212 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 7, NO. 3, AUGUST 1994

Design Considerations for a Gaudi Test Structure Which Can Be Used to Determine the Optimum Focus

Martin Fallon, Anthony J. Walton, J. T. M. Stevenson, and Alan W. S . Ross

Abstract- A test stucture is described which can be used to optimize the focus of wafer steppers. Simulation is used to examine the effect that some of the design parameters have on the sensitivity of the structure. Finally some practical measurements are presented.

I. INTRODUCTION

HE continual shrinkage of minimum feature size has T placed stringent demands on pattern transfer. Optimum stepper operating conditions are normally obtained using a focus-exposure matrix. The shift towards high numerical aper- tures has reduced the working depth of focus. Determining the optimum focus is time consuming unless a fast automated mea- suring instrument is available. Any change in the dimensions of the image due to focus variations may be only a very small percentage of a linewidth and, if this is the case, then both optical and electrical measurements will be very insensitive to this change. What is required is a structure which will amplify these small changes in dimension and thereby increase the sensitivity of the measurement to changes in focus.

11. RIE TECHNIQUE

As an image is defocused the minimum dimension between two lines that can be resolved is increased and this effect can be used to design a structure which is sensitive to changes in focus. A structure consisting of a number of parallel conducting bars with incremental changes in the gaps between them can be used to determine the minimum resolution possible [l]. However, if this is to be used for optimizing focus, a very large number of pairs of bars would be required. The separation between them would be incrementally varied with a dimension smaller than the best resolution expected from the exposure system. Such a structure would be very susceptible to any particulates and nonuniformities in the etch process.

Manuscript received October 13, 1993; revised April 18, 1994. This research was supported by SERC under Grant GR/F 38884; TMA, which provided the simulation software; and BBN, which provided RS/l and Cornerstone for use in the analysis of results.

The authors are with the Edinburgh Microfabrication Facility, Department of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 3JL, UK.

IEEE Log Number 94023 12.

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Fig. 1. Use of wedge to magnify distance in the y direction.

Fig. 2. the optimum focus.

Schematic layout of an optical structure that can be used to determine

The proposed Gaudi' structure uses the same principle as that employed in spreading resistance measurements [2] and the Murray dagger [3]. An angle less than 45' is used to amplify a small dimension, as indicated in Fig. 1. It can be observed that small changes in the 2 dimension result in large changes in the y direction and this ratio increases as the angle 0 is reduced. The wedge can be considered to be a variable width gap and the maximum depth to which it will be resolved is the horizontal distance ( I&), indicated in Fig. 1.

'The structure has been named after Gaudi who designed the Sagrada Familia Cathedral in Barcelona. The test structure resembles the profile of the spires and was originally proposed at the Barcelona meeting of ICMTS.

0894-6507/94$04.00 0 1994 IEEE

FALLON et al.: DESIGN CONSIDERATIONS FOR GAUD1 TEST STRUCTURE

/ Resistance controlling region

Parasitic region

Fig. 3. The areas of the structure that control the resistance.

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Voltage taps Tw

Fig. 4. determine the optimum focus.

Schematic layout of an electrical structure that can be used to

The resolution (R,) is simply given by

(1) 0

R, = 2d,tan- 2

where 0 and d, are indicated in Fig. 1. For a change in resolution of 0.1pm and an angle of 5' the value of d, is 1.145pm which gives a magnification factor of 11.45. It is a simple matter to see that as the angle 0 is reduced the magnification factor increases.

111. AN OFTICAL Focus STRUCTURE

Fig. 2 presents a schematic layout based on the Murray dagger that can be measured optically. The teeth at the right- hand side can be used to measure the size of the wedge using the cross hairs on a microscope while the bar at the top can be used with more automated equipment to measure the wedge dimension. However, automated measurement is not easy because of the difficulty in locating the position of the apex of the wedge using conventional optical linewidth measuring equipment.

IV. AN ELECTRICAL Focus STRUCTURE

It is possible to adapt the structure for electrical measure- ment by using the wedge to constrict the current flow and modulate the resistance of the device. This makes the design much more complex than the optical structure of Fig. 2 since a large number of parameters influence the resistance of the device.

Any structure using a wedge to constrict current flow may be thought of as consisting of two resistive components: the

273 4

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(a) The controlling resistance area. (b) The model sections. Fig. 5.

TABLE I STRUCTURE DIMENSIONS USED IN FIGS. 6 TO 10

Dimension Length (pm) Number of segments 5 Channel width (CW) 0.0 Tap width (TW) 1 .o

Length of segment (LSEG) Tap length (TL) 2.0

1 .o

resistance due to the change in the wedge dimension and the parasitic resistance, as-indicated in Fig. 3. The parasitic resistance at the ends can be eliminated by voltage taps while that at the lower region can be minimized by designing the apex of the wedge to be as close to the bottom of the structure as possible. The optimum size of this dimension (CW in h g . 4) will obviously be dependent upon the resolution capability of the exposure system and the variation of the process parameters that are experienced in the pattern transfer process. In order to optimize a structure's sensitivity the parasitic resistance must be made as small as possible so that the percentage change in resistance is maximized for a given alteration in focus. Fig. 4 gives the layout of a Gaudi structure which attempts to meet this objective. The measurement is made by forcing current between the two ends and measuring the voltage between the two taps. It can be deduced that, as CW tends towards 0, the range over which the resistance can vary is maximized and this increases the sensitivity of the structure to changes in focus. Obviously, it is important that a sufficient safety margin be built in to ensure that the structure does not become an open circuit should it be over etched.

Fig. 5 shows how the structure can be broken down into a very simple model consisting of rectangular and pentagonal sections, the resistances of which are added together to give the total resistance. This model has been used in Fig. 6 to give the change in resistance as a function of resolution for a 5" angle (R-model). Other key dimensions are given in Table I. As would be expected, the resistance decreases as the resolution increases. Interestingly enough the resistive contribution of the rectangular section remains constant as the resolution is reduced since its aspect ratio remains constant for the model presented in Fig. 5. The decrease in resistance is due to the reduction of the length and increase in the minimum width of the pentagonal sections. This simple model obviously has a limitation in that it forces equipotentials at the joints between the sections; this will significantly distort the current flow, especially when the resolution is near to its optimum value. The model is most accurate when the resolution is poor and the current flow becomes more uniform.

The limitation of the model requires that, for accurate results, Laplace's equation must be solved numerically. This

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274 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 7, NO. 3, AUGUST 1994

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Fig. 6. Comparison of the model with the simulated response for a 5' angle.

0

Resolut ion (microns)

-U- 1 degree -0- 5 degrees

--V- 10 degrees -0- 15 degress -m- 1 7 . 5 degrees ---t 25 Degrees

A 1 . 5 degrees

Fig. 7. The variation of resistance with angle for a focus structure.

has been performed using MEDICI, and these results are also presented in Fig. 6. The difference between the simple model and the full simulation is at the high resolution end of the curves. This divergence is due to the distortion of the current flow in the simple model and the effect of the voltage taps which are accounted for in the full simulation.

A MEDICI simulation has also been performed for the dimensions given in Table I and the variation of resistance with resolution is shown in Fig. 7 for wedge angles between 1' and 25'. Fig. 8 and 9 give the Current vectors and equipotentials for two different geometries, and the distribution of current can be observed to be nonuniform as the wedge constricts

FALLON et 01.: DESIGN CONSIDERATIONS FOR GAUD1 TEST STRUCTURE

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IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. I, NO. 3, AUGUST 1994

RES I STANCE, DRES-DRESN

O .+- - - - - * / * _----

0.5 _----- -I/- - - - - 0 . 7 4 * / * _---

Fig. 10. Response surface for a focus structure.

the current flow. It can also be noticed that more current TABLE II is forced into the voltage taps as the resolution increases. This effectively reduces the current constriction as the focus improves and reduces the sensitivity in this region. Fortunately this is the region of the device where the maximum change in resistance takes place (see Fig. 7), so it does not cause a major problem. The percentage effect that this has upon the change in resistance will be directly proportional to the number of segments, so there is a recommendation that this number be maximized. This will have the added bonus that the effect of any lithographic or etch nonuniformities will be minimized. The other option is to move the voltage taps to just outside the region where the segments are formed.

Fig. 10 shows the response surface relating the resistance of the Gaudi device to the angle of the wedge and the resolution. The change in resistance must be maximized for the structure to have maximum sensitivity to changes in focus. The response surface equation used to model the resistance has been differentiated with respect to the resolution and is also plotted in Fig. 10. It should be remembered that the response surface is only a best fit to the simulated results and any differentiation of this function will almost certainly magnify any small inaccuracies. This can be observed in the zero DR-DRN contour at the top of Fig. 10. Comparison with Fig. 7 indicates that the variation of resistance with resolution is small but nonzero as the resolution approaches LSEG. The zero value DR-DRN contour is simply an indication that even though the polynomial used to fit the response surface is a good fit it is not perfect. However, from Fig. 10 it can be observed that the change in resistance as a function of change of resolution is greater as the wedge angle increases. This information can be used to determine the geometry that gives

Focus CONDITIONS FOR THE STRUCTURE

Focus

-9.0 to +14.0 pm A = 1.0pm

the optimum sensitivity to changes in resolution. In practice the shape of the structure is more rounded, and the effect of this will now be discussed.

v. EXPERIMENTAL RESULTS

Some exploratory devices were fabricated with angles of lo-, 15, and 20-deg. and a designed channel width (CW) of 1.0 and 0.79pm. A thermal oxide was grown and 4500 A of polysilicon deposited and doped using a solid source (57R/U). The wafers were then coated with photoresist and the pattern defined using a l o x wafer stepper with a numerical aperture of 0.3. Aluminium silicon was then deposited on top of the polysilicon contact pads to guarantee that it would be possible to reliably probe the structure a number of times. Obviously for routine use the polysilicon pads would be probed directly, thereby reducing the amount of processing required. Initial investigations indicated that the resistance of the structures was very sensitive to changes in the width of the channel [4]. A wafer was exposed to examine the effect of focus on the resistance of the structures. The focus was varied in increments of 1.0pm in a serpentine pattern across the 24 sites on the wafers as detailed in Table 11. The exposure was set at twice the minimum necessary to clear the minimum feature.

Since nonuniformities in resistivity, etch, develop, and ex- posure will all affect the resistance, the structures were located

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FALLON et al.: DESIGN CONSIDERATIONS FOR GAUD1 TEST STRUCTURE

(C)

Fig. 11 . +13.0pm. (b) +l .Ojtm. (c) -5.Opm.

Changes in the structure dimensions as the focus changes. (a)

as close together as possible. To quantify this effect for each exposure increment, a reference device was also exposed at the nominal focus condition. Fig. 11 shows 3 photographs of the structure for three different focus settings. It can be observed that as focus changes there is a variation in CW and in the height of the apex of the triangular sections. The tap width can also be seen to reduce until it finally disappears at extremes of focus. The electrical measurement of

277 4

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Fig. 12. Variation of resistance with focus.

CJ c-' mn Fig. 13. Improved structure.

the devices was performed by forcing 0.5 mA and measuring the voltage between the taps. This current level was chosen after preliminary measurements indicated that no joule heating was taking place. Fig. 12 relates the resistance of the structures with angles of 10, 15, and 20" to incremental focus changes, as detailed in Table 11. It can be observed that all the structures appear to have a similar sensitivity to changes in focus and closely track one another. They indicate that the optimum focus condition is at +4.0 pm. Superimposed on these curves are the identical structures that were exposed at a constant focus. These give an indication of the noise. The nominal focus of these structures was 3 . 0 ~ m .

%

VI. CONCLUSIONS For the Gaudi structure to be of potential use it must be

capable of detecting changes in linewidth of 0.1 pm or better [ 5 ] . The results presented in Walton et al. [4] and this paper indicate that the resistance of the structure is very sensitive to changes in dimensions and can be used to set optimum focus conditions. However, the exercise has identified that there are improvements that can be made which will help improve the reliability of the structures.

278 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 7, NO. 3, AUGUST 1994

One very simple change is to increase the width of the taps which were designed with a width of 1 pm. At the extremes of focus these failed to resolve and it is suggested that this dimension should be increased to prevent this occurrence. Since the measurement of these structures is comparative this will not reduce the accuracy of the measurement.

In the experimental structure there were only 10 segments and this number could be increased to both minimize the effect of any etch nonuniformities and current distortion at the tap positions. Extending the number of segments is also of benefit since it increases the resistance of the devices thereby resulting in higher sensed voltages for a given current setting.

From Fig. 1 1 it can be observed that as the resolution of the structures degrades, the channel width CW increases, which reduces the resistance. At the same time the height of the apex of the triangular sections reduces by a factor AR. This effectively increases the resistance of the apex section identified in Fig. 6(b), which reduces the sensitivity of the measurement. Chamfering off the top of the apex does not help since this only increases the minimum resistance of the device and reduces the range of resolutions over which it will operate. Fig. 13 shows a schematic of a structure which both eliminates the effect of the apex height reduction without significantly increasing the minimum resistance of the device. This design is effectively two Gaudi structures in parallel but, since the two apexes overlap, any small change in dimension is not amplified in the manner observed in Fig. 1 1. Another possible solution is proposed in Walton et al. [6], whereby the structure is bent into a semicircle which enables the conducting regions which suffer apex reduction to take on a rectangular shape which eliminates the problem.

ACKNOWLEDGMENT

The authors would like to thank W. J. C. Alexander for CAD design, and the technical staff of the EMF who performed the processing. We would also like to acknowledge the support of SERC (Grant no. GFUF 38884), TMA who provided the simulation software and BBN for RSA and Cornerstone that were used for the analysis of results.

REFERENCES

A. J. Walton, W. Gammie, M. Fallon, and J. T. M. Stevenson, “An interconnect scheme reducing the number of contact pads on process control chips,” IEEE Trans. Semiconductor Manufacturing, vol. 4, no.

S. M. Sze, VLSI Technology. New York: McGraw-Hill, 1983, p. 189. K. Murray, “Measuring dimensions using Murray daggers,” Semicon- ductor Znt., pp. 69-73, Dec. 1982. A. J. Walton, M. Fallon, I. T. M. Stevenson, and A. W. S. Ross, “Design considerations for a test structure which can be used to determine the optimum focus,” in Proc. ZEEE Int. Con$ Microelectronic Test Structures, Barcelona, Mar. 1993, pp. 275-280. H. Koyama, private communication. A. J. Walton, M. Fallon, J. T. M. Stevenson, A. W. S. Ross, and C. M. Reeves, “An improved structure for the optimisation of focus and exposure for IC production,” Electron. Leff., vol. 29, no. 17, pp.

3, pp. 233-240, Aug. 1991.

1573-1574, Aug. 1993.

Martin Fallon received a degree in astrophysics (Hons.) from the University of Glasgow in 1980, and the Ph.D. degree from Edinburgh University. In 1980 he worked for General Instruments Microelectronics as a Process Engineer for 3 years before taking up a research post at Edinburgh University. He currently works as a Research Fellow in the Electrical Engineering Department. His research interests are in process and device modeling and parameter extraction.

Anthony J. Walton received the B.Sc. degree in electrical and electronic engineering from the Uni- versity of Newcastle-upon-Tyne, Newcastle-upon- Tyne, England, in 1974, the M.Sc. degree in 1976 for research into thin lumped elements at microwave frequencies, and the Ph.D. degree from Manchester Polytechnic.

After receiving his doctorate, he joined the De- partment of Electronic and Electrical Engineering, University of Technology, Loughborough, England, as a Research Fellow working on hybrid active

filters. In 1981 he joined the Department of Electrical Engineering, Edinburgh University, Scotland, as a Research Assistant and is now a Reader in the same Department. During that time he has been involved with the microelectronics industry in a number of areas which include silicon processing, process control, microelectronic test structures, design for manufacturability (DFM), and technology computer aided design (TCAD). His present interests also include the optimization of semiconductor processes through the integration of the experimental design and TCAD simulation tools. He has published over 90 papers and won best paper awards at the 1981 ISHM Conference in Chicago and also for the 1990 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING.

J. T. M. Stevenson received the B.Sc. degree in physics in 1967 and the M.Sc. degree in instrument design in 1969 from the University of Aberdeen, and the Ph.D. degree in 1988, from the University of Edinburgh.

He spent 5 years at Ferranti Ltd, Dalkeith, as a Development Engineer on moid fringe measuring systems. In 1974 he joined the Wolfson Microelec- tronics Institute to work on the design of an optical pattem generator for the production of integrated circuit masks. In 1980 he was appointed to a Re-

search Fellowship in the Edinburgh Microfabrication Facility, University of Edinburgh. His main research interests are in optical lithography and optical measurement techniques.

Alan W. S. Ross has worked with Ferranti Ltd. since 1959. As Senior Engineer he was involved in the design and development of a range of industrial metrology products. He became Cleanroom Man- ager in 1977, setting up processes for the production of diffraction grating measuring scales and a variety of photo fabricated products. He was promoted to Senior Production Engineer in 1982, within which post he was responsible for the manufacture of an expanding range of coordinate measurement ma- chines (CMM). In 1990 he joined the Edinburgh

Microfabrication Facility at the University of Edinburgh as a Research Fellow, where his interests included photolithography techniques for silicon wafers and the metrology errors associated with wafer steppers. He is currently involved in semiconductor manufacture training courses for industry as well as promoting the university-designed “schools chip” as a teaching package for school physics courses.