New wafer alignment technique

Mwrm'h'ctrom(s aml Rdiahilitv. Vol 15 pp.147 to 151. Pergamon Press. 1976 Printed in Great Britain

NEW W A F E R ALIGNMENT T E C H N I Q U E T. ROLAND FmaDRmSEN Computervision, Cobilt Division, Santa Clara, California, U.S.A.

INTRODUCTION

During many semiconductor processing steps the wafer must be accurately positioned so that the circuit patterns are aligned with respect to a predetermined object such as photomask, prior photoresist pattern, probe tips, markers, bonders, etc. Historically, this function has been performed by human operators through microscope viewing and manual equipment manipulation. The manual technique suffers from disadvantages in handling damage, operator error and inconsistent performance. In recent years, a successful method of automatic alignment was pioneered in the photomask/wafer alignment field. Here, purposely designed target patterns were introduced in certain die positions of the wafer to facilitate automatic machine alignment. The target technique has been implemented in volume production. This paper describes a more general wafer alignment technique which does not depend on special targets. The concept was specifically developed for wafer processing equipment used after the photolithographic processing is complete. Implementation of the unique pattern recognition scheme was made possible by the availability of new powerful microprocessors.

stage mechanism holding the wafer is only required to move in the Y direction. Assuming the mechanism and wafer to be perfect, the potential misalignment results from: (1) Wafer columns not being parallel to the Y axis. (A 0 error) and (2) Circuit patterns not correctly located in X or Y(AX, A Y error). It is logical to perform the initial probe alignment to one of the center die on a wafer and a 0 alignment then gives symmetrical error distribution as shown in Fig. 2b. Assigning half the tolerance to the 0 misalignment the maximum allowable A0 angle is 0.0001 360 A0=-D/2 " 2rr or

A0

(1)

It is interesting to note that the allowable angle of 0 misalignment is inversely proportional to the wafer size. This means that Larger wafer sizes require better 0 mechanization. Also, the theta misalignment gives rise to both X and Y error according to the formula: AXO = 2r sin 0 D

WAFER ENTRY CHARACTERISTICS

The automatic alignment system is part of a fully automatic prober where the load-guide system has to accommodate any wafer variation within a specific size, for example 4 + 81-". Thus, when we include working clearances the wafer center can easily be 1,, displaced from true chuck center. During loading, the wafer floats on an air cushion and spins freely within the guides; consequently, the final angle between scribe lines and machine Y axis could be any value from 0 to 360 ° (Fig. 1).

0.011465 - degrees. D

2r A YO = ~ cos 0

(2)

where r and 0 are polar coordinates of the die being tested. The AXp and A yp errors due to incorrect pattern recognition are constant for all die positions. Thus, max AXp = m a x Ayp = :L0.0001.

(3)

WAFER CHARACTERISTICS WAFER ALIGNMENT SPECIFICATION

A typical wafer contains a large number of identical microcircuits. The circuits (die) form rows and columns separated by "streets" or scribe lines (Fig. 1). Contact pads are located around the circumference of each die providing input-output connections for testing and later packaging leads. The die to die center dimensions are known quantities and the die positions are symmetrically arranged, thus, by accurately locating a street center all circuits can be reached by known displacements.

In the automatic prober the tolerance allowance for the alignment system is + 0.0002" anywhere on a 4" wafer. That is, a probe point may touch down within a square deviation zone as shown in Fig. 2a. The normal procedure for a wafer prober is to sequentially test individual die in a row or column. The wafer rows and columns are aligned parallel to the principal X and Y axis of mechanical motion. Thus, when probing a series of die in a column the 147

148

T. ROLAND FREDRIKSON

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Wafer orientation is defined by the flat location. Referring to Fig. 1, the circular outline of the wafer is broken by a straight cord cut. Sometines more than one fiat exists, but always one is larger and has a specific angular relationship with the scribe lines on the wafer. The probing of a wafer consists of accurately positioning a set of test probes on the circuit pads sequentially on each die. The probe pressure exerted against the pads is normally created by the cantilevering effect of the probe tips as the wafer is pressed up against the probe arrangement. Hence, the surface contour of the wafer represents a parameter necessary for consistent probing. Standard descriptions include: "taper", "bow" and "thickness" of the wafer. However, when the wafer is vacuum held against a fixed surface the three standard parameters are lumped in a AZ deviation with respect to the normally expected wafer surface elevation. THE A U T O L I G N ® C O N C E P T

The automatic wafer alignment scheme is based on the following concepts. (a) Locate the flat and rotate wafer into an approximate orientation ( 0 EMa, ..... = -4-5°) • Measure diameter• (b) Locate a central street by recognizing its special reflective characteristics as opposed to crossing circuit patterns.

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(c) Locate same street one device length away, calculate 0 error and correct. (d) Successive iteration of locating same street along principal axis followed by theta correction is concluded by final 0 orientation within minimum 0 increment. (e) Locate central cross street and calculate all whole die positions. AUTO-ALIGN MECHANIZATION The purpose of the auto align system is to place each microcircuit on the wafer in precise contact with a set of test probes and to implement this four types of motion arc rcquired, X, Y, 0 and Z. Figure 3 defines the coordinate axes and their relation. Note that the wafer can be moved in Z or 0 independent of the X, Y position. All four principal mechanisms are driven by stepping motors with micro stepping controllers. This gives both the high speed and the fine resolution performance required. Details are listed in Table 1. z

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New Wafer Aligmnent Technique

149

Table 1. Principal motions

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The height sensor and image sensor are located in the X - Y plane as outlined in Fig. 4 and since these devices are uniquely related to the alignment scheme a more detailed description is in order.

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Computer interface H E I G H T SENSOR O P E R A T I O N - - A D A P T I V E Z ®

The height sensor hardware consists of a capacitance probe (Fig. 5) and associated control hardware. When the wafer surface is positioned 22 mils away from the probe surface the amplifier output is zero volts. Any translation of the wafer in the Z axis gives a corresponding voltage output within the range of +10 mils at a rate of 1 V/mil. In the Autolign system described only + 5 mils is required, thus, to simplify the control only the positive range is used with the nominal wafer surface placed at Z0 giving 5 V output from the amplifier. The A - D converter produces a digital equivalent output when sampled by the computer control. The resolution of this measurement is 0" 1 mil giving a reading of 50 for the Z0 setting shown. I M A G E SENSOR O P E R A T I O N

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Fig. 5. The hardware applies adaptwe control to eliminate wafer to wafer differences as well as normal circuit aging effects. The simplified system shown in Fig. 6 indicates the conversion from instantaneous optical image to parallel digital data output. The computer interface has the ability to change the gain, initiate a sampling (converting the instantaneous analog value of the image to a digital number) and read the sampled data in digital form. The spot size on the wafer is approximately 0.5 mils in diameter and the maximum sampling rate is 25 kc. Hence, at the maximum speed of 5 in sec -~ the image number can be read once every 0.2mils of motion. The data rate is controlled directly by the computer and correlated precisely with wafer position. C O A R S E A L I G N I N G THE W A F E R

A neon-helium laser beam provides the light source in the pattern recognition system shown in Fig. 6. The microprocessor directly controls the image conversion parameters and the digital data stream is filtered by a sequential algorithm. Each step of the algorithm contains a multiple choice pattern comparison with iteration simplification.

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During the initial set up procedure the first wafer is used as probe height reference. Referring to Fig. 5, Zo is selected to give a reading of 50 (5 mils) at

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150

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approximately the center of the wafer. From there on. the variation of the wafer surface is recorded by always scanning the wafers at Z = Zo. Later, in the probing cycle, the variations are applied to the normal Z travel giving constant probe deflections. Finding the edge of the wafer is interpreted as a sudden and sharply defined surface discontinuity. Following a pattern indicated in Fig. 1 the controller operates a conbined X - Y and 0 motion to first measure the diameters along A B C D B E , then track the edge in a circular motion. A flat is defined as three or more points on a straight line and the length of every "fiat" is compared to a given criteria that relates to the particular wafer type. Having found the fiat, the computer calculates the angle and rotates the wafer into correct orientation. A f t e r rotation the wafer diameters are again measured and this time, knowing where the flats are, the precise wafer center can be determined.

THE FINE A L I G N PROCEDURE

During the last diameter determination, the image sensor output is also checked. If the numbers representing reflected images do not show a complete range with all numbers from 0 to F, the amplifier gain will be changed (see Fig. 6). W h e n the numbers are crowded in the high end, say, 10 to F the gain will be lowered and when only low numbers appear the gain will be increased. The first task of the fine align is to locate a street. This is a random search as the start point may be anywhere on the die (Fig. 7). Starting from point A in Fig. 7, the wafer is scanned in rapid short Y-strokes progressing in the X direction until a probable street condition is found at B. The stroke in now lengthened to one

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B Fig. 8. full die to die Y-dimension and the scan continued until the other side of the street is detected, F r o m the data collected the center of the street is calculated (point C) and the scan resumed displaced one whole die length in the Y direction. This procedure is continued up to five times and a line calculation made for the string of center points resulting in the first major theta correction. The initial street search was purposely conducted towards the center of the wafer reducing the chances for losing the street during O correction. However, depending upon the wafer center to chuck center relation a new street search may be required after the first major theta correction. The 0 alignment is typically only a fraction of a degree out after the first major correction and the scanning continues followed by minor theta corrections until the street is traced from one end of the wafer to the other as shown in Fig. 8. The street is detail scanned at points A and B until the center differences is less than one minimum theta increment (i.e. < 8 see.). The next step in the fine align is to locate the cross street in the X direction. The wafer is first positioned to center, then scanned in X until a street is found. Since the theta is already in perfect alignment, the center of the X street is readily found. FROM ALIGNMENT TO PROBING

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The first wafer in a test sequence means usually a new probe set up which, in turn, presents a new location of the probe cluster with respect ot the image sensor. During "set u p " the computer calculates the theoretical center of the probe cluster and moves the center of the wafer's most central circuit to this position. Referring back to Fig. 4, the X0 and Yo distances correspond to the theoretical probe cluster to image sensor displacement, and the operator can via the X - Y joy stick correct the X - Y position until the probes are centered on the pads. The new Xo, Y0 dimensions are recorded by the computer when " r e c h e c k " is initiated and a complete realign

New Wafer Alignment Technique operation performed to verify the positional accuracy, probe ring tightness and probe adjustment. The rest of the machine cycle is concerned with probing identical circuits. Here, the computer knows the exact locations of all streets since it has found two central streets and knows the die to die center distances. Also, the wafer outline is known and therefore the computer can calculate the position of all die and probe them sequentially or in any special order. At each circuit position the Z motion is adapted to give constant contact pressure and partial circuits on the edge are not tested to avoid probe damage.

151 CONCLUSION

This paper describes a new wafer alignment system which utilizes the street pattern for identification instead of special targets. Intended for high volume production machines, this method covers most wafer types and is relatively simple. However, the laser sourced image sensor arrangement is sufficiently accurate that a more general pattern recognition system can be developed should the need arise. As was shown in the earlier stages of development, the wafer surface can be faithfully reproduced to show most kinds of defects as well as regular patterns.