Wafer slicing by internal diameter sawing

Wafer slicing by internal diameter sawing W. F. Struth, K. Steffens and W. K6nig* Internal Diameter Sawing (ms) is the commonly used technique for slicing hard and brittle materials, such as semiconductive silicon and germanium or ceramics and glasses. Nevertheless, productivity and yield are relatively l o w on account of difficulty in handling the flexible t o o l Increasing workpiece dimensions leads to greater problems in realizing quality requirements, such as high flatness, l o w roughness, and l o w crystal damage. This paper deals with basic relationships in los, applying to all workpiece materials (in terms of geometry and kinematics), as well as those relating specifically to monocrystalline silicon (in terms of cutting mechanism, lattice damage and flatness). Finally, guidelines are given to improve process reliability.

Keywords: wafer slicing, internal diameter sawing, monocrystalline silicon, semiconductor technology

Internal Diameter Sawing (IDS) is a very precise cutoff-grinding technique for sectioning ingots into wafers (Fig 1 ). The tool is characterized by an annular steel blade 0.15 mm thick. The internal diameter of the blade is electroplated with an abrasive layer, consisting of nickel-bonded diamonds. Depending on the blade manufacturer, the abrasive layer thickness ranges from 0.3 to 0.5 mm. In order to ensure sufficient axial rigidity of the tool, the blade is tensioned like a drumskin and centred in the machine with an out-of-roundness error of 5 to 10 #m. WDSis mainly applied to expensive workpiece materials with high quality requirements. The dimensions of the internal diameter saws are directly defined by the workpiece diameter: 6-inch diameter ingots require an external tool diameter of 27 inch. The largest diameter is 40 inch, for cutting 10-inch diameter ingots +-3. The cutting peripheral speed of the internal diameter reaches 25 m/s, whereas the radial feed rate--dependent on workpiece material properties-can reach 3 mm/s 4'5. From an economical standpoint, 'wafering' is a crucial processing step, especially in the field of semiconductor technology. The most important quality criteria are ' b o w ' and 'damage' of the wafer: after cutting-off, the wafer is not perfectly flat, and the crystal structure is destroyed by microcracks, dislocations, and dislocation networks. Both bow and damage are highly influenced by workpiece characteristics (for example residual stress, impurities) and by the geometry and kinematics of the cutting tool. Without an exact knowledge and control of the toolshape and the kinematics there can be no precise choice of cutting conditions or tool preparation data and no process control. * Fraunhofer-lnstitut fi)r Produktionstechnologie--IPT, Steinbachstr. 17, 5100 Aachen, FederalRepublic of Germany

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In order to analyse the process conditions, it is necessary to characterize the grinding layer topography. The active surface of the grinding layer is determined by the total number, size, shape, and distribution of the diamonds. However, there is no exact and complete description of the real topography--either with the help of measuring equipment or with the help of mathematics: the first founders on the resolution of available sensors, the second on the extremely irregular shape of the natural diamonds. Hence, the topography has to be characterized on the basis of a combination of practicable approximations: the top of the grinding layer can be inspected with a conventional surface analyser. Twelve measuring tracks 48 mm long are uniformly distributed on the blade circumference so that mean topographical values and standard deviations can be calculated. The average peak-tovalley height Rzs corresponds with the diamond grit size; the peak-to-mean-line height Rps gives data about the chipspace. The standard deviation describes the uniformity of the wheel topography, and, hence, in the case of an ideal grinding layer condition, the value of standard deviation is zero.

0141-6359/88/010029-06/S03.00 © 1988 Butterworth & Co (Publishers) Ltd

29

Struth et al--wafer sficing by los The method is of restricted validity, as mentioned above, so an additional examination with the help of SEM is carried out.

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Basic relationships in IDS

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As mentioned above, the internal diameter saw blade is tensioned and centred on the rotor of an air bearing spindle. After mounting the saw blade the roundness error is about 20 to 30 #m. A further reduction of the roundness error is then achieved by dressing. But with increasing number of cuts, creep and relaxation of the blade material in connection with the blade texture once again lead to an increasing roundness error. A typical shape of this formal defect is an ellipsoid (Fig 2). In this case, the nominal radial feed has superimposed on it an additional speed component. The graph shows that the effective feed rate varies between a maximum of 5 m m / s and a minimum of - 3 m m / s during one tool revolution. This variation decreases with increasing tool diameter. On the other hand, increasing cutting speed leads to higher variations of the effective feed rate. The consequences are: • • • • •

chatter, interrupted tool engagement, increasing dynamic forces, excessive damage and bow, wafer breakage.

The roundness of the tool and the uniformity of the topographical features can be enhanced by sharpening, as shown in Fig 3: when sharpening, the internal diameter saw cuts a sharpening stick of vitrified bonded alumina grains. Even the first cut causes a clear correction of the topography and the

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roundness, proved by lower standard deviation and lower roundness error. Further sharpening results in further improvements. Generally, two or three sharpening cuts are sufficient for optimal blade conditioning. When departing from this optimum the blade wears out, as indicated by the increasing values of Rps. These mechanisms reflect the macro-shape of the abrasive tool layer. In addition, the micro-shape of the grinding layer, especially its cross-section, has to be taken into consideration too; the cross-section of the grinding layer has the shape of a teardrop or bowl. As a consequence, the guide motion of the single abrasive grains relative to the workpiece changes. Fig 4 shows an example of a bowl-shaped grinding layer. In the figure the variation of feed rate is expressed by the percentage ratio between effective local feed rate and the maximum feed rate, which occurs at a contour angle value of 90 °. The location of an abrasive grain is described by the contour angle, c~. According to their location there are different engagement conditions for the grains. A grain which is located at a position of ~ = 0 ° operates under face grinding conditions. It has zero effective feed rate and is influenced only by thrust forces. At c~= 90 ° the grains have the highest effective feed rates, and they operate under peripheral grinding conditions. In distinction to the contour angle, the engagement angle describes the instantaneous location of a cutting grain within the contact length. As can be seen from Fig 4, there is a large variation of kinematic conditions when varying the contour angle ~, while there is practically no change when moving along the contact length. While these considerations are based on ideal conditions, other disturbances, like irregular coolant flow, produce additional motion components. The resulting effect is an asymmetric velocity field

JANUARY 1988 VOL 10 NO 1

Struth et al--wafer slicing by los

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Fig 4 Influence of grinding layer shape on local feed rate causing force imbalance, which favours blade deflection. In consequence, the cutting layer changes its shape and thus supports additional blade deflection. Thus, sharpening aims to enhance workpiece quality by improvement of the grinding layer topography and shape, as illustrated in Fig 5. In the case of monocrystalline silicon, special attention has to be given to flatness, thickness, and damage to the wafer. The flatness error, the so called 'bow', is disadvantageous with respect to lithographical treatments and should be minimized as far as possible. Fig 5 illustrates the typical flatness error of an ID-sawn wafer: it shows a convex curvature with a valley in its centre. The opposite side of the wafer, which is not illustrated in the figure, is concave. Other shapes of flatness error, like saddles with a convex and a concave curvature on one side, also Occur.

There are two reasons for this flatness error, and both are related to the grinding layer topography. The first is the generation of residual stress. The lattice structure of silicon is the diamond type, consisting of two interwoven cubic face-centered structures (Fig 6). The float-zone-grown or Czochralski-drawn ingots are of extremely high purity and are, in technical approximation, nearly dislocation free. Mechanical processing like lOS generates new dislocations and dislocation networks under the influence of high local compressive and shear stresses. Dislocations generated by grinding mainly occur in the direction of the cutting-edge engagement traces. Fig 7 shows etching pits, which mark exits of dislocation lines. As opposed to many randomly distributed dislocation lines, the regularly arranged dislocations produce residual stresses, which tend to bow the wafer, with respect to the small thickness of the wafers (typical values are 0.3 to 0.5 mm).

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.,oeThe second reason for the flatness error is blade deflection during cutting, caused by force imbalance (Fig 8). The graph shows the engagement of the grinding layer. On the supposition that the condition of the grinding layer is uniform along the outline, there is a uniform distribution of the material removal rates. The maximum occurs at the top, the minimum at the flank of the layer. In this case, the local normal forces are balanced. With increasing number of cuts, the layer loads with abraded silicon. This implies increasing normal forces. Provided that these Ioadings are not symmetrical, the resultant action of unbalanced normal forces leads to blade deflection. Thus, two reasons for the flatness error are known: • •

residual stress caused by regular dislocation concentrations ('stress bow') ; blade deflection ('saw bow').

While residual stresses can be removed by mechanical processes with low effective cutting speeds (lapping) or by chemical treatments (etching), there still is a residual flatness error which can hardly be removed by any treatment: when lapping, the thin wafer deflects, forced by the pressure of the lapping wheel. After lapping, it relaxes. This type of deficiency is usually caused by blade deflection. Therefore, minimizing or, even better, preventing 'saw bow" is of major importance. As mentioned above, force imbalance is responsible for blade deflection and in consequence for the flatness error. This force imbalance is favoured by an asymmetrical grinding layer shape of the unsharpened blades or by a one-sided loading with silicon particles. Fig 9 shows SEM micrographs and the energy-dispersive x-ray analysis of the grinding layer. The left flank contains much more silicon than the right one. A tool like this generates axial force imbalance, causing blade deflection. Sharpening avoids this drawback ( Fig 10). The histograms illustrate the process forces in the tangential, axial, and normal direction, the flatness error on both sides of the wafer, and, as topographical values of the grinding layer, the peak-

31

Struth et al--wafer sficing by los

Atomic number Atomic weight Atomic radius Lattice spacing I nteratomic distance Atoms cm -3 Density (20°C) Density (1420°C) Melting point Specific heat (80°C-120°C) Linear thermal expansion coefficient (20°C) Hardness Young's modulus Bulk modulus

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Crystal structure and characteristics of silicon

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32

14 28 086 1.17 10 - 1 ° m 5.43 10 - 1 ° m 2.35 10 -10 m 4.96 1011 2.33 gcm -3 2.55 g cm -3 1420°C 0.754 J 9-1 deg- 1 C 2.33 10 - e deg- 1 C 7 Mohs, 1000 Vickers 1.9 10s N mm -2 7.7 104 N mm -2

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JANUARY 1988 VOL 10 NO 1

Struth et al--wafer slicing by IDS Grinding layer \

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to-mean-line height and its standard deviation. The new, unsharpened blade was used to cut 10 wafers. Triple sharpening leads to a clear enhancement of layer condition and flatness. But, with increasing number of cuts, axial forces increase because of asymmetrical loading. In the same way, flatness error increases too. The advantageous effect of triple sharpening is proved again: there is a significant reduction of flatness error. It should be mentioned that there is related to the change of forces and flatness only little change in topographical values: the effect of the sharpening is not breakage of diamonds and of the nickelbonding. As a matter of fact, sharpening mainly carries off silicon Ioadings and produces little additional wear. Thus, it would appear that sharpening is a helpful method to improve process reliability and workpiece quality.

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Conclusions Internal Diameter Sawing is a high precision cutoff-grinding process. In order to achieve the desired work result special attention has to be paid to the geometry and kinematics of the tool engagement. Bad tensioning and centring of the flexible tool lead to excessive dynamic forces in connection with an interrupted tool engagement: saw marks, excessive damage, and, in the worst case, wafer breakage are the consequences.

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Taking into account the characteristics of monocrystalline silicon there is a clear interaction between process stability, damage and flatness error: besides dislocation networks, blade deflection caused by force imbalance is of decisive influence on the flatness error of roD-sawn wafers. The socalled 'saw bow' can hardly be reduced by retreatments, either by mechanical or chemical action. It has been pointed out that asymmetric Ioadings of the grinding layer imply unidirectional force imbalance and blade deflection. Finally, directions have been given on how to reduce and prevent excessive flatness error by sharpening the grinding layer.

Acknowledgements The research work leading to this paper was meritoriously sponsored by Deutsche Forschungsgemeinschaft ( DFG ).

34

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45(2), 77-79 2 A n o n Diamond Tools for the Electronic Industry.

Werkzeugkatalog Ernst Winter & S~hne, Norderstedt, 1984 3 B r a n d t G. Slicing and grinding semiconductor materials.

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Steffisburg, Switzerland 5 A n o n Automatische Innenlochs~ige IDS 33. Firmenkatalog

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13(3), 234-242 9 M e e k R. L. a n d H u f f e t a t l e r U . C. ID-diamond sawing damage to germanium and silicon. JES, 1969, 116(6),

893-898 10 M e i s F. U. a n d S t r u t h W. F. Innenlochs~igen. Geometrie und Kinematik des Werkzeugeingriffs. Ind.-Anz., 1985,

107(98), 21-24

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