E L S E V I E R Microelectronie Engineering 41/42 (1998) 83-86

MICROELECTRONIC ENGINEERING

Sub-Resolu t ion Feature O P C as an Enabler for Manufac tur ing at 0 .2gm and Be low

J. Randall@, A. Tritchkov*, K. Ronse*, and P. Jaenen*

@Texas Instruments, Dallas, Texas - on Assignment at IMEC *IMEC, Kapeldreef 75, B-3001, Leuven, Belgium

Optical proximity correction (OPC) with sub-resolution assisting features (outriggers) aided by adjustable numerical apertures and partial coherence is shown to provide improvement in process windows through tightening printed critical dimensions (CDs) and improving the depth of focus. Printed CD variations due to mask errors are not exacerbated by the use of outriggers and may even be reduced by this type of OPC.

1. INTRODUCTION As the critical dimensions (CDs) printed by

optical lithography shrink below the physical wavelength of the exposing radiation, optical proximity effects (OPE) are increasingly hard to ignore. We address three important issues created by OPE: The shift in size even at best focus of linewidths as a function of the local pattern environment (the pitch of an array of lines), depth of focus (DOF) limitations, and the magnification of mask CD errors[l]. This paper will deal primarily with the printing of 0.2p.m lines at various pitches on a clear field mask with 248mn lithography. We assume that a manufacturable lithography process will require mutually overlapping process windows with at least a 0.6gm depth of field and 10% exposure latitude for all relevant pattern pitches, not just the extremes of the range of pitches.

The variable numerical aperture (NA) and partial coherence (if) as well as off-axis illumination options on state-of-the-art 248nm lithography systems am important tools in dealing with OPE. Because different pattems contain significantly different spatial frequencies, the optimum optical configuration for each pattern can be quite different. However, these variables create a large parameter space that must be explored to find suitable conditions for specific applications.

Feature size biasing is the most common type of OPC. However, feature biasing can only shift CDs and does little to improve the change in CD with defocus that is particularly acute with isolated lines. The use of outriggers has been shown to be effective in both correcting the CD and decreasing the change in CD as the image is defocussed. The selection of appropriate optical conditions and the use of outriggers is useful for establishing overlapping process windows for patterns with a wide variety of

pitches. The use of outriggers does require relatively small features on photomasks, but commercial mask makers have already demonstrated the ability to make such masks. Commercial software exists that automatically assigns placement of outriggers to mask layouts[2].

2. PROCESS WINDOW IMPROVEMENT A test reticle called RTP4 was loaned to IMEC

by the MicroUnity Systems Engineering Inc. for evaluation of its perfommnce at 248nm. The reticle contains many test structures and had outriggers placed according to several different rule sets for different target CDs. The 4X mask was fabricated by Photronics. We investigated primarily 0.2gm line patterns with fine-to-space ratios of 1:1, 1:2, 1:3, and 1:4. The outriggers applied to these patterns were nominally 75nm (IX) and placed (edge-to- edge) 0.251ares (IX) from isolated features. There were no outriggers placed between the 1:1 and 1:2 line arrays. The 1:3 array had a single 75nm outrigger placed in the center of the space between the lines. The 1:4 array had two 75nm outriggers in the space between the lines, each placed 0.25gms from the nearest 0.2gm line. By selecting these particular line to space ratios, we avoided the transition regions at approximately 1:2.5 and 1:3.5 where first one then two outriggers are placed between the 0.2gm lines. MicroUnity is exploring techniques to produce smooth transitions[3]. Prior to making test exposures with this mask on the ASML PAS 5500 /300 deep-UV (248nm) stepper, we used aerial image simulation to select settings for the NA, o, and for annular illumination conditions an inner boundary for the partial coherence (o-inner). In the simulations, NA was varied from 0.4 to 0.63, o was varied over 0.3 to 0.8, c-innercovered the range 0 to ~-0.3. We used

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84 J. Randall et al. / Microelectronic Engineering 41/42 (1998) 83-86

aerial image simulation with Depict 3.1 to determine optimized illumination conditions for the rule set descn'bed above on the RTP4 reticle. The goal of the optimization was to maximize the overlapping process window for the range of line to space ratios (1:1, 1:2, 1:3, 1:4) of 0.21.tm critical dimension (CD) lines with outrigger OPC. The process window for a particular line-to-space ratio under specific optical conditions was determined by calculating the aerial image intensity at positions plus and minus 10% of CD from the line edge over a range of focus settings. The negative value of the natural log of these intensities can be seen to be relative doses at which a high contrast resist would print lines at plus and minus 10% of the desired CD. Plotted at different values of focus, these doses define the boundaries of the exposure-defocus (ED) process window. For a particular set of illumination conditions, NA, ~, and 6-inner, the ED windows of different line-to-space ratios were calculated with Depict 3,1. A C program was used to calculate the rectangular mutually overlapping ED process window. Norman-Debora software was used to drive Depict and the C program to explore the realizable parameter space of optical settings on an ASML PAS 5500 /300, with the goal to maximize the mutually overlapping process window in units of lams (DOF) times percent (exposure-latitude).

For a clear field mask with 0.21xm CD lines, we found a relatively narrow range of exposure conditions that produced a good overlapping process window. For instance with conventional illumination at NA=0.63 and ~r=0.75, there is a rectangular process window with 0.6~tm DOF and 10.5% exposure latitude. For NA=0.6 and <;--0.5, on the other hand there was no overlapping process window. For annular illumination conditions there was an optimum near NA=0.61, t~=0.7, and ~ - inner=0.37. These conditions produced a smaller overlapping exposure latitude (8.5%) at a DOF=0.61.tm but DOF extended to 0.81.tm with at least some (~3%) exposure latitude. Focus exposure matrix ex2aeriments on the ASML PAS 5500 /300 with the 0.7pans ofTOK DP022 resist on DUV-18L bottom anti reflective coating (BARC) were carried out for both the conventional (NA=0.63, ~r=0.75) and annular (NA=0.61, ~=0.7, 6-irmer=0.37) illumination conditions. All measurements were with a top-down CD SEM before BARC etching. In both cases, the resulting overlapping process windows agreed well with the simulations. Figure 1 shows the experimentally determined process window for optimized conventional illumination.

0.2 . . . . . ~ . . . . ~ . . . . . . . . . '.. . . . . '.. . . . . '.. . . . . '.. . . . .

.......... i . . . . . . . . .

o . . . . . . . . . ! .......... h ....... ! . . . . . . . . .

. . . . . . . . . . .......... .......... . . . . . . . . .

+ , + i + . 2 . ... , t " - - . . . . . . , - -

-0.3 ......... ~ ......... ~..Ir..~,.~.......i. ......... & ...... ~.~...¢..~ ........ : . : I l i ~ | | - ~ . , i~

-0.4 : " ' " : " : .................... .,...~.'...~ ...................... ~..,~, .............. i" r; i ", ~ i , r ~ , - i "0'5~llllllllllllllllil llllll lllllllll l l l l l l l ~ 17 I~ t~ 2b 2"1 2~2 2

Dose Irr~l

Figure 1. Experimental (NA=0.63 and o'=0.75) ED process window for 0.21am lines with 1:1 (solid lines), 1:2 (long-dash lines), 1:3 (medium-dashed lines), and 1:4 (short-dashed lines) line-to-space ratios. The area between the lines on the left and right represent exposure and defocus conditions where lines will print within +/- 10% of 0.21am.

3. IMPACT OF OUTRIGGER CD ERRORS There has been concern about the required CD

control on the mask for the outriggers. We have investigated the issue of impact of mask writing errors on printed CDs with both simulation and experiment. We have found that size errors on outriggers have a weak effect on printed CDs.

220

- • 2 1 5 ........................ ~ ............. ~ ............. ~- ............ ~ ...........

210 ............ ~ ......... ~ ............. ~ ............. ~. ............ ~. ...........

205" ........... ~ ~ ~ % .- . . ~ - L ~ , , a ...........

2 0 0

Bar Separation irma]

Figure 2. Measured data showing the impact of outrigger size and spacing on printed 0.21xm lines. The curves that are solid, long-dashed, and short- dashed represent lines printed with 75nm, 100nm, and 125nm outriggers respectively. Exposures were with TOK DP022 resist on organic BARC with NA=0.63 and a=0.75.

The results, shown in Figure 2, can best be summarized by stating that a 10nm change in size of an outrigger results in a change of no more than lnm in the printed feature for this illumination setting. A change in outrigger separation of 10nm could result

J. R a n d a l l e t al. / M i c r o e l e c t r o n i c E n g i n e e r i n g 41 /42 (1998) 8 3 - 8 6 85

in a 2nm change in printed CDs for spacing below 250nm and little if any change in printed CDs for outrigger spacing above 250nm. Simulations for other conditions have indicated that printed CD variations to outrigger size changes can be larger when optical conditions produce stronger OPE. In the cases of strong OPE that we simulated a 10nm CD error in the scattering bar resulted in a printed CD change of less than 4nm. These results, indicate that specifications for outrigger CD control in mask making can be looser than the required CD control of the main features.

4. MASK ERROR REDUCTION Another effect that could be considered an

optical proximity effect is that mask CD errors can be exaggerated or magnified on the mask[l]. Since the error budget for CD errors on the mask is a substantial portion of the total CD error budget, any magnification of that error is a matter of concern. Using aerial image simulation with Depict 3.1, we can isolate the effect of CD errors on the mask. In the following, we will consider the effect of plus and minus 10nm (1X) CD mask errors on the printing of 0.21.tm isolated lines. These would be equivalent to 40nm or 50nm errors on 4X or 5X masks respectively. For a 248nm optical system the conditions NA=0.63 and cr=0.75, the magnification factor for a 10nm (IX) mask error on a 0.2/.tm isolated line is slight at best focus: a 10.4nm printed CD error for a 10ran mask error. The magnification factor increases at a defocns of 0.3ktm to produce a CD error of 11.5nm.

If 75rim outriggers are placed 250nm edge to edge from the nominal 0.2~m line and their size and center to center position does not change as a 10rim error is applied to the 0.2~tm line, then the printed CD errors are essentially the same except for a very slight increase in error as the image is defocussed. However, if we assume that whatever mask writing error created a 10rim error in the 0.21.tm line would also cause a 10nm error in the outriggers as well, then the simulations show that the printed CD errors are reduced from 10.4nm to 8.9nm at best focus and remain below the uncorrected line error out to 0.31.tm defocus. We believe that this mask error reduction is caused by the same optical bandwidth limitation that leads to optical proximity effects. Consider Figure 3 which shows the aerial simulations of CD response vs defocns of a 1:1 0.2t~m lines and spaces, an isolated 0.21am line, and an isolated 0.21.tm line with 75nm outriggers. The dense (1:l) lines are isofoeal, while the isolated line is printed at

--0.221xm at best focus (the iso-dense bias is 20nm) and the printed linewidth drops rapidly with defocns. The addition of 75nm outriggers adds spatial frequencies that makes the isolated line appear more like a dense line. The line prints closer to 0.2txms at best focus and the response with defocus is closer to the isofocal response of the dense line.

0.23

0.22 -

0.21

"~ 0.2 t21 r..)O. 19

0.18

0.17

. . . . . . . . . . . . . . * , . . . . . - - , , , . _ ~ . ~ . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . .

.-. .-. : -. : ................. ................................ . ~ . ~ ....... ,... .............

. . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . .

%

.................................................................. ~.~? .............

o!3 o!, o Defocus [uml

Figure 3. Aerial image simulation of the CD responses of an isolated 0.21.tm line (short-dashes), an 0.21.tm isolated line with 75nm outriggers (long- dashes), and 0.21.tm dense lines (solid-line). The conditions were X=248nm, NA=0.63, and 0"=0.75.

Consider the isolated line with outriggers where a mask writing or processing error results in the line to be written larger than 0.21.tm. The printed linewidth will be larger, however, if the outriggers also become larger, the combination will look "more dense" and OPE will tend to make the line print smaller therefore reducing the impact of the mask error. Similarly if mask errors result in both line and outriggers to print smaller, they will appear "less dense" and OPE will drive towards a larger printed line which also reduces the effect of the mask error on the printed CD. Two variations that support our contention that OPE produces the mask error reduction and demonstrate paths to enhance the effect are described below.

Because the outriggers are close to the resolution limit of mask writing processes, it is not unreasonable to assume that they are more sensitive to processing variations than the main features. If assume arbitrarily that the size changes in the outriggers are twice as large as the changes in the 0.21xm lines, the error reduction effect is enhanced. For instance, according to the Depict simulations, a 0.211.tm line (+10nm error) surrounded by 95nm outriggers (+20nm error) will print at best focus only 73nm larger than the case with no errors

86 J. Randall et al./Microelectronic Engineering 41/42 (1998) 83-86

Similarly a -10nm error in the line and -20nm error in the outriggers results in a -7.6ran error in the printed line. The exaggerated errors in the outriggers enhances the tendency to "more dense" and "less dense" for the same main feature error and strengthens the error correction effect.

If the driving force for the error correction is OPE, then the effect should be strengthened by selecting optical conditions that have stronger OPE. If the partial coherence factor is reduced from 0.75 to 0.5 then the iso-dense bias is increased from 201un to almost 50ran. In other words a 0.21.tin isolated line will print at 0.251ams for the same exposure conditions that will produce dense 0.21xm lines at the correct CD. For such strong OPE conditions, the addition of 75nm outriggers is not enough to produce 0.21.tm lines within specifications. However, biasing the isolated line to 0.181.tins and adding 75nm outriggers spaced 0.251ams from the edge of the biased line, the CD response is well within specifications from +/-0.31ares of best focus. For these conditions we simulated printed errors as a result of +/- 10nm (IX) mask errors on the biased line for the cases where the outrigger CD errors were +/-10nm and +/-20nm. These results are shown in Figure 4. For the case of +/-20nm outrigger errors at best focus, the resulting printed errors are less than 5nm from the CD of the line with no mask errors.

. . . . . . . . . . . : : : . . . . . . . . . . . .

o J ........... ......... i ............ • ".'- - ~ -- :~_"-- k i ~ "

o,9 - . . . . . . . . . . . . . . . . . . . . . . .

o,8 . . . . . . . . . . . i . . . . . . . . . . . . i . . . . . . . . . . . . . i . . . . . . . . . . . . i . ' , v . . . . i . . . . . . . . . . . -

O, or, i o!, o i o, i o Defocus luml

Figure 4. Aerial image simulation of the printed errors that result from +/-10ran (IX) mask errors for an isolated line biased to 0.181.tm and has 75xma outriggers. The solid line is the CD response with no mask errors. The short dashed lines are the result of simulations where the errors in the outriggers are the same as the line (+/-10nm). The long-dashed lines have +/- 20nnl errors in outriggers. The optical conditions were NA=0.63 and a=0.5.

While these results suggest dramatic reductions in mask errors are possible, several factors should be noted. At this point only simulations support the existence of this effect Outside of +/-0.31asns defocus the mask errors are magnified rather than reduced. However, since an 0.61xm depth of focus is considered adequate for 248nm lithography this should not be a problem. This effoct is achieved with outriggers, therefore no enhanced mask error reduction is possible for relatively tight pitches where outriggers may not be used. In this manner it is complimentary to the proposed pupil filtering method that achieves mask error reduction only for tight pitches[4]. Finally, the simulations showing the error reduction effect have assumed that errors in the outriggers are directly proportional to random errors in the size of the design features on a reticle. In real reticles, we would expect some correlation in main feature and outrigger errors, but we know that there will also be some component of uncorrelated size errors.

5. CONCLUSIONS We have shown experimentally that sub-

resolution feature OPC can provide a mutually overlapping process window with marly 0.61am depth of focus and more than 10% exposure latitude at best focus for a wide range of pitches of 0.21am lines. We have also shown that the specifications for size control of the outriggers on the mask can be significantly looser than the size control required for printed features. Finally we have shown that it may be possible to extend the capabilities of OPC to include mask error reduction.

6. ACKNOWLEDGEMENTS The authors are indebted to Luc Van den hove

and Gene Fuller for insight and encouragement and also to Fung Chert, Roger Caldwell, Kurt Wampler, and Tom Laidig of MicroUnity for guidance and the use of the RTP4 reticle.

REFERENCES 1. Wilhelm Mauer BACUS Proc. SPIE 2884 (1996) 562 2. MicroUnity Systems Engineering, Inc. 475 Potrero Avenue, Sunnyvale, CA 3. J. Fung Chen, Tom Laidig, Kurt Wampler, and Roger Caldwell, To be published JVST-B Nov. 97 4. Shuji Nakao, Akihiro Nakae, Kouichiro Tsujita, and Yasoji Matsui: Proc. SPIE 3051 (1997) 77.