640 480 image after development for different time periods (A) 1.8 s, (B) 6.3 s, (C) 10.7 s, (D) 16.9 s, (E) 21.7 s, (F) 26.2 s,(G) 30.8 s, (H) 35.9 s, (I) 40.2 s, (J) 45.5 s, (K) 50.4 s, and (L) 55.9 s.

ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234 2311500The histogram for 1.8 seconds development process1500The histogram for 30.8 seconds development processpixels1000500pixels10005000050100 150 200 250gray scale0050 100 150 200 250gray scalepixels1500100050000The histogram for a 6.3 seconds development process50 100 150 200 250gray scalepixels120010008006004002000The histogram for 35.9 seconds development process0 50 100 150 200 250gray scalepixels1200100080060040020000The histogram for 10.7 seconds development process50 100 150 200 250gray scalepixels120010008006004002000The histogram for 40.2 seconds development process0 50 100 150 200 250gray scalepixels120010008006004002000The histogram for 16.9 seconds development process0 50 100 150 200 250gray scalepixels150010005000The histogram for 45.5 seconds development process0 50 100 150 200 250gray scale1500The histogram for 21.7 seconds development process1500The histogram for 50.4 seconds development processpixels1000500pixels100050000 50 100 150 200 250gray scale00 50 100 150 200 250gray scale1500The histogram for 26.2 seconds development process1500The histogram for 55.9 seconds development processpixles1000500pixles100050000 50 100 150 200 250gray scale00 50 100 150 200 250gray scaleFig. 5. The histogram of images for development times (A) 1.8 s, (B) 6.3 s, (C) 10.7 s, (D) 16.9 s, (E) 21.7 s, (F) 26.2 s, (G) 30.8 s, (H) 35.9 s,(I) 40.2 s, (J) 45.5 s, (K) 50.4 s, and (L) 55.9 s.

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

234ARTICLE IN PRESSC.-S. Lin et al. / Materials Science in Semiconductor Processing 10 (2007) 227–234inside, causing the image to become fuzzy, andthe summation of the low gray-level pixels will belarger, the percentage ratio will be higher, and thehigh gray-level pixels will be lower for the selectedarea.In the beginning of the developing stage, the areapercentage ratio of low gray-level pixels is lower,which means that the photoresist pattern is notpresent or is unclear. After a certain time, thephotoresist pattern shows up gradually and the areapercentage ratio of the low gray-level pixels will belower and that of the high gray-level pixels will behigher. The main method is to check the comparisonof the image contrast for the exposed area andnon-exposed area, that is, the percentage ratiobetween the low gray-level area and the high graylevelarea, to find the optimum development time.The experimental results verified that this imagingsystem provides an economical and effective methodfor producing a micro-photo-etched product.The multipurpose image-based measurement applicationwith unsophisticated and economical equipmentis confirmed in the photo etching processing ofthe photolithography technology. We hope thissystem can bring higher quality and effect to themicro-fabrication.AcknowledgmentThis work was sponsored by the Taiwan NationalScience Council under Grant number NSC 96-2221-E-035-028-MY3.References[1] Chang RS, Lin CS, Hu CP. Fuzzy analysis of the lineareqivalent for fabricating a hoe in photoresist. Jpn J Appl Phys1995;34(3):1554–60.[2] Morton SL, Degertekin FL, Khuri-Yakub BT. Ultrasonicsensor for photoresist process monitoring. IEEE TransSemicond Manuf 1999;12(3):332–9.[3] Lin GR. Dynamical mapping and end-point detection ofphotoresist development by using plastic-fiber-bundle probearray. IEEE Trans Instrum Meas 1999;48(6):1319–23.[4] Lin CS. Dichromated gelatin hologram using digital imageprocessing technology to optimise processing. Imaging Sci J2000;48(2):93–105.[5] Jing X, Chau LP. An efficient three-step search algorithm forblock motion estimation. IEEE Trans Multimedia 2004;6(3):435–8.[6] Lin CS, Lay YL, Chan SX, Ho CW, Chiou YC. The highspeedmeasurement of partial area imaging system applied todevelopment processing. J Chin Inst Eng 2005;28(4):721–6.[7] Cho J, Lin CS, Jan BJ, Lin CH, Chang NC. An optoelectricalmethod for measuring the gap of LCD glass plates.Opt Laser Technol 2005;37(8):623–30.