contrast enhancement lithography

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1 Comparson of modelng and expermental results n contrast enhancement lthography Comparson of modelng and expermental results n contrast enhancement lthography Tom Brown General Electrc Co. Phoenx, AZ Chrs A. Mack Natonal Securty Agency Fort Meade, MD Tom Brown Chrs A. Mack General Electrc Co. Natonal Securty Agency Phoenx, AZ Fort Meade, MD Introducton Introducton Characterzng the effects of the many varables that are a part of photolthography s not an easy task. As more sophstcated patternng technques, such as contrast enhancement lthography (CEL), are ntroduced the number of varables ncreases. Two tools are commonly used to deal wth the complex task of understandng the nterrelatonshps of these many varables: expermentaton and computer smulaton. Whle expermentaton, and n partcular statstcally desgned experments, can gve a great deal of nsght nto a partcular system, t s often dffcult to extrapolate trends outsde of the expermental condtons (e.g., to other lthographc systems). Computer modelng, on the other hand, s rather poor at predctng exact outcomes of specfc condtons, but s at ts best when predctng trends whch may exst over a broad or narrow range of condtons. There exsts, however, an uncertanty as to how accurately the model reflects condtons of the real world. Characterzng the effects of the many varables that are a part of photolthography s not an easy task. As more sophstcated patternng technques, such as contrast enhancement lthography (CEL), are ntroduced the number of varables ncreases. Two tools are commonly used to deal wth the complex task of understandng the nterrelatonshps of these many varables: expermentaton and computer smulaton. Whle expermentaton, and n partcular statstcally desgned experments, can gve a great deal of nsght nto a partcular system, t s often dffcult to extrapolate trends outsde of the expermental condtons (e.g., to other lthographc systems). Computer modelng, on the other hand, s rather poor at predctng exact outcomes of specfc condtons, but s at ts best when predctng trends whch may exst over a broad or narrow range of condtons. There exsts, however, an uncertanty as to how accurately the model reflects condtons of the real world. Any process smulator has strengths and weaknesses. In optcal lthography modelng the strong ponts nclude: - the ntensty dstrbuton of the aeral mage, - the exposure (bleachng) of the photoresst flm, Any process smulator has strengths and weaknesses. In optcal lthography modelng the strong ponts nclude: - the ntensty dstrbuton of the aeral mage, - the exposure (bleachng) of the photoresst flm, - the formaton of standng waves, and - the photoactve compound (PAC) gradents (called the latent mage). - the formaton of standng waves, and - the photoactve compound (PAC) gradents (called the latent mage). The weak areas are: - a knowledge of the aberratons present n a projecton system, - accurately determned development parameters, - the effects of processng condtons on the development parameters, and The weak areas are: - a knowledge of the aberratons present n a projecton system, - accurately determned development parameters, - the effects of processng condtons on the development parameters, and - the development model tself. - the development model tself. In general, the process of exposure s better understood than development. Thus, expermental confrmaton of smulated results s an mportant part of a comprehensve lthography modelng effort. In general, the process of exposure s better understood than development. Thus, expermental confrmaton of smulated results s an mportant part of a comprehensve lthography modelng effort. Ths paper compares the results of expermental work to computer smulatons n contrast enhancement lthography. Effects to be studed nclude development lattude, feature sze and proxmty effects, exposure lattude, and focus lattude. In all studes, sngle layer resst and CEL are compared both expermentally and wth the computer model PROLITH, v [ -3]. 'Also, an nterestng anomaly has been observed expermentally. A mechansm to explan the phenomenon s proposed and computer smulatons are used to confrm the predcted mechansm. Ths paper compares the results of expermental work to computer smulatons n contrast enhancement lthography. Effects to be studed nclude development lattude, feature sze and proxmty effects, exposure lattude, and focus lattude. In all studes, sngle layer resst and CEL are compared both expermentally and wth the computer model PROLITH, vl [-3]. 'Also, an nterestng anomaly has been observed expermentally. A mechansm to explan the phenomenon s proposed and computer smulatons are used to confrm the predcted mechansm. 390 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) 390 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) Downloaded From: on 06/02/204 Terms of Use:

2 Relatve PAC Concentraton Relatve PAC Concentraton I * r " r r r r Horzontal Dstance (pm) Fgure : Latent mage (PAC concentraton) as a functon of normalzed CEL thckness (Ad = 0, 2, 4, and 8) as predcted by PROLITH. Development Lattude Horzontal Dstance (um) Fgure : Latent mage (PAC concentraton) as a functon of normalzed CEL thckness (Ad = 0, 2, 4, and 8) as predcted by PROLITH. Development Lattude Although most lthography engneers tend to equate process lattude wth exposure lattude, development lattude s also an mportant part of the total process lattude. A major source of lnewdth errors s the nablty to adequately control the development process. For example, resst "senstvty" changes are almost always a result of a change n the resst development propertes. Also, the mportance of prebake s due to the extreme senstvty of development to the prebake process condtons. A process wth good development lattude wll be resstant to these changes. Although most lthography engneers tend to equate process lattude wth exposure lattude, development lattude s also an mportant part of the total process lattude. A major source of lnewdth errors s the nablty to adequately control the development process. For example, resst "senstvty" changes are almost always a result of a change n the resst development propertes. Also, the mportance of prebake s due to the extreme senstvty of development to the prebake process condtons. A process wth good development lattude wll be resstant to these changes. For a development process wth gven propertes, development lattude s determned by two factors: the gradent of PAC and the magntude of the standng waves. A resst wth a large PAC gradent near the mask edge wll exhbt good development lattude. Shown n Fgure s the effect of CEL thckness on the latent mage. As the normalzed CEL thckness (defned as the bleachable absorbance, the absorpton coeffcent A tmes the CEL thckness d) goes from zero to Ad = 8, the PAC gradent near the mask edge mproves dramatcally. (The exposure energy was adjusted to gve the nomnal lnewdth for fxed development condtons). The mprovement n the latent mage s a result of an mportant, though often overlooked, property of CEL: the center of a space receves a hgher exposure than normal snce the CEL bleaches n the center frst. Ths "over- exposure" of the center results n mproved PAC gradents and thus, mproved development lattude. For a development process wth gven propertes, development lattude s determned by two factors: the gradent of PAC and the magntude of the standng waves. A resst wth a large PAC gradent near the mask edge wll exhbt good development lattude. Shown n Fgure s the effect of CEL thckness on the latent mage. As the normalzed CEL thckness (defned as the bleachable absorbance, the absorpton coeffcent A tmes the CEL thckness d) goes from zero to Ad = 8, the PAC gradent near the mask edge mproves dramatcally. (The exposure energy was adjusted to gve the nomnal lnewdth for fxed development condtons). The mprovement n the latent mage s a result of an mportant, though often overlooked, property of CEL: the center of a space receves a hgher exposure than normal snce the CEL bleaches n the center frst. Ths "over-exposure" of the center results n mproved PAC gradents and thus, mproved development lattude. The hgher exposure of the center can also reduce the effects of standng waves. As the resst becomes more exposed, the replcaton of the standng wave pattern begns to bleach out. Ths effect s shown n Fgures 2a and 2b. These fgures are gray -scale reproductons of the PAC concentraton as a functon of x and z. Darker colors mean hgher concentratons of PAC. Fgure 2a shows the case wthout CEL. The standng waves are evdent as the horzontal lght and dark regons. Fgure 2b shows the equvalent case wth CEL (agan, the exposure was adjusted n each case to gve the same lnewdth). As can be seen, the The hgher exposure of the center can also reduce the effects of standng waves. As the resst becomes more exposed, the replcaton of the standng wave pattern begns to bleach out. Ths effect s shown n Fgures 2a and 2b. These fgures are gray-scale reproductons of the PAC concentraton as a functon of x and z. Darker colors mean hgher concentratons of PAC. Fgure 2a shows the case wthout CEL. The standng waves are evdent as the horzontal lght and dark regons. Fgure 2b shows the equvalent case wth CEL (agan, the exposure was adjusted n each case to gve the same lnewdth). As can be seen, the SP/E Vol. 920 Advances n Resst Technology and Processng V (988) / 39 SP /E Vol. 920 Advances n Resst Technology and Processng V (988) / 39 Downloaded From: on 06/02/204 Terms of Use:

3 standng waves have been somewhat "bleached out" by by the hgher exposure n n the center of the space. As a result, development n the center s less hndered by the standng waves. Note from Fgure that the exposure near the mask edge s nomnally the same regardless of CEL thckness. Expermental results verfy the above conclusons. Shown n n Fgure 3 s s both the expermental and smulated varaton of lnewdth wth development tme, wth and wthout CEL. The expermental and modelng condtons are gven n Table I. Development tme was chosen as as beng a representatve parameter of the development process. Varatons n n development rate (caused by by lot lot-to-lot -lot photoresst dfferences, prebake, etc.) should have a smlar effect on lnewdth as varatons n development tme. Thus, development tme lattude s a good measure of the overall development lattude. As can be seen n Fgure 3, the CEL case shows greatly mproved development lattude. Vertcal Dstance z (pm) (ym) Horzontal Dstance x (pm) (urn) (a) (b) Fgure 2: Gray-scale plot of resst PAC concentraton a.) wthout CEL, and b.) wth CEL. (Darker means more PAC, lower exposure.) Feature Sze and Proxmty Effects An mportant aspect of lthography s the ablty to accurately reproduce the mask dmensons on the wafer. A typcal mask wll have features of many types and szes. A plot of the prnted lnewdth versus the mask feature sze wll show any feature sze effects that may be present n a lthographc process. Fgure 4 compares the feature sze effect for sngle layer resst and CEL processes usng equal lnes and spaces. At large ((> pm) features, both processes fathfully reproduce the mask sze. But as the lnewdths go submcron, the prnted lne sze becomes greater than that of the mask. As can be seen n Fgure 4, the standard resst process shows worse feature sze effects than the CEL process. In fact, the smallest lne sze, 0 pm, does not clear n the standard process, but s prnted n the CEL process. PROLITH smulatons confrm the mprovement n n feature sze effect wth the addton of of CEL. The 392 // SPIE Vol. 920 Advances n n Resst Technology and Processng V (988) Downloaded From: on 06/02/204 Terms of Use:

4 Lnewdth (pm) Lnewdth (ym) I T I I I I I I I I I I I I I I I I I I I I I Experment Model n CEL NoCEL Development Tme (sec) Development Tme (sec) 0 20 Fgure 3: The effect of CEL on development lattude. Both expermental data and PROLITH results are presented for 0 pm lnes (see Table I for condtons). Fgure 3: The effect of CEL on development lattude. Both expermental data and PROLITH results are presented for 0 pm lnes (see Table I for condtons). Table I Expermental and modelng condtons for the study of development lattude Table I Expermental and modelng condtons for the study of development lattude Expermental Parameters Modelng Parameters Expermental Parameters Modelng Parameters.23 pm Shpley 83 resst on slcon.23 pm Shpley 83 resst on slcon 60sec, 5 C hotplate prebake GOsec, 5 C hotplate prebake Shpley MF32 developer n spray/puddle mode (GOsec nomnal development tme) Shpley MF32 developer n spray /puddle mode (60sec nomnal development tme) Optmetrx 800D stepper NA= 0.32, 0 =0, X =436nm Optmetrx 800D stepper = 0.32, o = 0, A = 436nm 0 pm equal lnes and spaces 0 pm equal lnes and spaces lnewdth measured at bottom of resst usng SEM lnewdth measured at bottom of resst usng SEM 0pmCEM pm CEM -420 Rmax = 00 nm/s 00nm/S Rmn = 2 nm/s Rmn = 2 nm/s mth = 0.3 mth = 0.3 n = 3.0 n = 3.0 Acel 7-5 Acel = 7 pm- Bce = 0.3 Bcel = 0.3 pm- Cce = 0.06cm2/mJ Ccel = 0.06 cm2/mj SPIE Vol. 920 Advances n Resst Technology and Processng V (988) / 393 SP /E Vol 920 Advances n Resst Technology and Processng V (988) / 393 Downloaded From: on 06/02/204 Terms of Use:

5 Prnted Lnewdth (pm) Prnted Lne wdth (um) '' ' '' ' ' ' '' ' ' ' '' ' '' ' '' ' ' '' ' ' ' '' - Experment Model Experment D CEL No CEL Model I I I.,.. - p. ~,,.,.,,,, r _ I I I I I I I I Mask Lnewdth (pm) Mask Lnewdth (urn) Fgure 4: Feature sze effect - the ablty to reproduce the mask lnewdth wth and wthout CEL. Both expermental data and PROLITH results are presented. Fgure 4: Feature sze effect - the ablty to reproduce the mask lnewdth wth and wthout CEL_ Both expermental data and PROLITH results are presented. Proxmty Rato Proxmty Rato I " F I I I I I FI I I I I I I I f F I I T I I I I I I I I I T I I I T~ Mask Lnewdth (on)..2 Mask Lnewdth (pm).3 Fgure 5: Proxmty effect - the rato of the lnewdths of solated to packed lnes, wth and wthout CEL. Values closer to one mean less proxmty effect. (Sold lnes represent curve-fts through the data). Fgure 5: Proxmty effect - the rato of the Inewdths of solated to packed lnes, wth and wthout CEL. Values closer to one mean less proxmty effect. (Sold lnes represent curve -fts through the data). 394 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) 394 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) Downloaded From: on 06/02/204 Terms of Use:

6 reason for ths mprovement s agan the hgher exposure at the center of the space. As the feature sze decreases, the peak ntensty of the aeral mage of the space decreases. Ths reduces the development rate n the space and results n a larger lnewdth (smaller spacewdth). Snce the use of CEL ncreases the exposure n the center of the space, the reduced peak ntensty of the aeral mage s less mportant. A lthographc process must also reproduce the mask regardless of feature type, e.g., packed versus solated lnes. A change n lnewdth due to the presence of nearby features s called the proxmty effect. The most common example of the proxmty effect n n optcal lthography s the dfference n lnewdth between an solated lne and a lne n the center of a lne/space array. Ths dfference can be quantfed as sze ofsola solated lne /,. x proxmty rato =. ( U sze of packed lne Proxmty effects are a strong functon of feature sze. In general, smaller features exhbt larger proxmty effects. Fgure 5 shows the proxmty rato as a functon of mask feature sze for resst wth and wthout CEL. Smulated results are not shown because PROLITH does not adequately predct ths effect. The cause of ths dscrepancy s s unknown, but under nvestgaton. As can be seen, the CEI. CEL case shows less proxmty effect (a proxmty rato of means no proxmty effect). Ths effect s also shown n Fgure 6a and b, where 0 pm lnes have been maged wth and wthout CEL. Exposure Lattude The effect of CEL on exposure lattude s determned by measurng the change n lnewdth wth exposure energy. Fgure 7 shows that the expermental data does not ndcate any mprovement n exposure lattude wth the use of CEL. The modelng results, however, ndcate otherwse. Wllams, et al., [4] have reported that exposure lattude was mproved when pm of CEM-420 was used, but that no mprovement was seen for 0 lam pm of CEM (0 (0 pm pm was was used n n ths ths study). Clearly, more work, both expermental and modelng, must be done n order to adequately resolve ths ssue. Focus Lattude Focus s another crtcal parameter n optcal lthography. Poor focus condtons have two effects: a change n lnewdth and reducton of the resst sdewall angle. Snce one of the man benefts of CEL s an mprovement n resst sdewall angle, t seems lkely that the use of CEL would make the resst sdewall less senstve to focus changes. Fgure 8 shows smulatons of the effect of focus on the sdewall angle for varous thcknesses of CEL. Obvously, the use of CEL reduces the degradaton of the resst sdewall due to defocus. Thus, for a process whch s sdewall lmted CEL can greatly mprove focus lattude. The effect of CEL on lnewdth changes wth defocus s s less obvous. Fgures 9a and b show lnewdth versus focus for 0, 0,.0, and.2 jm pm lnes wth and wthout CEL. Both the expermental data and the smulatons show some mprovement n focus lattude wth CEL. (Note: PROLITH vl v wth the enhanced focus model was used for these smulatons [3]). From the perspectve of modelng, the focal poston has been defned as zero at the top of the resst (or CEL). Thus, a focal poston of-0 pm means that the aeral mage s beng focus 0 pm nto the resst/cel. It should be noted that exposure lattude s a strong functon of focus and focus lattude s a strong functon of exposure. The best way to characterze the effects of of focus and exposure s s to measure lnewdth over a large matrx of exposure and focus values and determne process wndows of lnewdth control. Ths work s currently n progress. SPIE Vol. 920 Advances n Resst Technology and Processng V(988) V // 395 Downloaded From: on 06/02/204 Terms of Use:

7 (a) 0,a 220J MD 2MM S,03000 P,tjcGa4 SUM (b) ym lnes, 0 um of 0 FgureS: Fgure 6: SEM SEMmcrographs mcrographsdepctng depctngthe theproxmty proxmty effect of CEL. a.) wthout wthout CEL, wthcel. b.) wth and b.) CEL, and a.) Processng V (988) andprocessng Technology and ResstTechnology nresst Advances n 920Advances Vol. 920 SPIE 396 / /SP 396 /E Vol. Downloaded From: on 06/02/204 Terms of Use:

8 Lnewdth (um) Lne wdth (urn) If I I I I ' II I Experment CEL No CEL I I II I I Model _ -'- Experment Model ; a CEL NoCEL ' ' 'a..2.3 Relatve Exposure Energy..2.3 Relatve Exposure Energy Fgure 7: The effect of CEL on exposure lattude (0 ym lne/space). Both expermental data and PROLITH results are presented. Fgure 7: The effect of CEL on exposure lattude (0 pm lne /space). Both expermental data and PROLITH results are presented. Sdewall Angle Sdewall Angle 90,.,.., 90 I I I I I II I I I I I I I I I I I I I II I I II I I I I II I II I I In! ' _ Ad=8.. Am. Ad=4 _ '. -. Ad=2'. NoCEL - I I I I I I I I I I I I I I I I I I I I'I I I I I Iy O.2.2 Defocus Dstance (pm) Ad = Defocus Dstance (um) Fgure 8: The mprovement n resst sdewall angle (0 ym lne/space) wth CEL also mproves focus lattude (from the pont of vew of sdewall angle). Fgure 8: The mprovement n resst sdewall angle (0 pm lne /space) wth CEL also mproves focus lattude (from the pont of vew of sdewall angle). - SPIE Vol. 920 Advances n Resst Technology and Processng V(988) / 397 SPIE Vol. 920 Advances n Resst Technology and Processng V (988) / 397 Downloaded From: on 06/02/204 Terms of Use:

9 Lnewdth (pm).0 pm, 00 pm,.0 pm, x.2 pm, model Lnewdth (vm)»0 on, no ym, A.0pm, x.2pm, model.3.2 (a). 0 ym 0 urn > -4 t l l _ L J L L L l I I r I I I I I.3.2 (b) Relatve Focus Settng (pm) Relatve Focus Settng (vm) Fgure 9: Focus effect - varaton of Inewdth wth defocus: a.) wthout CEL, and b.) wth CEL. Fgure 9: Focus effect - varaton of lnewdth wth defocus: a.) wthout CEL, and b.) wth CEL 398 / SPIE Vol. 920 Advances n Resst Technology and Processng V(988) 398 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) Downloaded From: on 06/02/204 Terms of Use:

10 Dffracton Effects Dffracton Effects Controllng the shape of a resst profle can be as mportant as mantanng control of ts dmenson. Fgure 0 depcts several common resst profles. The normal profle, obtaned by a typcal sngle layer postve resst process, has sloped sdewalls due to absorpton and the nature of the development process [5]. The negatve profle can be obtaned usng a negatve resst (or mage reversal of a postve resst) where absorpton may overcome development propertes and make the sdewall negatvely sloped. The hourglass profle s obtaned when forces compete to make the profle postve and negatve. Usually, vertcal resst profles are the most desrable for purposes of pattern transfer and dmensonal control. However, negatve profles are sometmes purposely created for metal lft -off. In a normal process, negatve profles can cause problems durng etchng and crtcal dmenson measurement. Also, hgh aspect rato features tend to topple over f the profle s too negatve. Controllng the shape of a resst profle can be as mportant as mantanng control of ts dmenson. Fgure 0 depcts several common resst profles. The normal profle, obtaned by a typcal sngle layer postve resst process, has sloped sdewalls due to absorpton and the nature of the development process [5]. The negatve profle can be obtaned usng a negatve resst (or mage reversal of a postve resst) where absorpton may overcome development propertes and make the sdewall negatvely sloped. The hourglass profle s obtaned when forces compete to make the profle postve and negatve. Usually, vertcal resst profles are the most desrable for purposes of pattern transfer and dmensonal control. However, negatve profles are sometmes purposely created for metal lft-off. In a normal process, negatve profles can cause problems durng etchng and crtcal dmenson measurement. Also, hgh aspect rato features tend to topple over f the profle s too negatve. Normal Profle Hourglass Negatve Profle Normal Profle Hourglass Negatve Profle Fgure 0: Typcal photoresst profle types. Fgure 0: Typcal photoresst profle types. Under certan condtons, the use of CEL wll contrbute to the creaton of hourglass or negatve profles. These anomales n profle shape are not always experenced, but are a result of the nteracton of the exposure tool, photoresst system and CEL used. We propose that the mechansm for the creaton of these effects s dffracton. When a separate bleachable layer s placed on top of photoresst and s exposed to an aeral mage, the bleached areas of the CEL act as apertures whch dffract lght. Typcally one thnks of dffracton as occurrng wth a perfectly opaque mask wth transparent apertures. However, dffracton also occurs wth CEL, whch acts as a dffuse mask. Under certan condtons, the use of CEL wll contrbute to the creaton of hourglass or negatve profles. These anomales n profle shape are not always experenced, but are a result of the nteracton of the exposure tool, photoresst system and CEL used. We propose that the mechansm for the creaton of these effects s dffracton. When a separate bleachable layer s placed on top of photoresst and s exposed to an aeral mage, the bleached areas of the CEL act as apertures whch dffract lght. Typcally one thnks of dffracton as occurrng wth a perfectly opaque mask wth transparent apertures. However, dffracton also occurs wth CEL, whch acts as a dffuse mask. To verfy ths mechansm, a modelng study was ntated to quantfy the dffracton by the CEL and to determne f ths phenomenon could be responsble for the creaton of negatve profles. A modfed Krchoff's dffracton approach, prevously ntroduced to model contact prntng [], was agan modfed to model dffracton by a dffuse mask [6]. The "mask" transmttance s determned usng the standard model for CEL [7]. Because the transmttance vares wth exposure tme, these calculatons must be repeated over a seres of small tme ncrements. Smultaneously, exposure of the photoresst s calculated. Development s carred out usng the standard PROLITH algorthms. Fgure s a typcal output of the CEL dffracton model showng a negatve profle. To verfy ths mechansm, a modelng study was ntated to quantfy the dffracton by the CEL and to determne f ths phenomenon could be responsble for the creaton of negatve profles. A modfed Krchoffs dffracton approach, prevously ntroduced to model contact prntng [], was agan modfed to model dffracton by a dffuse mask [6]. The "mask" transmttance s determned usng the standard model for CEL [7]. Because the transmttance vares wth exposure tme, these calculatons must be repeated over a seres of small tme ncrements. Smultaneously, exposure of the photoresst s calculated. Development s carred out usng the standard PROLITH algorthms. Fgure s a typcal output of the CEL dffracton model showng a negatve profle. The model can now be used to predct the effects of processng varables on the formaton of negatve profles. The model accurately predcts that hgher optcal denstes (Ad) of the contrast enhancement layer contrbute to greater dffracton effects. Fgure 2 demonstrates the expermental results of varyng the optcal densty of the CEL. Fgure 2a s a SEM of a resst profle where a CEL of hgh optcal densty was used. Note the extreme negatve profle. Fgure 2b s a SEM of a resst profle where a moderate optcal densty CEL was used. Fgure 2c shows a profle where a lower optcal densty CEL elmnates the negatve profle. The model also predcts that hgher numercal aperture (NA) steppers wll exhbt greater dffracton effects. Most expermentally observed negatve profles have been on 02 NA systems. The model can now be used to predct the effects of processng varables on the formaton of negatve profles. The model accurately predcts that hgher optcal denstes (Ad) of the contrast enhancement layer contrbute to greater dffracton effects. Fgure 2 demonstrates the expermental results of varyng the optcal densty of the CEL. Fgure 2a s a SEM of a resst profle where a CEL of hgh optcal densty was used. Note the extreme negatve profle. Fgure 2b s a SEM of a resst profle where a moderate optcal densty CEL was used. Fgure 2c shows a profle where a lower optcal densty CEL elmnates the negatve profle. The model also predcts that hgher numercal aperture (NA) steppers wll exhbt greater dffracton effects. Most expermentally observed negatve profles have been on 02 NA systems. SPIE Vol. 920 Advances n Resst Technology and Processng V(988J / 399 SPIE Vol. 920 Advances n Resst Technology and Processng V (988) / 399 Downloaded From: on 06/02/204 Terms of Use:

11 Resst Heght (pm) Resst Heght (ym) Mask Poston (pm) Mask Poston (urn) Fgure : Typcal negatve profle smulated usng the CEL dffracton model. Fgure : Typcal negatve profle smulated usng the CEL dffracton model. One of the most mportant varables n formng negatve profles s the development characterstcs of the photoresst. Fgure 3 s a contour plot showng lnes of constant PAC wthn the resst after exposure through a CEL as predcted by the dffracton model. Dependng on the threshold of PAC converson needed by the photoresst/developer system for development, anythng from postve to negatve profles can result. Resst/developer systems wth a low PAC threshold concentraton do not exhbt negatve profles. Also, over -exposure ncreases the degree of negatve profles. One of the most mportant varables n formng negatve profles s the development characterstcs of the photoresst. Fgure 3 s a contour plot showng lnes of constant PAC wthn the resst after exposure through a CEL as predcted by the dffracton model. Dependng on the threshold of PAC converson needed by the photoresst/developer system for development, anythng from postve to negatve profles can result. Resst/developer systems wth a low PAC threshold concentraton do not exhbt negatve profles. Also, over-exposure ncreases the degree of negatve profles. The new CEL dffracton model presented here has accurately predcted the contrbutors to negatve profles. By usng ths model, t was possble to quckly screen several possble varables for ther effect on the profle shape. Negatve profles can be best controlled by adjustng the optcal densty of the CEL, or the development parameters of the photoresst. Snce n many cases the propertes of the photoresst are fxed for all practcal purposes, G. E. has ntroduced a new materal, ALTILITH'" CEM -420Z, whch has a reduced optcal densty. Ths materal has been successful n controllng negatve profles. The new CEL dffracton model presented here has accurately predcted the contrbutors to negatve profles. By usng ths model, t was possble to quckly screen several possble varables for ther effect on the profle shape. Negatve profles can be best controlled by adjustng the optcal densty of the CEL, or the development parameters of the photoresst. Snce n many cases the propertes of the photoresst are fxed for all practcal purposes, G. E. has ntroduced a new materal, ALTILITH CEM-420Z, whch has a reduced optcal densty. Ths materal has been successful n controllng negatve profles. Conclusons Conclusons A study usng both expermentaton and computer smulaton was ntated to determne the benefts of contrast enhancement lthography. An ncrease n development lattude was predcted by the modelng and observed expermentally. Ths ncrease s due to a hgher than normal exposure of the center of a space causng a greater PAC gradent near the lne edge. Experment and model also agreed that CEL reduced the feature sze effect, allowng better reproducton of the mask sze for submcron features. The proxmty effect was seen to be reduced wth CEL, but PROLITH smulatons faled to predct ths trend. A study usng both expermentaton and computer smulaton was ntated to determne the benefts of contrast enhancement lthography. An ncrease n development lattude was predcted by the modelng and observed expermentally. Ths ncrease s due to a hgher than normal exposure of the center of a space causng a greater PAC gradent near the lne edge. Experment and model also agreed that CEL reduced the feature sze effect, allowng better reproducton of the mask sze for submcron features. The proxmty effect was seen to be reduced wth CEL, but PROLITH smulatons faled to predct ths trend. 400 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) 400 / SPIE Vol. 920 Advances n Resst Technology and Processng V (988) Downloaded From: on 06/02/204 Terms of Use:

12 7,6KX NM 30KUW0,5MM S P <POS> CEP, 420 EX EXP 555 M' WAFER' POSITI (a) 7,2KX KU W0 2MM CO N M <POS> T P s+tt (b) Fgure Fgure 2: 2: SEM SEMmcrographs mcrographsdepctng depctng negatve negatve profles profles for CEL CEL materals materalswth: wth: b.) moderate moderate optcal optcal densty, densty, and c.) c.) low low optcal densty. a.) a.) hgh hgh optcal optcal densty, b.) SP /EVol. Vol20 920Advances AdvancesInnResst ResstTechnology Technologyand andprocessng ProcessngVV(988] (988)/ / SPIE Downloaded From: on 06/02/204 Terms of Use:

13 30KU WD 8MM 7,5KX {POS} BEN SAFER MSEX ' POSIT (c) (0 Fgure 2: SEM for CEL CEL materals Fgure 2: SEMmcrographs mcrographsdepctng depctng negatve negatve profles for materals wth: wth: a.) hgh b.) moderate moderate optcal optcal densty, densty, and and c.) c.) low low optcal optcal densty. a.) hgh optcal densty, densty, b.) densty. The The effect effect of ofcel CELon onexposure exposurelattude lattudessstll stll n n queston, queston, wth wth smulatons smulatons and prevous expermental current data datashowng showng lttle lttle dscernable data predctng an mprovement and current dscernable dfference. dfference. CEL CELwas was found found to to have depth-of-focus, have greater depth -of- focus,though thoughonly onlyslghtly, slghtly,usng usnglnewdth lnewdthasasa ameasure, measure,and andsgnfcantly sgnfcantly greater greater depth-of-focus usng sdewall angle asas the depth -of -focus usng sdewall angle themetrc. metrc.under Undercertan certan crcumstances crcumstances negatve negatve profles profles have and a dffracton mechansm mechansm has has been been proposed proposed to to explan explan the the phenomenon. been observed observed expermentally expermentally and phenomenon. Usng new dffracton dffracton model, Usng aa new model, the the effects effects of ofthe the CEL, CEL, photoresst, photoresst,and and exposure exposuretool toolon onthe the onset onset of of negatve Based on analyss, aa new new materal, materal,altilith'" ALTILITH negatve profles profles has has been been accurately accurately predcted. predcted. Based on ths analyss, CEM -420Zhas hasbeen beenntroduced ntroduced to to control control negatve negatve profles. CEM-420Z Computer smulaton Computer smulaton ss helpful helpful nn predctng predctngtrends trendsnn contrast contrast enhancement enhancement lthography lthography and and n n dentfyng Wth the the excepton excepton of of the proxmty proxmty effect, effect, the the computer computer dentfyng the the physcal physcal causes causes of of these these trends. trends. Wth model model PROLITH PROLITH was wasfound foundtotoclosely closelymatch matchthe theobserved observedbehavor behavorofofcontrast contrastenhancement enhancement lthography. lthography. Acknowledgments The authors would would lke the followng followng ndvduals for there help help n n collectng collectng expermental expermental The lke to thank the Bonne Yost, Ron Dak data: Bonne Ron Seehoffer, Seehoffer, Laure Laure Peterson, Peterson, and John Lee of G. G. E. E. Mcroelectronc Mcroelectronc Materals, Materals, Dak Knght of ASM and Tm Tm Henry Henryof oflsi LSI Logc. Logc. Knght of ASM Lthography, Lthography, and SPfEVol Vol20 920Advances AdvancesnnResst ResstTechnology Technology and and Processng Processng V (988) 402 //SPIE (988) Downloaded From: on 06/02/204 Terms of Use:

14 Vertcal Dstance (pm).3.2. o Space I I I I I I I I I I I I I I I Mask Edge Vertcal Dstance (ym) Mask Edge! Lne \ 7 I = I I - = I I t t I 'l Horzontal Dstance (pm) Fgure 3: Contour lnes of constant PAC concentraton smulated usng the CEL dffracton model. References. C. A. Mack, "PROLITH: A Comprehensve Optcal Lthography Model," Optcal Mcrolth. IV, Proc., SPIE Vol. 538 (985) pp C. A. Mack, "Advanced Topcs n Lthography Modelng," Adv. Resst Tech. III, Proc., SPIE Vol. 63 (986) pp C. A. Mack, "Understandng Focus Effects n Submcron Optcal Lthography," Optcal /Laser Mcrolth., Proc., SPIE Vol. 922 (988). 4. R. E. Wllams, et al., "Contrast Enhancement Materals. Effects of Process Varables on Crtcal Dmenson Control wth ALTILITH CEM- 420," Adv. Resst Tech. III, Proc., SPIE Vol. 63 (986) pp C. A. Mack, "Photoresst Process Optmzaton," KTI Mcroelectroncs Semnar Interface '87 (987). 6. to be publshed n ] 7 ' : Space / j L : ' I :.. _ - L I... I j 7. C. A. Mack, "Contrast Enhancement Technques for Submcron Optcal Lthography," Jour. Vac. Sc. Tech. A, Vol. 5, No. 4 (Jul. /Aug. 987) pp TT I I f * I VI I III I. \ Lne V \ : j -7 \ \ j \ \ : \ \ T \ ^ l * : : MI ^-." Horzontal Dstance (um) Fgure 3: Contour lnes of constant PAC concentraton smulated usng the CEL dffracton model. References.C. A. Mack, "PROLITH: A Comprehensve Optcal Lthography Model," Optcal Mcrolth. IV, Proc., SPIE Vol. 538 (985) pp C. A. Mack, "Advanced Topcs n Lthography Modelng," Adv. Resst Tech. HI, Proc., SPIE Vol. 63 (986) pp C. A. Mack, "Understandng Focus Effects n Submcron Optcal Lthography," Optcal/Laser Mcrolth., Proc., SPIE Vol. 922 (988). 4. R. E. Wllams, et al., "Contrast Enhancement Materals. Effects of Process Varables on Crtcal Dmenson Control wth ALTILITH CEM-420," Adv. Resst Tech. Ill, Proc., SPIE Vol. 63 (986) pp C. A. Mack, "Photoresst Process Optmzaton," KTIMcroelectroncs Semnar Interface rs7(987). 6. to be publshed. 7. C. A. Mack, "Contrast Enhancement Technques for Submcron Optcal Lthography," Jour. Vac. Sc. Tech. A, Vol. 5, No. 4 (Jul./Aug. 987) pp SPIE Vol. 920 Advances n Resst Technology and Processng V(988) / 403 SPIE Vol. 920 Advances n Resst Technology and Processng V (988) / 403 Downloaded From: on 06/02/204 Terms of Use: