Proc IEEE Ink Conference on Microelectronic Test Structures, Vol6, March 1993 275

Design Considerations for a Test Structure which can be used to Determine the Optimum Focus

A.J . Walton. M . Fallon, J.T.M Stevenson, A.W.S. Ross

Edinburgh Microfabrication Facility, Department of Electrical Engineering,

University of Edinburgh, King's Buildings,

Edinburgh, EH9 3JL, UK.

Abstract: This paper describes a test structure which can be used to optimise the focus of wafer steppers. Simulation is used to determine the optimum setting for some of the design parameters to ensure maximum sensitivity of the device.

1. Introduction As smaller geometries have been introduced the

importance of ensuring that the image on the wafer is in focus during exposure has become more critical. One of the problems with determining whether the optimum focus conditions have been achieved is that over or under exposure to a first order approximation exhibits very similar effects. Measurement of a full exposure-focus matrix to determine the optimum process operating conditions is time consuming unless a fast, automated measuring instrument is available. Any change in the dimensions of the image 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.

2. The 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 smcture 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 opthising 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 non- uniformities in the etch process.

The proposed 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 figure 1. It can be observed that increases in the x 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 indicated in figure 1. The resolution (R,) is simply given by

e R, = 2 d, tan -

2 where 0 and d, are indicated on figure 1. For a change in resolution of 0.1 pn and an angle of 5" the value of d , is 1.145 p 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.

3. An Optical Focus Structure Figure 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.

4. An Electrical Focus Structure

It is possible to adapt the structure for electrical measurement 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 figure 2 since a large number of parameters influence the resistance of the device.

Any structure using a wedge to constrict current flow consists of two resistive components; the resistance due

@-7803-0857-3/93/$3.00 01993 IEEE

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

Figure 2. Schematic layout of an optical structure that can be used to determine the optimum focus.

to the the change in the wedge dimension and the parasitic resistance as indicated in figure 3. The parasitic resistance at the ends can be eliminated by voltage taps while that at the lower region can be minimised by designing the apex of the wedge to be as close to the bottom of the structure as possible (CW=0 in Figure 4). The objective in trying to optimise the sensitivity of this type of structure is to minimise the parasitic resistance so that the percentage change in resistance is maximised for a given change in focus. Figure 4 gives the layout of such a structure which attempts to meet these objectives. Current is forced between two ends and voltage measured between the two taps. Figure 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 figure 6 to give the change in resistance as a function of resolution for a 5" angle. Other key dimensions are given in table 1.

Dimension Length arm) Number of segments Channel Width (CW) Tap Width (TW) Tap Length (TL) 2.0 Length of segment (LSEG) 1 .0

Table 1. Structure dimensions used in figures 6 to 10.

216

Resistance controlling region

dm

Parasitic region Figure 3. The areas of the structure that control

the resistance.

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Figure 4. Schematic layout of an electrical structure that can be used to determine the optimum focus.

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(a) (b) Figure 5. (a) The controlling resistance area.

(b) The model sections.

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

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 figure 5. The decrease in resistance is due to the the reduction of the length and increase in the width of the pentagonal sections. This simple model obviously has a limitation in that it that it forces equipotentials at the joints between the sections and 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.

This limitation of the model requires that for accurate results Laplace's equation must be solved numerically. This has been performed using MEDIC1 and, for the dimensions given in table 1, the variation of resistance with resolution is shown in figure 7 for wedge angles between 1' and 25". Figures 8 and 9 give the current vectors and equipotentials for two different geometries and the distribution of current can be observed to be non- uniform as the wedge constricts the current flow. It can also be noticed that the voltage taps distort the current flow far more when the resolution is 0.1 p in figure 9. This effectively removes the effect of the segment 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 figure 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 should be maximised. This will have the added bonus that the effect of any lithographic or etch non-uniformities will be minimised. The other option is to move the voltage taps to just outside the region where the segments are formed.

The difference between the simple model and the full simulation is at the high resolution end of the curves and is due to the distortion of the current flow by the simple model and the effect of the voltage taps which are accounted for in the full simulation.

Figure 10 gives the response surface indicating how the resistance of the device depends upon the angle of the wedge and the resolution. The change in resistance must be maximised for the structure to have maximum sensitivity to changes in focus. The response surface equation used to model the resistance can be differentiated with respect to the resolution and this has also been plotted in figure 10. It should be remembered that the response surface is just the 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 figure 10 and comparison with figure 7 indicates that the variation of resistance with resolution is small but non zero 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

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

good fit it is not perfect. However, from figure 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 the optimum sensitivity to changes in resolution. In practice the shape of the structure will be more rounded and the effect of this will now be discussed.

5. Experimental Results

Some exploratory devices were fabricated with angles of 10, 15 and 20 degree angles and a designed channel width (CW) of 1 .O and 0.79 pm. A thermal oxide was grown and 4500 A of polysilicon was deposited and doped using a solid sonrce. The wafers were then coated with photoresist and the pattern defined using a lox wafer stepper. Aluminium silicon was then deposited for the contact pads. Two wafers were exposed to examine the effect of exposure time and focus on the resistance of the structures. Theexposure was varied in increments of 50 ms and the focus in increments of 0.5 pn in a serpentine across the 38 sites on the wafers as detailed in table 2.

EXPOSURE FOCUS WAFER 1 1200ms -5 to +12.5p

A=50ms

Table 2. Exposure and focus conditions for the structure.

Figures 11 and 12 show how the structure varies as the exposure and focus change. It can be observed that for both exposure and focus changes there is a variation in CW and in the height of the apex of the triangular sections. A Bio-Rad Quastor was used to measure these dimensions by printing out A4 pictures and using these to extract CW and the reduction in the apex height (AR). This machine was also used to automatically measure the widths of the taps and these measurements used to calibrate the measurements of CW and AR. It should be

D I D t M - ,Mlcron=i) L YY "" " ""

Oilcf-- <mLsr-a> Y Y Y I "U

Figure 8(a). The equipotential for a focus Figure 9(a). The equipotential for a focus structure with a resolution of 0.5 m. structure with a resolution of 0.1 p.

l . ' O O 2." 3." , 5 ,1m _3 -. . 00 ornrmica <aicrar.-,

- - , . . , , . . D l n t M s e < M l c m - r . 3 > -5 i 6,

Figure 8(b). The current vectors for a focus structure with a resolution of 0.5 p.

Figure 9(b). The current vectors for a focus structure with a resolution of 0.1 p.

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noted that the nominal tap width was 1.0 p and the minimum intensity position was used to determine the edge of the tracks so the measured linewidth is exaggerated. Figure 13 shows the variation in these dimensions as the focus is varied. It should be remembered that the identification of the edge of the polysilicon is somewhat subjective and the error for CW and AR is estimated to be in the region o f f 0.15 p. It is interesting to note that the tap width remains surprisingly constant until about +8 pn of defocus after which it was no longer resolved. Figure 14 and 15 shows how the resistance of the structures varies as the focus and exposure are altered. It can be observed that as the exposure time is increased there is an increase in the resistance. However, this increase is not monotonic. The

ANGLE

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Figure 10. Response surface for a focus structure.

219

Figure 11. Changes in the focus changes. (a)

I the structure dimensions as -4 PI, (b) -2 m, (C) +4 P

curves for the different angles track each other and this suggests that there is some non-uniformity that may be related to variations in sheet resistivity or etch. Unfortunately, in this preliminary study no structures were available to confirm this, although optical examination suggest that spurious changes in resistance are related to dimensional variations. A similar situation exists with focus where it had been expected that the resistance would rise to a peak at the optimum focus condition and fall in a monotonic fashion either side. Examination of figure 14 shows a peak region in the same

Figure 12. Changes in the structure dimensions as the exposure changes (a) 850 ms, (b) 1100ms, (c) 1300 ms

position as the minimum for CW in figure 13. However, the non-uniformity makes these preliminary results somewhat inconclusive.

6. Conclusions For this structure to be of potential use it must be

capable of detecting changes in linewidth of 0.1 j m ~ or better [4]. The preliminary results presented above indicate that the resistance of the structure is sensitive to changes in both exposure and focus but the 'noise' on the measurement masks the true sensitivity. It is possible that

:I /

Figure 13. Variation of dimension with focus. this problem is inherent in the structure but the tracking of the curves of the different angled structures suggests that this is probably not the case. However, the exercise has identified that there are improvements that can be made which will help improve the reliability of the structures.

One very simple change is to increase the width of the taps which were designed with a width of 1 p. At the extremes of exposure and focus these had a tendency to disappear 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 minimise the effect of any etch non-uniformities and current distortion at the tap positions. This will have the added bonus of increasing the resistance of the devices so that the sensed voltage will increase for a given current level.

From figure 13 it can be observed that as the resolution of the structures degrade 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 parasitic resistance which reduces the sensitivity of the measurement. Much of this variation could be removed by chamfering off the top of the triangle but this would only have the effect of increasing the parasitic resistance to a higher constant value. Figure 16 shows a schematic of a structure which minimises both the increase in parasitic resistance while also eliminating the effect of the apex height reduction.

7. Acknowledgements The authors would like to thank W.J.C. Alexander for

CAD design, W. Gammie for help with the electrical measurements and the technical staff of the EMF who performed the processing. We would also like to acknowledge the support of SERC (Grant no GR/F 38884) and "MA who provided the software.

Figure 14. Variation of resistance with exposure time.

I O O T I A

Current

Figure 16. Improved structure 8. References

A.J. Walton, W. Gammie, M. Fallon, J.T.M. Stevenson, "An Interconnect Scheme Reducing the Number of Contact Pads on Process Control Chips", IEEE Trans On Semiconductor Manufacturing, Vol 4, no 3, August 1991, pp 233-240. S.M. Sze, "VLSI Technology", p189, McGraw Hill, 1983 K. Murray, "Measuring Dimensions Using Murray Daggers", Semiconductor International, December

H. Koyama, Private Communication 1992, pp 69-73.