method for measurement of full-field alignment

Microelectronic Engineering 4 (1986) 269-283 North-Holland

269

Method for measurement of full-field alignment

Bruce Heflinger, Steven Eaton, Rolf Jaeger and Marcos Karnezos Hewlett-Packard Laboratories, Palo Alto, CA 94304, U.S.A.

Received 12 November 1986

Abstract. The alignment performance of a full-field X-ray exposure system is discussed. A

“single-mask alignment” technique is presented, which extracts an array of local misalignments from two exposures of the same mask. A computer program is described which decomposes the array of vector misalignments into mask- and system-related components. The system overlay precision is shown to be 0.06 micron in x and 0.06 micron in y (1 u), across a 40 mm x 40 mm array, using the Hewlett-Packard prototype X-ray mask aligner to overlay one pattern onto itself. Sources of error in overlays of two different masks are categorized. With feedback control of in-plane mask distortion, system overlay accuracy of 0.12 micron in x or y, for two-mask overlays, is achievable. The random component of die placement by the Perkin-Elmer MEBES II, used to generate the X-ray masks, is inferred to be 0.05 micron, RMS.

Keywords. X-ray, lithography, alignment, pallets, masks, distortion, precision, accuracy.

Bruce Hetlinger is a native of Alaska, born January 21, 1947, now at Hewlett-Packard Company in California. Since earning his Ph.D. in solid-state electronics at MIT, he has worked in optics, lithography, and recently semiconductor packaging. He plans to explore the world by bicycle after ten more years in the workforce.

Steven Eaton is a native of Kansas, now living in Mountain View, California. He joined Hewlett-Packard Laboratories in 1980 after receiv- ing his B.S. degree in engineering and his M.S. degree in electrical engineering from the California Institute of Technology. In the field of lithography, he has worked on alignment for X-ray lithography and on simulation programs for electron beam and X-ray exposures. He has also worked on projects in the fields of printing and mass memory. Bicycling heads his list of avocations.

0167-9317/86/$3.50 @ 1986, Elsevier Science Publishers B.V. (North-Holland)

270 B. Heflinger et al. / Measurement of full-field alignment

Rolf Jaeger is a native of the Federal Republic of Germany. He studied physics at the Technical University of Munich where he received a Ph.D. degree in surface science in 1979. He worked as a postdoctoral fellow at the Stanford Synchrotron Radiation Laboratory of Stanford University from 1979 to 1983 where he studied photon stimulated desorption and X-ray absorption of adsorption layers on metal and semiconductor single- crystal surfaces. In 1983 he joined Hewlett-Packard Laboratories in Palo Alto to work on the application of X-ray lithography to MOSFET fabrication. Since 1985 he is project manager in the High-Speed Device

Laboratory working on GaAs MODFET ICs. In his spare time he enjoys ss guitar, reading, windsurfing and snow skiing.

Marcos Karnezos received his B.S. in physics from University of Athens, Greece and his Ph.D. in solid state physics from Carnegie-Mellon Uni- versity. Following two years of postdoctoral research on very low tem- perature magnetism, in 1978 he joined the Magnetic Bubble Memory Development Group at National Semiconductor. As a senior engineer he worked on garnet materials and thin film process development. In 1981 he joined Hewlett-Packard Laboratories and worked on X-ray mask development. In 1985 he became manager of the X-ray Lithography Program and currently he is involved in high performance electronic packaging. Dr. Karnezos has numerous publications and patents.

1. Introduction

A method and results are presented for measurement of the alignment performance of an automatic X-ray exposure system [l, 21. The system is a lab-prototype full-field proximity aligner for three-inch wafers, utilizing a unique alignment method. The alignment concept is proprietary and will not be discussed, except to state that six degrees of freedom are held (under feedback control throughout exposure of the wafer (at three in-plane and three gap sensors). Alignment marks on the wafer are etched into the silicon prior to any circuit processing. Circuit geometries are delineated in a series of masks [3], each of which is a taut membrane, spanning a circular hole in a square glass frame. The mask is held in close proximity to the wafer, with electrical feedback between the alignment marks on the wafer and matching structures on the mask.

Electrical probing of titanium films on silicon wafers is employed to measure the misalignment between exposures of a pair of masks. A computer program was written to analyze the data. The program decomposes an array of x- and y-misalignments into several components: errors at three single-axis alignment marks, errors at three gap sensors, and in-plane distortion. The program is fast (no evident lag on a Hewlett-Packard Model 1000 minicom- puter) and efficient, in that residual misalignment vectors, after extracting the systematic errors, appear randomly oriented. The random component includes e-beam stage placement errors and measurement noise.

Because observed misalignments, between pairs of masks, were initially

B. Hefiinger et al. / Measurement of full-field alignment 271

larger than we had hoped, we devised a method to measure the misalignment between two exposures of one mask. In this way, we removed from immediate consideration (a) distortion of the mask resulting from clamping in a pallet; (b) random die placement errors resulting from imprecision of stage motion. Errors sources (a) and (b) arise during patterning of the mask by the e-beam. Eliminating these error sources allowed us to characterize and improve system performance, then to identify the sources of misalignment in conventional two-mask overlays.

Although the full aperture of the system is a three-inch circle, we have found that overlay precision is optimized within a central 40 mm x 40 mm zone, which we refer to as the “exposable field” of the system. This zone would be a good aperture choice for a step-and-repeat system.

Alignment statistics are reported at one standard deviation (1 a), in microns, from the average across the exposable field. Independent error sources are “RMS-summed,” using the Common Error Propagation Law, meaning that the combined result is the square root of the sum of the squares.

2. Description of single-mask alignment @MA) test pattern

Ordinarily, we measure misalignments between a pair of masks by creating an array of “female” test patterns in Mask 1 and an exactly coincident array of “male” test patterns in Mask 2. Exposure of a wafer (coated with negative X-ray resist) through Mask 1, and etching of a titanium film, is followed by exposure through Mask 2 into new resist, and etching again. The intersection of the male and female patterns comprises a pair of resistors which may be electrically probed to detect misalignment between them. This works satis- factorily for pairs of masks, but cannot be applied to the overlay of a single mask with itself.

To measure misalignments between two exposures of a single mask, we employ a unisex linewidth-measurement test pattern (in a novel way) rather than an interlocking pair. An intentional misalignment during the second exposure of the test pattern (Fig. 1) is necessary to preserve sign information. We introduce a +1.5 micron offset, to both x and y, narrowing the 5.0 micron resistors (etched after the first exposure) to 3.5 microns. The offset is generated, in software, by ordering feedback voltages at the alignment sensors (which have been determined, by laser interferometry, to correspond to 1.5 microns alignment error). Vector-misalignments are calculated by subtracting the measured x- and y-linewidths from 3.5 microns, the nominal measurement resulting from perfect overlay. We can measure misalignments to approximately -1 and +l micron.

A pair of satellite test patterns is included in each die, to separately measure the 5.0 micron (nominal) linewidth, printed at each exposure step. Selective ultraviolet exposure ensures that each satellite pattern is etched only once (Fig. 2). A chrome-quartz mask was created for this purpose, comprising


SECOND EXPOSURE AND ETCN

REMAININQ LINEWIDTH

FIRST

AND ETCN

Fig. 1. Single-mask test pattern. Two exposure/etch cycles are done, with a 1.5 micron offset in x and y on the second exposure. This is a slightly modified linewidth-measurement pattern, for

electrical probing. Negative resist is used for the wafer exposures.

an array of clear apertures in an opaque background. After the first-level exposure of the die array onta negative resist, a partial development is done, to render visible the exposed pattern. The chrome-quartz mask is aligned to

expose one of the satellite patterns within each die. Throughout a zone

! UU EXPO I !

NO UU

Ei# OFFSETS x: +I .5pM Y: +I.5 M

P

I I

5 fJM1 ----- ---------_A

Fig. 2. Single-mask alignment die. All three patterns are probed to calculate the local misalignment. The rectangles within the die delineate the ultraviolet exposure zones, which contain the satellite patterns. Each satellite is exposed to UV once. The pattern labeled “no UV” is etched twice, with a 1.5 micron offset between exposures. This die is located on the mask, at

5 mm centers, across the exposable field.

B. Heflinger et al. / Measurement of full-field alignment 273

including that pattern, we crosslink the negative resist with UV radiation. This protects the pattern from the resist development step, and from the subsequent titanium etching. The other satellite pattern, in each die, is similarly protected after the second-level exposure of the die array. Each satellite is thereby etched only once, providing an essential record of the actual linewidth which was printed at each of the two exposure steps.

3. Calculation of misalignment components

The SMA pattern and the two satellite patterns are probed at each die site, yielding, for each pattern: local sheet resistance, x-linewidth, and y-linewidth. The sheet resistances at the three test patterns are compared as a consistency check. The local misalignment in each axis (x or y) is calculated from the three test pattern linewidths which are oriented perpendicular to that axis, as illustrated in Fig. 3. Misalignments sampled over a 9 x 9 array, at 5 mm centers, are fed into the data reduction program. The program fits a chosen subset of eight parameters (X, YI, Yz, Z, PI, Pz, O,, 0,) to equations (la) and (1 b), minimizing C (ez + e;) over the 9 x 9 array.

e,=X++(Y2- Y,)-2P2+(PI/r+Z/h)x+[(~(YI- Y,)+2PJ/r]y

+(0,/W + (&JWxy, (la)

(a) SATELLITE PATTERNS

(b) SMA TEST PATTERN

ETCHED AWAY AFTER FIRST EXPOSURE

+ L1 - - CENTROID OF Ls CENTROID OF L,

I S (REMAINING LINEWIDTH, MEASURED)

ETCHED AWAY AFTER SECONDEXPOSURE

Fig. 3. Misalignment calculation. (a) Satellite patterns are preserved and measured for linewidth calibration. (b) The distance P-e is the preload shift, P (=lS microns), less the local misalignment, e. The final linewidth is S =$L, +;L,-(P- e). Rearranging, e =

S + P - &(L, + I& and can be either positive or negative.

274 B. Heflinger et al. 1 Measurement of full-field alignment

34NCH WAFER

WAFER FLAT

Fig. 4. Positioning of alignment marks. Three single-axis sensors detect misalignment between

marks on mask and wafer, providing a feedback signal during exposure of the wafer. Mask-to-wafer gap is detected and held constant at three positions (not shown) adjacent to the alignment marks. The data reduction program quantifies the mispositioning of the mask in the second exposure with respect to the mask in the first exposure, by extrapolating the array fit to

the alignment mark positions.

e,=i(Yl+ Y2)+(-P1/r+Z/h)y+[i(Y2- Y#r]x

+ @x/h)y’+ (8ylh)xy. (lb)

Where (referring to Fig. 4): x and y are coordinates, with the origin at mask center; e, and e,, are the x- and y-components of the overlay error; r is the radius of the alignment sensors from the wafer center (32 mm); h is the height of the X-ray source above the mask and wafer (300 mm); X, Y,, and Y2 are errors at the respective lateral alignment sensors; 2 = i(Z, + ZJ is the gap error (average at gap sensors near Y, and Y2; the third gap sensor, Z3, is near the X sensor); 0, = (Z3 -;(Z, + z))/ r is the mask tilt about the x-axis; 0, = i (&- Z,)/r is the mask tilt about the y-axis; P, and P2 are in-plane distortions, evaluated at the alignment sensors, illustrated in Fig. 5. P, distortion is of the form +ax in the x-direction and -ay in the y-direction. P2 is of the form +by in the x-direction and +bx in the y-direction.

Individual contributions to equations (la) and (lb) are listed in Table 1. The entries in Table 1 describe the displacement of the shadow of a mask feature due to an error at a particular sensor. The exceptions are the errors (P, and P,) quantifying in-plane distortion, which we infer from the pattern of misalignments. We do not have sensors to detect these distortions, although that capability could be included in subsequent versions of the hardware. The nonlinear terms due to mask tilt have been suppressed in the following analysis, since they are almost always smaller than the translational errors. Use of the full fitting routine demonstrates that mask tilt typically introduces only 0.02 micron error, RMS, within the die array. (Higher-order terms are even more negligible.) The remaining terms are all linear functions of position (x, y) within the exposable field.

B. Hefinger et al. / Measurement of full-field alignment 275

Fig. 5. In-plane distortion: (a) P, component, (b) Pz component. These distortions arise because the square mask frame is subjected to squeezing along one symmetry axis and consequent expansion along the perpendicular axis, Since the shapes of the two plots are fixed by their mathematical formulation, each type of distortion can be quantified by the signed magnitude of a

single vector. We select a vector coinciding with one of the alignment marks.

Table 1 Misalignment at a general position (x, y), resulting from individual errors at the six sensors, and from in-plane mask distortion. Geometric factors (h and r) and misalignment parameters (X, YI, Y2, 2, PI, Pz, O,, 0,) are defined in the text

Error source Local misalignment components

e, eY

(4 Y,lr? y - r) (-;Y,lr)(y-r)

(Z/h)x (&/h)xy (Bylh)xZ (PJr)x

2(PJr)(y- 4

none

(-fYJr)(r-r) G YJr) b + r)

(Z/NY (%lh)y* (e,lh)xY (--P,lr)y

none

4. Single-mask alignment results

Table 2 combines data from four lots, totaling 32 wafers. Individual lot results are given in Table 3. These four lots were subjected to several variations in wafer processing. The variations, described in the “comments” column of Table 3, were an attempt to determine whether our alignment method is sensitive to thin films, covering the alignment marks on the wafer. Since the four lots gave identical alignments, within 0.1 micron, we conclude

276 B. Hefhger et al. / Measurement of full-field alignment

Table 2 Single-mask alignment results, combined data from four lots of wafers (all entries in microns). The substrate was titanium over oxidized silicon. A gap error of one micron produces 0.07 micron expansion at the side of a 40 mm square. P, and Pz are the distortion factors, in microns, measured at the alignment marks

Mean misalignment 1 u scatter

Alignment errors

X Y, YZ

-0.0 1 t-O.06 +0.09

0.03 0.04 0.04

Gap error

Z

+0.2 1 0.67

Distortions

PI pz

+0.006 +0.010 0.014 0.026

that our single-mask alignment method is insensitive to thin film coverage over the marks, at least to that precision level. Mean misalignment is nonzero in the SMA results, due primarily to inaccuracy in our method of offsetting the mask 1.5 microns for the second exposure (plus system inaccuracy, apart from the offset).

Additional single-mask results demonstrate the necessity for careful clamping of masks to avoid in-plane distortion. Although single-mask overlays automatically cancel any distortion caused by patterning of the mask in the e-beam, they can exhibit in-plane distortion when the mask is removed from a pallet, then reloaded for a second exposure in the X-ray system. (The pallet is a square frame, with clips to hold the mask in a fixed position above the wafer.) Early single-mask overlay work showed in-plane distortions as large as 0.50 micron when reclamped between exposures. An improved pallet was designed to avoid inducing a bending moment when clamping the mask. Subsequently, pallet-induced distortion was measured by unclamping the mask after first-level exposure of a wafer, then reclamping the same mask before second-level exposure of the same wafer. Four lots of wafers were exposed in this fashion and the results are shown in Table 4. The distortion values are less

Table 3 Results of individual lots which were summarized in Table 2 (all entries in microns)

X

Mean

1U

Mean 1U

Mean 1U

Mean 1U

0.00 0.03

-0.04 0.02

-0.02 0.03

-0.01 0.01

Y,

+0.07

0.05

+0.07 0.03

+0.05 0.04

+0.02 0.03

Y*

+0.09

0.03

+0.07 0.02

+0.12 0.03

+0.08 0.04

Comments

Reclamped in pallet between exposures. Alignment marks protected from coverage by Ti/resist. 13 wafers.

Same as above, but separately processed. 8 wafers.

Mask was not reclamped. Alignment marks protected from resist, but not from titanium. 5 wafers.

Mask was not reclamped. Alignment marks not protected from either titanium or resist. 6 wafers.

B. Heflinger et al. I Measurement of full-field alignment 277

Table 4 In-plane distortions induced by reclamping mask between exposures. These results measure the capability of our new pallet design to minimize in-plane distortion of masks. This experiment illustrates one of the unique measurement capabilities of the single-mask alignment technique (all entries in microns, evaluated at one of the alignment marks)

Lot average

p,

Intralot

@I)

Lot average

PZ

Intralot

o(P2) Comments

+0.017 0.005 +0.046 0.012 -0.016 0.005 +0.019 0.006

+0.023 0.005 -0.026 0.009 -0.022 0.034 -0.020 0.015

3 wafers, 12116185 8 wafers, l/20/86 8 wafers, l/30/86 7 wafers, 3118186

Mean

+0.016

o(P,)

0.026

Mean

-0.018

o(P2)

0.026 Combining the above lots

consistent between lots than they are within a lot of wafers (since the reclamping was done lot-by-lot rather than wafer-by-wafer). Yet, the numbers are a tenfold improvement over distortion results (not shown) of the earlier pallet design. We have determined the means and standard deviations of in-plane distortion, after combining all four lots into one data set. Those standard deviations, (T(P,) = 0.026 and u(PJ = 0.026 micron, are used in Section 7.

5. Estimate of alignment precision

The overlay error, from a particular set of parameters, can be expressed as an RMS vector magnitude, C,, averaged over the die array. A good ap- proximation of E. is obtained by integrating e’, + ec over the exposable area, dividing by the area, then taking the square root. The result of this procedure is:

QRMS)

= 1 2(2x+ Yz- Y, -4P,y++(Y, + Yd’

K Y1- ~+4p,)‘+2(~~+2(~~+(y’2ry’)I]]“2,

where f is the overall width of the array of dice. We have restricted our subsequent discussion to an exposable field of

f = 40 mm, beyond which errors grow excessively. Using the system geometry facors, f = 40 mm, r = 32 mm, h = 300 mm, we obtain:


E,(RMS) = {:(2x+ Y*- y, -4&)*+;(y1 + Yz)’

+&[(v,- Y,)‘+(Y,- Y~+4~2)2+8p:]+~~}1’2. (2b)

E, is a conservative figure of merit for a particular set of misalignment parameters. It includes the entire exposable field, but is weighted quadratically, emphasizing the most extreme misalignments (usually at the corners).

Statistics on the parameters fitted to the 32 wafers allow us to estimate the system alignment precision. For this purpose, we assume that the mean value of each parameter will be determined and corrected via software. Neverthe- less, each individual wafer’s vector plot will generally reduce to a nonzero value in every parameter, because the alignment is not perfectly repeatable. We assume that each of the six misalignment parameters is Gaussian-dis- tributed, over a large sample of wafers. Our measurements of standard deviations of the misalignment parameters (via test exposures) determine a Gaussian function, G:

1 x2

[ (

y: g 22 Pf P: exp -- -+y+y+2+2+2

G= 2 0: UY, c+Y, uz UP, UP, >I

87~~ uxuu, uu_ azap, up2 (3)

Each subscripted u, in this expression, is the “1 u scatter” value listed under the corresponding parameter, in Table 2.

We wish to estimate system alignment precision across the exposable field. In (2a), E, is the average magnitude of all misalignment vectors, throughout the exposable field, for a given set of parameter values. The Gaussian function, G, quantifies the normalized probability of any such set of parameters. In- tegrating Ez x G, in the six parameters, --co to +a, yields mean-square system overlay precision, CT, the square root of which is the average magnitude of misalignment vectors expected in an ensemble of wafers, across the exposable field. The resulting expression is:

l ,(RMS)

= u~+~u:,+~u~,+4u(T2,,+~u2y,+~u2y, [

f 2 ( ) f 2 0 1 l/2

+A 5 (u’,,+u~,+u2,,+&+16u;,+8u;,)+~ h u$ .

(44

Using the system geometry factors, f = 40 mm, h = 300 mm, r = 32 mm, we obtain

E,(RMS) = [a: +$&UC, +u~U,+~C&)+$&& +i&#‘*. (4b)

Inserting standard deviations from Table 2 (“1 u scatter”) into (4b), we find

B. Hefiinger et al. / Measurement of full-field alignment 279

that E, = 0.084 micron. This number includes contributions from two ortho- gonal vector components (x and y) at each sample point. The RMS value for each vector component is less by fi. In summary, the system overlay precision, across a 40 mm x 40 mm exposable field, is:

E,(RMS) = 0.060 micron in x

0.060 micron in y for single-mask overlay.

The reader is cautioned that system overlay precision disregards mean misalignments, which would be included in a specification for system overlay accuracy. The overlay accuracy can be made to approach the precision limit by using information derived from test exposures. The results of the test exposures determine software-driven offsets, which can be used to zero the mean misalignments. We currently do this with X, Y,, and YZ: an example is given later in this article. The gap correction is hardware-ready, but not yet implemented in software. The P1 and PZ corrections are not presently avail- able, even in hardware. Sensing P1 could be done with four sensors at the right, left, top, and bottom of the exposable field. Sensing P2 could be done with four sensors at ends of the diagonals. The mask would be intentionally squeezed, under feedback control, during exposures, to cancel the distortion.

6. Two-mask alignment results

Subsequent to the SMA data, conventional two-mask overlay errors were measured. Two wafer lots were analyzed with the data reduction program. The results of the first lot of five wafers are shown in Table 5. Raw data plots of these five wafers are illustrated in the upper half of Fig. 6. Plots of the residual vectors after correction with the fit parameters (rigid motions X, Y1, Y2; gap error 2; distortions P,, P2; evaluated in Table 5) are in the lower half of Fig. 6. Several conclusions can be drawn from Table 5:

(1) There are offsets between the aligned positions of the two masks. The

Table 5 Two-mask overlay data, prior to adjustment of mean misalignments (all entries in microns)

Wafer ID

Alignment errors X Y, Y*

Gap error Distortions Raw data Full fit Z P1 PZ (RMS) (RMS)

41-12 -0.07 +0.49 +0.22 +1.1 -0.75 -0.16 0.57 0.06 41-11 -0.02 +0.4s +0.21 +1.2 -0.75 -0.16 0.58 0.06 41-10 +o.oP? +0.42 +0.27 +2.3 -0.79 -0.15 0.66 0.06 41-9 -0.01 +0.39 +0.21 +1.1 -0.75 -0.17 0.58 0.06 41-8 +0.02 +0.43 +0.22 +1.0 -0.77 -0.16 0.60 0.07

Mean 1U

0.00 +0.44 +0.23 0.05 0.04 0.02


RAW DATA

WAFER 41-12

RESIDUAL MISAUQNMENTS, FIT EXTRACTED

---it--- 1 fim

t WAFER 41-l 1

1

. ‘. , \ , \. , i.

-. _, ‘x \ 1 \ \ \

RAW DATA

t WAFER 41-10

1

RESIDUAL

1 fim t

WAFER 41-S

1

0.1 pm YISAUQNMENTS, FIT EXTRACTED 0.1 pm

WAFER 41-8

RESIDUAL MISALIONMENTS, FIT EXTRACTED

Fig. 6. Two-mask overlay data, before and after parameter fit. The upper row shows vector misalignment raw data from five wafers, at a scale of 1 micron per grid spacing (tail-to-tail between adjacent arrows, as indicated), sampled on 5 mm centers, over a 40mm X 40 mm exposable field. The evident in-plane distortion, which is common to all five plots, results from unequal stresses induced in the two masks, by the pallets in which they were written in the e-beam system. The lower row shows residuals, after subtracting the error components listed in the text, at a scale of 0.1 micron per grid spacing. The residual errors show a repeating pattern

of local misalignments, corresponding to random writing error of the MEBES.

mean shifts (from Table 5) are 0.00 micron at X, +0.44 micron at Y1, and +0.23 micron at Y2, within this limited sample. This is unlikely to have resulted from mispositioning of the alignment keys in the two masks: the relevant MEBES specification states, “Level-to-level overlay position accuracy for a 1.0 micron address size is +0.30 micron, 3 a” [4]. (We wrote our die patterns with a 0.25 micron spot size, for best resolution; we wrote the alignment keys with a 1.0 micron spot size, to minimize e-beam writing time.) Offsets were expected as an artifact of our proprietary alignment system, but they are thought to be a stable characteristic of a given pair of masks. Such offsets may be determined via overlay test prints. The long-term stability of intrinsic mask offsets was not studied.

(2) The scatter of the alignment errors (1 U, under X, Y,, and YJ is approximately equal to the corresponding scatter in the single-mask alignment data, by comparison of Table 5 to Table 2.

(3) In-plane distortion constitutes most of the raw data (RMS) misalignment. This distortion has several sources. The predominant P, and P2 com-

B. Heflinger et al. / Measurement of full-field alignment 281

ponents, illustrated in Fig. 5, are a pallet-induced record of the difference in strain between the two masks when they were clamped to be written in the MEBES. The pair of masks used in the reported two-mask overlay data was e-beam-written in old-style mask pallets-which caused the in-plane distortion-but was X-ray-exposed in the redesigned mask pallet. Higher-order distortions caused by mask processing and absorber stress [5] were not removed by the data reduction program.

(4) The residual error is approximately 0.06 micron, RMS. The column labeled “full fit (RMS),” in Table 5, includes the random e-beam writing error, local mask distortions (caused by processing and thin film stresses), and the measurement noise.

The measurement noise results from rough edges on the electrical resistor lines, due to their delineation in (swelling-prone) negative resist. To evaluate the measurement noise, we subtracted vector misalignment files from two identically exposed and processed wafers, then ran the difference file through the data fitting program. After removing the discrepancies in alignment and gap setting, the residual error in the difference was 0.01 to 0.02 micron (for various pairs of wafers). We attribute this to the roughness of the line edges rather than probing-hardware errors, because repeated probings of the same wafer generated virtually identical data with few discrepancies as large as 0.01 micron.

The procedure just described was utilized to infer that the e-beam mask- writing error is 0.05 micron, RMS. Cleanly developed and etched single wafers had different residual errors (after parameter fitting) depending upon whether they were exposed with two masks, or twice with the same mask. Single-mask overlays yielded residuals as low as 0.024 micron, after fitting (RMS, across the wafer). Two-mask overlays yielded residuals as low as 0.054 micron. We assert that the square root of the difference of the squares, 0.048 micron, is the MEBES contribution. The evidence for this is that residuals in difference files are 0.01 to 0.02 micron, regardless of whether each of the two wafers was twice exposed with a single mask, or with two interlocking masks (once with each). The difference file eliminates the local mask-writing errors in the two-mask overlays, since they are common to the two wafers. (It is essential to this argument that the same masks be used to expose both wafers. Any pair of interlocking masks will do: ours were independently written over a span of several months.) Therefore, the excess random component in the (single- wafer) two-mask overlays (0.048 micron) is attributed to the e-beam system. Our masks, in this respect, are better than the relevant MEBES specification, which states, “Level-to-level overlay position accuracy for a 0.25 micron address size = kO.25 micron, 3 a” [4]. That is equivalent to 0.08 micron, RMS. It is reasonable to assume that a fraction of the MEBES specification cor- responds to simple translations (and other low-order systematic patterns), which have already been removed from our full fit (RMS) data.

A final lot of wafers was exposed to demonstrate the use of test exposures to correct intrinsic mask offsets. We introduced intentional offsets of

282 B. Heflinger et al. I Measurement of full-field alignment

Table 6 Two-mask overlay with mask offset, to correct in-plane misalignments of Table 5

Wafer ID

21-4 21-5 21-6

Alignment errors Gap error Distortions Raw data Full fit X Y, YZ Z PI p* (RMS) (RMS)

+0.01 +0.01 -0.01 +0.8 -0.73 -0.17 0.56 0.07 +0.02 -0.02 0.00 +0.9 -0.73 -0.15 0.56 0.05 +0.02 +0.02 0.00 +1.0 -0.74 -0.15 0.55 0.06

-0.44 micron in Y1 and -0.23 micron in Y2 to compensate the mean misalignments listed in Table 5. The results of the three wafers in that lot are shown in Table 6. The Y, and Y2 errors have been greatly reduced, confirming that mean misalignments may be minimized via software. We have not to date tried to correct the mean misalignments in Z, PI, and P2. (The ‘<raw data (RMS)” errors remain large due to clamping distortions introduced in the original patterning of the pair of masks, in old-style e-beam pallets.)

7. Estimate of two-mask overlay accuracy

Because all our interlocking masks were made in the old pallets, we use our single-mask overlay results to estimate the PI and P2 distortions expected in our current hardware. Reclamping the SMA mask caused mean PI = +0.016; o(PI) = 0.026; mean P2 = -0.018; a(P,) = 0.026, according to Table 4. Let us suppose that we e-beam-write a new pair of interlocking masks and use them to expose a lot of wafers. This involves four mask clampings: two in the e-beam and two in the X-ray system. Each clamping contributes to the final measured distortion. Allowing 1 (T of P, and 1 (T of P2 for each clamping yields 4 x 0.026 = 0.104 micron as an estimated measurement of PI or P2. (This is conservative: the total range of PI was -0.02 to +0.07; that of P2 was -0.05 to +0.04 in the 26 wafers summarized in Table 4. Since it is a worst-case estimate, we don’t add any contribution for statistical variation of P, or P2, in the following.)

Tables 5 and 6 jointly indicate that X, Y,, and Y2 can be corrected to 0.02 f 0.05, 0.02 f 0.04, and 0.02 f 0.02 microns, respectively. We have not attempted a Z-correction, but feel that we can hold it to at least O.O* 1.0 micron from optimum. Now, we can insert these values into (2b) and (4b) to estimate the system two-mask overlay accuracy. The contribution from nonzero mean values is (worst case, all terms having same sign):

E: = i(O.04 + 0.02 + 0.02 + 0.416)2 + i(O.02 - 0.02)’

+ &(0.02 + 0.02)2 + (-0.02 - 0.02 - 0.416)2 + 8(0.104)‘]

+ & (o.o)2

= 0.0711.

283 B. Heflinger et al. I Measurement of full-field alignment

The contribution from statistical variations is:

E,2 = (0.05)* + g [(0.04)* + (0.02)*] + A( 1 .o)*

= 0.0066.

The final contribution is 0.0036 micron* from the residual errors. The system overlay accuracy is:

E = JO.07 11 + 0.0066 + 0.0036 = 0.285 micron

This is split between two vector components, 0.202 micron per component (x or y). The result is strongly influenced by P2 distortion. If P2 were zero, the result would be 0,128 micron per component. (If P, were also zero, the result would be further reduced only to 0.123 per component.) Thus, in future embodiments of this aligner, it will be essential to implement feedback control of in-plane distortion. In summary, our system overlay accuracy for a pair of masks, across a 40 mm x 40 mm exposable field, is:

0.202 micron in x

E(RMS) = (0.202 micron in y for two-mask overlay.

This is a conservative estimate of the performance achievable with the current hardware.

Acknowledgment

We wish to thank all who contributed to the design of the Hewlett-Packard X-ray lithography system, the mask-fabrication team, and the personnel of the IC process laboratory, who provided necessary support and equipment for this work. Jim Kruger was particularly helpful.

References

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121

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G. Garrettson and A. Neukermans, X-ray lithography at Hewlett-Packard, in: Microcircuit Engineering 83 (Academic Press, London, 1983) 247-254. A. Neukermans, Status of X-ray lithography at H-P, in: Electron-beam, X-ray, and Ion-beam Techniques for Submicrometer Lithographies II: SPIE 393 (1983) 93-98. M. Karnezos, R. Ruby, B. Heflinger, H. Nakano and R. Jones, Tungsten: An alternative to gold for X-ray masks, in: Proceedings International Symposium on Electron, Ion, and Photon Beams, Boston, MA, 1986. Performance specifications from Technical Proposal for MEBES IB to MEBES II upgrade, Hewlett-Packard Internal Memo. R. Ruby, D. Baldwin and M. Karnezos, The use of diffraction techniques for the study of in-plane distortions of X-ray masks, in: Proceedings International Symposium on Electron, Ion, and Photon Beams, Boston, MA 1986.

method for measurement of full-field alignment

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