Influence of shifter errors on the printability of L/S structures using the alternatedphaseshifting design: Simulations and experimentsMaaike Op de Beeck, Kurt Ronse, Kazuya Kamon, Masato Fujinaga, Hal Kusunose, and Hiroaki Morimoto Citation: Journal of Vacuum Science & Technology B 10, 2468 (1992); doi: 10.1116/1.586041 View online: http://dx.doi.org/10.1116/1.586041 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/10/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Influence of roughness in the phase-shifting speckle method: An experimental study with applications Rev. Sci. Instrum. 85, 015114 (2014); 10.1063/1.4861909 Multiplelayer blank structure for phaseshifting mask fabrication J. Vac. Sci. Technol. B 14, 63 (1996); 10.1116/1.588435 Exposure characteristics of alternate aperture phaseshifting masks fabricated using a subtractive process J. Vac. Sci. Technol. B 10, 3055 (1992); 10.1116/1.585928 Fabrication of phaseshifting masks with shifter overcoat J. Vac. Sci. Technol. B 9, 3150 (1991); 10.1116/1.585329 A PhaseShift Refractometer Rev. Sci. Instrum. 22, 162 (1951); 10.1063/1.1745870

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Influence of shifter errors on the printability of LIS structures using the alternated phase-shifting design: Simulations and experiments

Maaike Op de Beeck LSI Laboratory, Mitsubishi Electric Corporation, 4-1 Mizuhara. ltami, Hyogo 664, Japan

Kurt Ronse Imec vzw, KapeldreeJ 75, 3001 Leuven, Belgium

Kazuya Kaman, Masato Fujinaga, Hal Kusunose, and Hiroaki Morimoto LSI Laboratory, Mitsubishi Electric Corporation, 4-1 Mizuhara, ltamt, Hyogo 664, Japan

(Received 23 March 1992; accepted 1 September 1992)

It is well-known that many improvements concerning resolution and focus latitude can be obtained by the use of phase-shifting masks. Different phase-shifting mask designs are proposed already, and one of the most suitable designs for periodical structures is the alternated shifter design. If such phase-shifting masks are investigated, it is assumed that aU shifter parameters are ideal. A major problem, however, is the production of such an ideal phase-shifting mask with perfect shifter parameters. In this work, the influence of shifter parameters such as transparency, thickness, and sidewall slope are studied for the alternated phase-shifting design. The investigation is carried out by simulations and by experiments, and for various coherence factors. It is found that all shifter deviations cause linewidth differences between shifter and quartz lines, often resulting in a reduced depth-of-focus (DOF). Especially if various deviations occur simultaniously, the DOF reductions are important, putting stringent demands on mask processing in order to obtain tolerable shifter deviations. In order to print LIS down to 0.3 pm with reasonable DOF for production applications, the shifter transparency and thickness should not deviate more than 5% from their ideal values.

I. INTRODUCTION

It is obvious that many improvements concerning resolu­tion and focus latitude can be obtained by the use of phase­shifting masks. Different phase-shifting mask designs are studied in detail by many scientists, each design with its advantages and drawbacks. 1-4 One of the most suitable de­signs for periodical structures is the alternated shifter de­sign. By simulations and experiments, it is shown that this mask design improves both resolution and focus latitude of small resist features. 5

If phase-shifting masks are investigated by simulations, it is assumed that the shifter thickness corresponds to an optical phase shift of 180°, and that the shifter transpar­ency is equal to that of the quartz areas on the mask. Further, the shifter sidewall slope is perpendicular to the quartz substrate, shifter edges are straight, and no errors occur during alignment of the shifter to the metal layer.

A major problem, however, is the production of such an ideal phase-shifting mask. Deviations of the optical shift from 180·, transparency differences between shifter and quartz areas and imperfections of the shifter edges will have an influence on the lithographic performance of the phase-shifting mask.

In this work, the influence of such mask imperfections is studied by simulations and experiments for the alternated shifter design.

II. SIMULATION CONDITIONS

All simulations are carried out using the three­dimensional (3D) simulation program MULSS, developed at Mitsubishi Electric.6 An exposure wavelength of 248 nm and a numerical aperture (NA) of 0.42 is assumed for all simulations. Variables are the defocus condition, the shifter transparency, the shifter thickness (optical shift), the co­herence factor, and the dimensions of the shifter and quartz areas. Simulations are carried out for periodical 0.3, 0.4, and 0.5 pm LIS, with special attention for 0.3 pm LIS. The transparency and optical shift of the shifter layer is expressed with quartz as a reference (100% transparency and zero optical shift is supposed for quartz).

The depth-of-focus (DOF) is calculated from the image intensity profiles using the following criteria.

(i) An "illumination width" maximum variation of 10% (measured at I nom' this is the intensity which corre­sponds to the nominal Iinewidth at optimum focus).

(ii) An image contrast minimum of 60%. (The re­quired image contrast for proper imaging depends on the particular photoresist and its processing.)

(iii) No scum or residues between the lines for negative resists or no surface damage for positive resist (the inten­sity in the opaque areas shoulc be lower than I nom) .

The third criterium is less stringent if positive tone re­sists are used. Since the experiments are carried out with negative resist, the interpretation of the simulation results is also with negative resists.

2468 J. Vac. Sci. Technol. 8 10(6), Nov/Dec 1992 0734·211 X/92/062468-12$01.00 ©1992 American Vacuum Society 2458

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2469 Op de Beeck et ill.: Influence of shifter errors on the printability of L/S structures 2469

TABLE L Various masks with important shifter parameters and resulting DOF for 0.3 !-1m LIS.

Shifter Refractive Shifter material index shifter thickness

(A)

Mask A Cytop 1.42 2850 Mask B SOG 1,45 2800 Mask C SOG 1.45 3100 Mask D SOG 1.45 3600 Mask E SOG 1.49 2400 Mask F SOG 1.47 3800 Mask F SOG 1.47 3800

aOptical shift as calculated from thickness and refractive index.

III. EXPERIMENTAL CONDITIONS

A. Mask making

Phase-shifting masks are fabricated by sequential pat­terning of the metal layer and the shifter layer. Different shifter materials are the subject of investigation. The first material is a teflonlike polymer commercially available un­der the name CYTOP. This polymer is fabricated by Asahi Glass Corp. and is conventionaly sold as a pellicle mate­rial. CYTOP has a refractive index of 1.42 at 248 nm and shows almost no absorption at this wavelength. The second shifter material is organic spin-on-glass (SOG), fabricated by Allied Signal Corp. Various types of SOG and curing processes are used, resulting in a different refractive index and absorbance at 248 nm. In Table I, the various masks are listed together with some important shifter parameters; the shifter material, the refractive index of this material at 248 nm, the shifter thickness and the transmission relative to the quartz substrate. The optical shift is calculated for each mask, using the measured values for the refractive index and the shifter thickness. More details concerning mask making are described in Refs. 5, 7, and 8.

80 Exposure conditions and resist proceSSing

Two negative tone deep ultraviolet (DUV) resists are used: Shipley SNR 248-1.0 and UCB-JSR Plasmask 302-u. Experimental work using SNR 248-1.0 resist is carried out at Mitsubishi Electric, while experiments using Plasmask resist are performed at Imec. Both resists are spin-coated to a thickness of 1 tLm after development.

Shipley SNR 248-1.0 is a wet developable resist based on the chemical amplification principle. Development is carried out in puddle mode using Shipley XP 89-114 (N =0.14) as a developer.

Plasmask 302-u resist is based on surface imaging com­bined with dry development. After exposure, wafers are silylated (gas phase silylation) using tet~amethyldisilazane (TMDS).9,10 The dry development consists of two steps: first, a nonselective removal of a very thin top layer of the resist, followed by a selective removal of nonexposed resist during the second development step.

All experiments are carried out using excimer laser step­pers with a NA of 0.42, equipped with a KrF excimer laser

J. Vac. Sci. Techno\. B, Vol. 10, No.6, Nov/Dec 1992

Shifter Optical Total DOF of transparency shift' O.3/Lm LIS

(%) (deg) (!Lm)

100 175 1.75(0'=0.5) 100 182 1.65 (0'=0.5) 100 199 1.3(0'=0.5) 100 233 0.9(0'=0.5) 98 175 1.65(a-=O.53 ) 105 260 0.75(0'=0.53) lOS 260 0.4(0'=0.43)

(248 nm wavelength). The coherence is fixed at 0.5 for the Shipley experiments, and is 0.43 or 0.53 for exposures us­ing Plasmask resist.

IV. THE IDEAL ALTERNATED SHIFTER DESIGN

The mask layout of this type of phase-shifting mask is simple: every other transparent part in a periodical design is covered by a shifter layer. Light amplitudes of opposite phase cancel out each other in ideally dark regions, causing a clear improvement in image contrast.

Different realizations of this mask design are possible (see Fig. 1); the shifter can be formed from a coated or deposited layer, or it can be etched in the quartz substrate. Shifter patterning can be done prior to or after the metal patterning.

For the ideal alternated shifter mask, the shifter thick­ness corresponds to an optical shift of 180°, and the shifter transparency is equal to that of the quartz areas on the mask In case the shifter edges are not located on the non­transparent metal patterns, the shifter sidewall slope should be perpendicular to the quartz substrate, shifter edges should be straight, and no errors are allowed during

(al

(b)

.... " "'\

(c)

(d)

FIG. 1. Various realizations of the alternated shifter design (dark = metal, gray=shifter, white=quartz substrate). (a) Overlapping shifter edges, shifter coated or deposited on the metal patterns. (b) Nonoverlapping shifter edges, shifter coated or deposited on the metal patterns. (c) Metal deposited on the shifter layer. (d) Optical shift realized by selective etch­ing of the quartz substrate.

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2470 Op de Beeck et al.: Influence of shifter errors on the printability of L/S structures 2470

standard mask, (T ~O.3 standard mask, (T ,.(l.5 standard mask, (T ,.(l.7

0.4 0.4 0.4

~ ~ ~ .<: .c: £;

i 0.3 i 0.3 31 ~ \l.3

c: c: '" ~ ~ .S! <U

.S c: '" E ·e ·e ~ ~ ~

.(J.s 0.0 O.S L2 -0.6 0.0 0.6 L2 -0.6 0.0 0.6 1.2

Defocus (pm) Defocus (pm) Defocus (1JlTl)

altern. sh.,;' .180°,T_100%, IT ~O.3 altern. sh., Ii .180°,T .100%, (1" ~O.5 ahem. sh., ~ .180°,T -1 ()O%, cr- sO.7

Q.4 0.4 0.4

@

~ ~ ~ t

.r;

~ 0.3 i 0.3 ~ 0.3

<: c: '" .2 .1:1 ~ OJ OJ .s .s c: E E ·e '" ~ ~ ==

0.2 -1.2 -0.6 0.0 0.6 1.2 -0.6 0.0 0.6 1.2 -0.6 0.0 0.6 1.2

Defocus (1JlTl) Defocus (1JlTl) Defocus (1JlTl)

FIG. 2. Illumination width vs focus for 0.3 fLm LIS using a conventional mask [(a),(b),(c)] and the alternated shifter design [(d},(e),(f)], with the coherence as a parameter: 0-=0.3 [(a),(d)], 0-=0.5 [(b),(e)], and u=O.7 [(c),(f)].

alignment of the shifter to the metal layer (or metal to shifter layer).

For such an ideal phase-shifting mask, important im­provements in resolution and nOF are obtained. To illus-

trate these improvements, the illumination width versus focus for 0.3 /Lm LIS structures is plotted in Fig. 2 for a conventional transmission mask and for an ideal phase­shifting mask. These simulations are carried out for differ-

standard mask, IT -0.3 standard mask, IT" -0.5 standard mask, 0- =0.7 0.6 0.6 0.6

~ ~ ~ t t .c:

0.5 0.5 ~ 0.5

t: <: c: .2 ~ .2 OJ OJ .5 .5 .5 E E E ~ ~ :!

0.4 0.4 -0.9 ~.3 0.3 0.9 1.5 -1.5 -0.3 0.3 0.9 1.5 0.3 0.9 1.5

Defocus (1JlTl) Defocus (pm) Defocus (tun)

altern. sh., ~ .180',T ~100%, IT =0.3 altern. sh., ~ .180',Tal00%, IT-O.5 altern. sh., ~ .180',T =100%, cr .0.7

0.6 0.6 0.6

@)

~ ~ ~ .<: .c: .c -0 0.5 -0 0.5 -0 - 0,5 ·i ·i ·i c: c: c:

~ .9 ~ c;; .5 c: .5 E ·E E ~

0.4 § :§

0.4 -1.5 -O,g -0.3 0.3 0.9 1.5 -ItS -0.3 0.3 0.9 '1.5 -1.5 -0.9 -0.3 0.3 0.9 1.5

oelocus (jJ.m) Defocus (jJ.m) Defocus (tun)

FIG. 3. Illumination width vs focus for 0.5 fLm LIS using a conventional mask [(a),(b),(c)] and the alternated shifter design [(d),(e),(O), with the coherence as a parameter: u=0.3 [(a),(d)J, u=O.5 [(b),(e)], and 0-=0.7 [(c),(f)].

J. Vac. Sci. Technol. B, Vol. 10, No.6, Nov/Dec 1992

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2471 Op de Beeck et Ill.: Influence of shifter errors on the printability of LIS structures 2471

3

-E :1. • ~ . - 2 • • LI.. • e

0 IJ Q e as IJ -0 l- •

0 0.2 0.3 0.4 0.5 0.6

LIS size (Ilm)

FIG. 4. DOF vs LIS size using SNR 248·1.0 resist (0'=0.5) for a con­ventional mask (squares) and the alternated shifter design (circles).

ent coherence factors (0'=0.3, 0.5, and 0.7). The dark­shaded areas indicate an image contrast lower than 50%, the light-shaded area represents an image contrast between 50% and 60%. From Fig. 2, it is obvious that the nOF of 0.3 f.lm LIS will enlarge by the use of the alternated shifter mask, especially if a higher coherence (lower a) is chosen.

For larger linewidths however, improvements in nOF are smaller, as is clear from Fig. 3 which shows the illu­mination width versus focus for 0.5 JLm LIS for a standard transmission mask and the alternated shifter design. Again, the simulations are carried out for coherence factors ofO.3, 0.5, and 0.7. For standard transmission masks, the best result is obtained for a coherence of 0.7. For the phase­shifting mask, the image contrast will. incr~ase wi~h .in­creasing coherence (lower a), but the hneWldth vanatlOn with defocus is minimized if a medium coherence (0-=0.5) is used. This result indicates that a larger coherence (lower 0-) for the alternated shifter design is only beneficial for the smallest linewidths. 1 J

Experimental work is in clear agreement with these .sim­ulation results. Fi.gures 4 and 5 shows the nOF for dIffer­ent linewidths, obtained by experiments using SNR 248-1.0 and Plasmask 302-u resist, respectively. Note that the ex~

FIG. 5. DOF vs LIS size using Plasmask 302-u resist (a=O.43) for a conventional mask (squares) and the alternated shifter design (circles).

J. Vac. Sci. Techno!. S, Vol. 10, No.6, Nov/Dec 1992

1.6

-E 1.4 • ::1. -I.&. 1.2 • • 0 Q

1.0 l- e (ii -0

0.8 .... e

0.6 0.4 0.5 0.6 0.7 0.8

coherence factor cr

FIG. 6. DOF vs coherence factor u using Plasmask 302·u resist for a conventional mask (squares) and the alternated shifter design (circles).

posures of SNR 248~ 1.0 resist are carried out with a co­herence factor of 0.5 for both mask types, while the coher­ence is 0.43 for the Plasmask experiments. The higher coherence (lower a) for both mask types results in even larger DOF improvements for the alternated shifter design. In Fig. 6, the nOF of 0.25 J.1.m LIS (Plasmask resist) is listed for both types of masks as a function of coherence. While the nOF improves with decreasing coherence for a standard transmission mask, the opposite is true for the alternated shifter design, at least for these small linewidths.8

Not only the DOF but also significant resolution im­provements are obtained by the use of the alternate~ shi~ter design. Our experiments showed that the resolutIOn Im~ proved from 0.28 to 0.25 f.lm for SNR 248-1.0 (0-=0.5), and from 0.25 J.1.m (0'=0.53) or 0.26 J.1.m (0'=0.43) to 0.2 JLm (0'=0.43) for Plasm ask 302-u resist (see Fig. 7).

FIG. 7. 0.2 jLm LIS in Plasmask 302-u resist, printed using the alternated shifter design (a=0.43).

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2472 Op de Beeck et sl.: Influence of shifter errors on the printability of LIS structures 2472

... .,

FIG. 8. Definition of In=' illumination widths Wq and W,h' and line­widths Lq and L,h.

V. DEVIATIONS OF THE IDEAL PHASE-SHIFTING MASK

As is explained above, significant improvements are ob­tained with an ideal phase-shifting mask applying the al­ternated shifter design, but it is not straightforward to fab­ricate an ideal phase-shifting mask with perfect shifter thickness, transparency, and shifter edges. Coating nonre­producibility and nonuniformity will result in deviations of the optimum shifter thickness, and hence the optical shift will differ from 180°. Since it is difficult to obtain an accu­rate value of the refractive index of the shifter material, the ideal shifter thickness can even not be calculated precisely. In case the phase-shifting mask consists of a shifter layer coated on the metal patterns, the topography formed by the metal features will often cause local differences in shifter thickness, depending on the size of the metal pat­terns (Fig. 1).

The transparency of shifter areas can be different com­pared to that of quartz areas due to mismatching of the refractive indexes of the shifter material and the quartz (multiple interference) or due to the absorbance of light by the shifter material. Further, roughness of the shifter or quartz surface as a result of the mask processing should be avoided, since it will cause scattering of light and hence reduce the transparency too. Finally, process errors can introduce some reduction in transparency. for example an incomplete removal of the antistatic material deposited on

J. Vac. ScI. Technol. B, Vol. 10, No.6, NovlDec 1992

altern. sh., ¢ ",1600 ,T .. 100cro. (j ",,0.5 altem. sh .• j1I.,.180°,T .. 95%, (J" ~.5

0.4 ,.,..,..,.,,,.-----"""""'"" 0.'

~ ~ ~ ~ 0 .• '" :2

~ 0.3

.g

.~ E ,jj

~ a .~

~ -0.8 0.0 0.6 1.2 -0.6 0.0 O.S 1.2

Defocus (i-lm) Dsfocus (~"T!)

altern. sh., ~ .. 180",T",90%, cr ",,0.5 altern. sh .• 1IIi ",180o,T:0s85%. a ~.5

0.4 .----,----_-=''''' 0.'

, ~ E

.;!,

ti i; .~ 0 .• '" 1 c., " :1

~ ~ V 0.2

-j ~

-0.6 0.0 0.6 1.2 -1.2 -0.6 0.0 O.S 1.2

Defocus (jJ.m) Defocus (,un)

FIG. 9. Illumination width vs defocus for 0.3 JIm LIS using the alternated shifter design with the shifter transparency as a parameter: (a) 100%, (b) 95%, (c) 90%, and (d) 85%.

the shifter to avoid charging during e-beam exposure for shifter patterning.

Shifter patterning is still at its initial stage, therefore the sidewalls can be sloped and shifter edges become somewhat irregular instead of straight. It should be mentioned that the slope of the shifter edges is only important if the edges are not located on the nontranparent metal areas (see Fig. 1) .

Alignment of shifter to quartz is a new task too, and the facilities of standard e-beam equipment are often not suf­ficient to match the required overlay accuracy necessary for some type of phase-shifting mask designs.

It is obvious that a real phase-shifting mask will deviate from the ideal mask, and it is important to know the con­sequences of all these deviations on the lithographic per­formance of such a mask. With the knowledge of these consequences, scientists working on mask processing can focus their effort in order to optimize the most critical parameters to assure a high lithographic performance of the mask, and eventually people involved in mask design­ing can correct their designs to compensate for nonoptimal mask processing.

VI. SIMULATIONS: RESULTS AND DISCUSSION

In order to evaluate the influence of nonoptimum shifter conditions on the printability of LIS structures for the alternated shifter design, simulations are carried out with shifter transparency, optical phase, and sidewall slope as a parameter.

From the simulated intensity profile at zero defocus, the nominal intensity Inom is calculated for each mask under consideration. The nominal intensity Inom is defined as the relative energy for which the width of the illumination peak (Wq) corresponding to the quartz aperture of the mask, is equal to the width of the quartz aperture in the mask divided by the mask reduction (Wnom ) (see Fig. 8).

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2473 Op de Beeck st al.: Influence of shifter errors on the printability of lIS structures 2473

altern. sit., tP ~180c,T -100%, cr cO.S altern. SIt.,; _1130°,T~9S%, cr zO.5

0.5 ..,,..----------. ... Q5~--------------__ _

I , c .2 OJ .5

~

0.4

-£

~ 0.4

I 0.3 "'"""_-'-~ ...... __ "__---OIW

.o.S 0.0 0.6 1.2 -1.2 .a.S 0.0 O.S 1.2

Defocus (llffll Delows (JUI1) Defocus /11m)

FIG. 10. Illumination width vs defocus for 0.4/-im LIS using the alternated shifter design with a shifter transparency of (a) 100%, (b) 95%, and (c) 90%.

This nominal intensity Inom is always calculated from the illumination profile under optimum focus conditions (Ai =0). The illumination width Wsh corresponding to the shifter aperture is calculated at the same intensity Inom' For defocus conditions (!l.f=f=O), both Wq and W:~h are calcu­lated at the nominal energy I nom' For an ideal alternated shifter mask, the widths Wq and Wsh are equal for all defocus conditions. For non optimum shifter conditions, the illumination widths Wq and Wsh can differ from each other, causing an illumination width difference (Wq- Wsh)IWq between the shifter-intensity and quartz­intensity of the same set of LIS structures.

Except from the illumination width differences, the im­age contrast of the simulated intensity profiles is calculated too. The maximum intensities corresponding to the quartz and shifter aperture can differ from each other for nonop­timum shifter conditions, in such a case the lowest peak intensity is used for the calculation of the image contrast.

DOF graphs are made by plotting the illumination widths Wsh and Wq of the shifter and quartz apertures versus defocus, using a thin curve for the shifter width Wsh

and a thick curve for the quartz width WiT Additionally, the image contrast is indicated on these DOF graphs: a light-shaded area corresponds to an image contrast lower than 60%, a dark-shaded area to an image contrast lower than 50%.

A. Influence of shifter transparency

In order to evaluate the influence of the transparency, simulations are carried out with the transparency of the shifter varying between 100% and 85% with respect to the quartz transparency. It is obvious that the shifter illumi­nation profile will decrease with decreasing shifter trans­parency, and therefore the shifter width Wsh will be smaller than the quartz width W q• The shifter and quartz widths are plotted in Fig. 9 for 0,3 J.Lm LIS (0'=0.5).

If the transparency of the shifter and quartz areas is not equal, the illumination widths Wsh and Wq will differ for all focus conditions. For a shifter transparency of 90%, the shifter illumination width is only 0.27 ;.tm in optimum fo­cus, while the shifter and quartz illumination widths differ even more for defocus conditions. It is obvious that the nOF of 0.3 J.Lm LIS is strongly reduced if the transparency of the shifter and quartz differs more than 10%.

Simulations for larger LIS show similar results: the shifter illumination width and hence the DOF of the LIS structure is strongly reduced by a lower shifter transpar­ency (see Fig. 10).

The effect of reduced transparency is very similar for different coherence factors. Figure 11 shows the illumina­tion widths for 0.3 ;.tm LIS with the coherence as a param­eter and a shifter transparency of 90%. The reduction of

altem. sh.,;' a100C ,Tm90%. rr ~O.3 altem. SIt., _ ~i80·,T-9O'Y., 0' ~O.!S altern. sh.,;' ~180°,T~9O%. rr ~O.7 0.4

@

~ ~ t 0.3 i ·8 0: ,g ...

.~ .5 S ~ ~

0.2 -1.2 -0.8 0.0 0.8 1.2

Defocus (JUI1)

0.4

0.3

0.2 -1.2 -o.a 0.0 0.6 12

Defocus (p,m)

~ t 0.3 c

I i

-0.6 0.0 O.S 1.2

Defocus (p,m)

FIG. 11. Illumination width vs defocus for 0.3 /-im LIS using the alternated shifter design with a shifter transparency of 90% and with the coherence as a parameter (a) 0'=0.3, (b) 0'=0.5, (e) 0'=0.7.

J. Vac. Sci. Technol. B, Vol. 10, No.6, Nov/Dec 1992

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2474 Op de Beeck et al.: Influence of shifter errors on the printability of LIS structures 2474

MASK: T=lOO%, 0 = 150°, Wsh=Wq

Defocus = 0.6J.lffi I 1\ ' -j------'\----f---'

\ J 0.0

X-coordinate (Ilm) 1.2

FIG. 12. Intensity profiles of 0.3 ttffi LIS for various defocus values for the alternated shifter design with optical shift of 150" (a=0.5).

the shifter width Wsh is slightly more pronounced for a lower coherence (higher 0').

In the discussion above it is assumed that the shifter transparency is equal or lower compared to the quartz transparency. It should be mentioned here that the oppo­site situation is possible too; due to processing effects or to the refractive index of the shifter material, the transpar­ency of the quartz areas can be smaller than that of the shifter areas. It is clear that this will result in a smaller illumination width of the quartz compared to that of the shifter.

B. Influence of shifter thickness (optical phase)

If the shifter thickness does not correspond to an optical shift of 180', the illumination profiles corresponding to the shifter and quartz areas will be different. Figure 12 shows this situation for an optical shift of 150· instead of 180· (0.3 p'ffi LIS, 0'=0.5). In optimum focus, the shifter and quartz widths are equal, but they differ from each other in defocus conditions. For a negative defocus, the quartz illumination width will be smaller than the shifter width, For positive defocus conditions, the opposite is true.

In Fig. 13, the shifter and quartz illumination widths are plotted versus focus for 0.3 pm LIS, with the optical shift as a parameter (a=0.5). The larger the deviation of optical shift from 180°, the larger the differences in shifter and quartz width for defocus conditions. The image con­trast of 0.3 /Lm LIS is strongly reduced for optical shift deviations larger than 40°, for smaller optical shift devia­tions the image contrast is almost unaffected. For 0.4 {Lm

J. Vac. Sci. Techno!. e, Vol. 10, No.6, Nov/Dec 1992

0 .• =".,..-____ _

altern. sh., II .160',T.l00%, CT .0.5 altern. sh., ~ -2OO°,T_100%, r:r ..0.5

0.' ='IT""-----,,= 0.'

E <!.

~ ;a 0.3 c .2 1ii .E

! 0.0 0 .• 1.2

Defocus (jun l

altern. sh.o,s .14OI>,T .. 1000/ .. , rr .0.5 aHem. sh .• jl.220',T-l00%. 0' -0.5

0.4 0.'

I I 1 0.3

~ • 0.3

~ ~ ~

.~

'" E ~

0 .• 1.2

Defocus (JJ.m) Defocus (~m)

ahom. sh,,;, -240',T.l00%, tr ..0.5

0.4 d.'

E <!. I i 0.3 i 0.'

I E

01!

J .~

'" -D.e 0.0 0.8 1.2

Defocus (jun)

FIG. 13. Illumination width vs defocus for 0.3 pm LIS using the alter­nated shifter design with the optical shift as a parameter: (a) 180·, (b) 160·, (cl 200', Cd) 140·, (e) 220·, (0 120', and (g) 240· (a=0.5).

altern. sh.o ¢ =140°,T ",100"/0, C1 =0.5

0.5

0.3 -1.2 -0.6 0.0 0.6 1.2

Defocus (J.llT1)

FIG. 14, Illumination width vs defocus for 0.4 ttm LIS using the alter­nated shifter design with an optical shift of 140' (a=O.5).

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2475 Op de Beeck et 81.: Influence of shifter errors on the printability of LIS structures 2475

allem. sh., ~ ~140°,T ~100%. 0" _0.7 0.4 ..---------.............

O.2.j.'-2~--o"".6~-O.'-O--O"".6~...........J1.2

Defocus (jI.fIl) Defocus (p.m) Defocus (Jim)

FIG. 15. Illumination width vs defocus for 0.3 J.!ffi LIS using the alternated shifter design with an optical shift of 1400 and with the coherence as a parameter (a) 0"=0.3, (b) 0"=0.5, and (c) u=O.7.

LIS structures, the differences in shifter and quartz width due to optical shifter deviations are similar but less pro­nounced compared to the 0.3 i.Lm case (see Fig. 14). For LIS of 0.5 /.Lm and larger, the illumination width differ­ences are very sman for optical shift deviations smaller than 40°.

Simulations carried out for various coherence factors «(1=0.3, 0.5, and 0.7) show similar trends: the shifter and quartz illumination widths differ more for larger deviations in optical shift, and the contrast is strongly reduced for shifter deviations larger than 40·. For 0.3 /.Lm LIS, the differences in illumination width are more pronounced for a higher coherence (lower (7), but because of the contrast improvement the DOF is still larger for lower a values (see Fig. 15). For 0.5 /.Lm LIS, the illumination width difference due to optical shift errors is very small, and is minimized by the use of a medium coherence.

C. Influence of the shifter or quartz sidewall slope

As mentioned earlier, the slope of the shifter or quartz pattern will influence the imaging properties of a phase­shifting mask in case the sloped edge is not overlapping with nontransparent metal patterns [see Figs. 1 (b), 1 (c), and 1(d)]. As an example, the influence of the shifter slope for the mask realization of Fig. 1 (b) is investigated. For the simulations, it is supposed that the shifter edge is

sloped and that the sloped edge is nontransparent. As a consequence, the useful width of the shifter area is reduced.

For shifter materials with a refractive index around 1.4, the required shifter thickness for exposures at 248 nm is around 300 nm. For a nontransparent shifter slope of 80°, the shifter width will be reduced by 0.05 /.Lm at each shifter edge, which corresponds to a shifter width loss of 0.10 /Lm (or 0.02 f..Lm at wafer level) for all LIS sizes. Simulations corresponding to this particular example are carried out for 0.3 /Lm LIS supposing a 5 X reduction stepper is used for the exposures. The DOF graphs for different coherence factors are plotted in Fig. 16.

The illumination width differences as a result of shifter slope are quite comparable to the effects of a reduced shifter transparency: the shifter illumination width is smaller than the quartz width for all focus conditions. For a slope smaller than 75°, the shifter width is reduced by more than 10% in optimum focus, reducing the DOF to zero. For larger LIS sizes, the effect of the same shifter slope is less important, since the shifter width reduction is constant for all LIS sizes.

In the previous discussion, it is supposed that the shifter patterns are sloped. The opposite can be the case too, for example if the difference in optical shift is realized by dry etching in the quartz substrate. Further it is possible that the slope is partially transparent, which will result in smaller illumination width differences as is indicated in the

ahem. sh., IT ~.3, Sh ~.28 Jim altern. sIl., IY ..0.5, SIl 50.28 J!l11 ahem. sh., CT ~O.7, Sh DO.28 j.Im

0.4 r-------------,

! .s:: :2 ill 0.3 c .2 Oi .5 E :§

0.2 ':-'-_-'-_-'-_ ........ _.......J ·1.2 ·0.6 0.0 0.6 1.2

Celoous (p.m) Defocus (1Ifll) Defocus CIun)

FIG. 16. Illumination width vs defocus for 0.3 ,.m LIS using the alternated shifter design with shifter width error of D.l/Lm (mask dimension) for a 5X reduction stepper, (al a=O.3, (b) a=O.5, and (c) a=D.7.

J. Vac. Sci. Technol. B, Vol. 10, No.6, NovlDec 1992

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2476 Op de Beeck et al: Influence of shifter errors on the printability of LIS structures 2476

allem. sh., ~ =190·,T =95%, C1 ,.0.5

0.4

~ .s::

~ 0.3 c::

~ c:: 'E :§

-0.6 0.0

Defocus (J..Lm)

FIG. 17. Illumination width vs defocus for 0.3 !LID LIS using the alter­nated shifter design with a transparency of 95% and an optical shift of 190' (a=0.5).

simulations discussed above. It is obvious that the effect of a shifter or quartz slope is strongly dependant on the re~ duction of the exposure tool: the influence of such a slope is less pronounced for a 10 X reduction stepper, but be­comes especially important for 1: 1 exposures.

.... i ...

J ... D.O u ...

-'CIom) ... ,.......~-~-~-...,

...

.... '--~-~---'----' -102 -GA U 0.8 1.1

>< I.' .1.2 00.1 "0 U 1.2

~ j" u~~~~ __ ~~~

ct.o ... 1.2

Do_CIomI

FIG. 18. Linewidth measurements vs defocus of 0.3 p,m (left) and larger LIS sizes (right) printed using masks A, B, C, and D (17=0.5) and SNR 248-1.0 resist.

J. Vac. Sci. Techno!. B, Vol. 10, No.6, Nov/Dec 1992

{al

(bl

leI

FIG. 19. 0.28 J.tm LIS printed using mask D in (a) -0.3 p.m, (b) 0 J.l.m, and (c) +0.3 {tm defocus condition (SNR 248-1.0, a=0.5). The second, fourth, and sixth line starting from left correspond to a shifter aperture of the mask.

As is mentioned above, the sidewall slope for the alter­nating shifter design is only important if this sidewall is located in a quartz area, which is also the case at the end of LIS structures on a lightfield mask (used to print LIS in positive resist). In such a situation, a straight shifter side­wall will cause an intensity drop and therefore the ends of resist lines will be connected two by two (so called bridg~ ing). This problem can be solved at several ways5,12 and will not be discussed in this work.

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2477 Op de Beeck et af.: influence of shifter errors on the printability of lIS structures 2477

... .... ~ I ... f I ... ~ I

~ ! I J: I ~ ... .... .oO ... ... ·u .... ... ... . ..

-'f!un) -(J<m)

FIG. 20. Linewidth measurements vs defocus ofO.3/-Lm (left) and larger LIS sizes (right) printed using mask B with a shifterwidth reduction of 0.1 {lm (mask dimension) for all LIS sizes (a=O.5). SNR 248-1.0 resist is used.

D. Combination of shifter deviations

In real processing, various shifter deviations will occur simultaneously. The total influence of these deviations will depend on the particular combination of shifter errors. Most combinations will result in even more decreased im­aging properties of the phase-shifting mask. As an exam­ple, simulations are carried out for a phase-shifting mask with a shifter transparency of 95% and an optical shift of 190°. These small errors (5% deviation from the ideal shifter thickness and transparency) are often used as pro­cessing criterion for mask fabrication. The shifter and quartz illumination widths for 0.3 /Lm LIS are plotted in Fig. 17. Due to the combination of both shifter errors, the DOF graphs become asymmetrical. The differences in shifter and quartz illumination width are significant, but the contrast is hardly affected, hence the DOF of 0.3 /Lm LIS is not decreased yet. For smaller linewidths, the effects are more pronounced, and the DOF will be reduced sig­nificantly due to these rather small shifter errors. A shifter transparency smaller than 95% together with an optical shift deviation of 190· will result in an obvious DOF re­duction for 0.3 JLm LIS and even for larger LIS.

... ....

~ ...

Xu I

I I: ...

·u .... ... ... U ·1.0 .u u eLf. 1.0 '.I DofoI>oa (1""1 -(!wI

... ...

~ ...

1 .... 1 I .. 1M I ......

... .... ..... ... ... , .. ~wm) -(J<m)

FIG. 21. Linewidth measurements 'IS defocus of 0.3 fLm (left) and larger LIS sizes (right) printed using masks E and F and Plasmask 302-u resist (a=O.53).

J. Vac. Sci. Technol. B, Vol. 10, No.6, Nov/Dec 1992

.,. ",. '-" - ••• - ••••• .-,.-•••• ;> •••• ; ••• ;.:.;.:.:.:;;:.: ••• ' •••••••••• ;.:.~·:······ ..... ·.v···-·~·-·· ••••••••••• -.";> ••• ':.:.:-:.:.~ ••••••••••••••• -•••• -.-.-.-.','~""" -_.

...

~ Xu I

i J I: OJ! . ... oM ... os u ·u u1.o 41 CUI' U 1.0 '.I

-(J<m) 00_"",,)

FIG. 22. Linewidth measurements vs defocus of 0.3 fLm (left) and larger LIS sizes (right) printed using mask F and Plasrnask 302-u resist and applying a higher coherence (a=0,43).

VII. EXPERIMENTS: RESULTS AND DISCUSSION

In order to evaluate the alternated shifter design under non optimum shifter conditions, several masks are fabri­cated and the shifter properties are measured for each mask (see Table I). Since it is extremely difficult to make ideal phase-shifting masks or to vary only one shifter pa­rameter at a time, it is inevitable that most masks represent a combination of shifter errors.

Wafers coated with SNR 248-1.0 or Plasmask 302-u resist are exposed and developed and studied by scanning electron microscope (SEM). The linewidth corresponding to "shifter" and "quartz" lines (Lsh and L q, respectively) is measured for different LIS sizes under different focus con­ditions (see Fig. 8). The results are plotted in linewidth versus focus graphs, in which a thick curve corresponds to the width of quartz lines, and a thin curve to the width of shifter lines. 0.3 lim LIS are indicated by circles, 0.4 p,m by triangles and 0.5 /-Lm by squares. Linewidths are not plot­ted if scum occurs between the lines. In the linewidth graphs of 0.3 JLm LIS, scum is additionally indicated by a shaded area.

A. Influence of shifter thickness (optical phase)

Since the transparency of their shifter material is equal to that of the quartz substrate, masks A, B, C, and Dare perfectly suitable to study the influence of the optical phase caused by the shifter layer. The optical shift is indicated in Table I for each mask.

For the experiments SNR 248·1.0 resist is exposed using a coherence factor a of 0.5. After development, the width of the shifter and quartz lines is measured for 0.3, 0.4, and 0.5 JLm LIS for all four masks. The linewidths are plotted versus focus in Fig. 18. For mask A with an optical shift of 175°, the differences between shifter and quartz lines are small, with a larger width of the quartz lines for positive defocus values. The linewidth differences are less pro­nounced for larger LIS sizes. For mask S, having a shifter with optical shift of 182·, again rather smalilinewidth dif­ferences are measured. Opposite to mask A, the shifter lines are wider than the quartz lines for positive defocus conditions. Linewidth measurements for mask C with an optical shift of 1990 indicate a larger deviation from the optimum shifter thickness, since linewidth differences are more pronounced. The optical shift of 233" for mask D causes even more difference between shifter and quartz

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2478 Op de Beeck fit al.: Influence ot shifter errors on the printability of LIS structures 2478

lines. The correspondence between the simulations and these experiments is obvious.

The simulations showed further that, except for line­width differences, a deviation in optical shift causes a de~ crease in image contrast too. Such a decreasing image con­trast will result in scum between the resist lines. This result is confirmed by our experiments. Indeed, in the left graphs of Fig. 18 (0.3 lim LIS), scum is indicated by a shaded area. It is easily seen that scum is more likely to occur for larger deviations in optical shift. The linewidth differences caused by optical shift deviations are more important for smaller LIS sizes. The SEM photographs of Fig. 19 show 0.28 lim LIS exposed with mask D in different focus con~ ditions. The influence of the deviation in optical shift is obvious.

To illustrate the total result of optical shift errors, the DOF of 03 jLm LIS is determined and listed in Table I. The DOF is reduced significantly with increasing deviation in optical shift. For larger LIS sizes, the reduction in nOF is less pronounced,

B. Influence of the shifter or quartz sidewall slope

On mask B, additional LIS structures with reduced shifter width are designed in order to evaluate the influence of a nontransparent shifter or quartz slope. Exposures are carried out with a coherence factor of 0.5 on a 5 X reduction stepper using SNR 248-1.0 resist. In Fig. 20 the linewidth measurements are plotted for a shifter reduction of 0.1 jLm (mask dimensions) for all LIS sizes, corre­sponding to a shifter slope of 80° (supposing a refractive index of 1.4 ). The reduction in shifter linewidth is obvious, even for the larger LIS sizes. The fact that the shifter linewidth is slightly larger than the quartz width is caused by the fact that the optical shift of mask B is slightly larger than 1800 (see Fig. 18, mask B, linewidths for perfect shifter sizing). For larger LIS sizes, the shifter linewidth is significantly reduced due to the shifter slope, while the small deviation in optical shift does not have a significant influence anymore.

C. Combination of shifter deviations

Mask E is an almost ideal phase-shifting mask for nuv exposures (see Table I). Mask F has a large deviation in shifter thickness and some error in transparency too. In fact, mask F is originally an i-line phase-shifting mask which is used for this work for nuv exposures, hence the shifter thickness deviates a lot from the required thickness for 248 nm. Both masks are used for exposures (248 nm) of Plasmask resist (a= 0.S3). Linewidth measurements are plotted in Fig. 21. (Note the differences in scale between the plots for larger LIS sizes for Shipley and Plasmask resist. )

Mask E has very small shifter deviations (see Table I), resulting in minor linewidth differences too. The shifter lines are slightly smaller in positive defocus and larger in negative defocus, which confirms that the optical shift is somewhat below 180°. No scum was observed between 0.3 ?lm resist lines over an entire focus range of 3 !lm. Since

J. Vac. ScI. Techno!. S, Vol. 10, No.6, Nov/Dec 1992

Plasmask resist has a high resist contrast, the required im­age contrast for proper imaging is lower in comparison with Shipley resist. Therefore, nOF limits are reached due to Iinewidth differences for 0.3 ItID LIS in Plasmask resist, while they are reached by scum for 0.3 Ilm Shipley resist lines.

Mask F, with a shifter transparency of 105% and a thickness corresponding to 2600 optical shift for 248 nm exposures, deviates significantly from the ideal phase­shifting mask. Linewidth measurements make this very ob­vious too (see Fig. 21); the shifter and quartz lines differ a lot and scum is more likely to occur. The higher transpar­ency of the shifter parts on the mask results in wider shifter lines under optimum focus conditions. The deviation in transparency is most obvious for the larger LIS, since the influence of the optical shift is less important for these structures. For all LIS sizes the DOF is reduced drastically (See Table n.

D. Influence of coherency

Mask F is used for exposures of Plasmask resist for two coherence factors: u equals 0.53 (Fig. 21) and 0.43 (Fig, 22). If the linewidth measurements for both coherence fac­tors are compared, it is observed that scum is reduced by the use of a higher coherence (lower a), but meanwhile the linewidth differences between shifter and quartz lines are enlarged. Therefore, the total result of the use of a higher coherence (lower a) in combination with a mask with a large deviation in optical shift is a reduction in nOF, due to more pronounced linewidth differences. Especially for Plasmask resist the gain in image contrast by applying a higher coherence is of minor importance, since Plasmask has a high resist contrast itself. Again, these results are in perfect agreement with the simulations.

VIII. CONCLUSIONS

The influence of shifter parameters such as transpar­ency, thickness, and sidewall slope are studied for the al­ternated phase-shifting design by simulations and experi­ments. It is found that deviations in optical shift cause linewidth differences between shifter and quartz lines for exposures out of optimum focus, The linewidth differences result in a significant reduction in DOF for small LIS sizes, For large deviations in optical shift, scum will limit the DOF even more, The influence of optical shift errors is less pronounced for larger LIS sizes. A reduction in shifter transparency will cause a reduction in shifter linewidth too for all focus conditions. The influence of transparency is also obvious for larger LIS sizes, The nOF of all LIS structures will be significantly reduced for a transparency lower than 90%.

Unless the shifter is made overlapping with the non­transparent metal patterns, mask processing should be op­timized in order to avoid undersizing of the shifter or quartz width due to sloped sidewalls, since it will reduce the DOF too. The importance of the shifter or quartz slope is strongly dependent on the reduction of the stepper. While a transparency deviation of 10% is a limit in case

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2479 01' de Beeck et 81.: Influence of shifter errors on the printability of LIS structures 2479

other shifter parameters are optimal, it is an intolerable deviation in combination with other shifter errors, even when these errors are rather small. Indeed, if two or more deviations occur simultaneously, linewidth differences are usually more pronounced, resulting in a reduction in DOF. Therefore, stringent demands on mask processing are re­quired in order to obtain phase-shifting masks with toler­able shifter deviations.

The influence of shifter deviations is similar for coher­ency factors varying between 0.3 and 0.7. A higher coher­ence (lower 0") results in an improved image contrast for small LIS sizes, for both an ideal phase-shifting mask and for a mask with some deviations. Therefore it is recom­mendable to choose for a lower 0", unless the optical shift of the mask differs more than 30· from 180·, since this devi­ation will cause very big linewidth differences if a Iowa is applied. Unfortunately, a medium coherence results in the highest DOF for the larger LIS sizes for both an ideal and a less optimal phase-shifting mask, so a compromise may be required.

ACKNOWLEDGMENTS

The authors are indebted to S. Aoyama, S. Matsuda, N. Yoshioka, and R. lonckheere for their assistance in phase­shifting mask fabrication. Thanks also to A. Tokui, T. Ha-

J. Vac. Sci. Technot e, Vol. 10, No.6, Nov/Dec 1992

nawa, and K. H. Baik for their help in resist processing, and to J. Miyazaki for useful discussions. The attention and encouragement of Y. Akasaka, K. Tsukamoto, and L. Van den hove towards this work is also acknowledged.

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