ID-Cut-off Grinding of Brittle Materials

E. Brinksmeier, W. Von Schmieden, University of Hanover/Germany - Submitted by H. Trumpold (1) Received on February 25, 1987 - Accepted by the Editorial Committee

Abstract: The inner diameter(1D) cut-off grinding is primary used for the cutting of silicon single crystals into thin sliaes. These slices, so called wafers, are the basic material for the manufacturing of semiconductors. Larger wafers with an increased density of components and an improved quality can only be manufactured economically, if the basics of the slicing operation are known. The paper descri- bes the major problems in ID cut-off grinding and derives strategies which can make the process more predictable. There is applied a special kind of tool with a centred hole. It is a 170 micron thick fine steel rod plated with diamonds on its inner diameter. The stiffness of this tool is achieved by a ra- dial loading in a circular clamping frame: so especially the load stresses of the blade core are of great interest. To get values of radial and tangential stresses, finite element calculations have been carried out. One possibility of measuring these stresses is to use electromagnetic methods: the neces- sary calibration tests are described. The process behaviour during the cutting-off is investigated on the influence of cutting speed, feed speed and the kind of coolant which is utilized.

Key words: diamond tool, cut-off grinding, silicon wafer, electromagnetic stress measurement, finite element calculation.

1. Introduction

The inner diameter(1D) cut-off grinding is primary used for the slizing of semiconductor materials. At present time ultrapure silicon is the most applied substrate material for the production of components like transistors, thyristors, diodes and integrated circuits (IC's) /l/. Fig. 1 shows important steps during the production of electronic chips. A cut-off grinding is necessary for the slicing of the silicon single crystal into wafers.

During the 1950's conventional cut-off tools were first used for slicing and also dicing of silicon single crystals. This method, however, was ineffi- cient due to the high kerf loss. The development of the so called Inner Diameter Blade (ID-blade) reduced the cutting loss to nearly 20% compared with the conventional cutting methods. The first ID-blade featured a 206 mm outer diameter (OD) with an ID- cutting surface of 8 3 mm in diameter. Due to the in- creasing demands of the semiconductor industry the silicon wafers became larger in diameter. Therefore the tools had to be provided with larger diameters, too / 2 / .

Fig. I: Processing of wafers

In the near future silicon ingots with a diameter of 200 nun have to be cut. The necessary tools and ma- chines are existing /2.3/. In order to assure the wafer-quality and the process reliability, a syste- matic analysis of the ID cut-off grinding has to be carried out. Surface damages, cracks, scratches and form deviations of the wafers have to be avoided, because this will lead to rejected material. Faults in the electronic circuits may occur later. The most unconventional component in the ID cut-of f grinding is the tool and it's properties. This will be shown in this paper.

2.

The slizing of silicon single crystals by ID cut-off grinding is achieved by a thin rotating tool which plunges with it's active cutting edge profile into the silicon ingot with a certain feed speed (Fig. 2 ) .

Technology of ID cut-off grindin7

lomping ring enstoning r ing 0 - blode

sect ion A-8

Si -c rys ta l

v: cutting speed v i r rod io l feed speed

F, F, F, cutt ing l o w s

Pig. 2 : Principle of ID cut-off grinding

The core of the ID-blade is made of a high-strength cromium-nickel steel sheet with a thickess of 100 to 170 microns dependant on the blade-diameter. Diamond grit is electroplated at the edge of the internal hole in the circular core. They are embedded in a metallic nickel bond and act as the cutting edge.

The blade is tensioned like a drumskin at the outer diameter with a special clamping head. The tensioning procedure is of extreme importance for the complete process, because it determines the stiffness of the tool and the truth of rotation / 4 / . Fig. 3 gives a general view of input quantities on the process. They are of major importance on the wafer quality, the economics and the tool wear.

- i

Fig. 3 : Major quantities in ID cut-off grinding

It is obvious that the unconventional tool leads to disturbance variables, which are unknown in conven- tional grinding processes. The radial and tangential blade stresses are additional parameters of tremen- dous importance for the performance and the reliabi- lity of the process. Therefore the ID cutting-off can only be controlled, if the state of stress is known. As the blade is tensioned till the yield stress, ra- dial and tangential stresses change during the pro- cessing period. For that reason the stresses have to be known local and per unit time.

Annals of the ClRP Vol. 36/1/1987 21 9

In the following, conventional and new methods for the determination of blade stresses are described. Accordingly, technological results from the ID cut- off grinding are introduced.

3 . Determination of blade stresses

The load stresses of an ID-blade determine it's stiffness. In order to achieve a maximum value of stiffness, the blade is tensioned up to the yield stress limit. It is important to know, when this li- mit is reached and which state of stress occurs in the blade. Thus the stresses have been determined in a first approach by a finite element calculation.

3.1 Finite element calculation

The aim of the FEM-calculation was to find out radial and tangential blade stresses for different values of blade elongation at the inner diameter. As the state of stress is an two-axial one, a two-dimensional FEM- network was generated in the computer. The disk-like model yielded a fine segmentation and different thickness in the area of the cutting edge. For the Young's modulus of the cutting edge the values of nickel (bond material) have been used. The results are plotted in fig. 4 .

x.1 t in i fe element c a l c u ! a ~ ! ~ ~ mi 0 - b l a d e . Ihe w e r diameter

blade elanpation 01

ield - the bin& -aJ----

.

UI J TO r I D C 150 rnm 211

I D - b l a d e rndtus r

Fig. 4: Calculated radial and tangential stresses

While the radial stresses are zero at the inner dia- meter, a maximum value for the tangential stresses can be observed. A failure of the blade will thus start at the inner diameter due to an exceeding of the tangential stresses. The slight decrease of tan- gential stresses at the inner diameter is caused by the higher thickness of the cutting edge. he yield point, which was determined to be 1750 N/mm' indica- tes the limit for an elastic blade elongation.

3.2 Measurement of axial blade deflection

For the FEM-calculation the blade material was assu- med to be of homogenious nature. It is known, how- ever, that the unconventional high values for the yield point and the ultimate tensile strength are achieved by a high degree of cold rolling. This was confirmed by structure analysis. It can be assumed that the elastic properties and thus the Young's mo- dulus of the material are dependant from the rolling direction.

60 10-blode 5 358 422-153-0.29-015-01 a x i o l b l a d e d e f l e c t i o n under constant load

I 180' 270' 360'

J O 0 9 0'

angle of r o t a t i o n (P

Fig. 5: Influence of rolling texture on axial blade deflections

This was proved by the measurement of axial blade deflections under constant load. The principal test set-up and the results are shown in fig. 5.

The axial blade deflections are dependent from the angle of rotating which means the direction of rol- ling. Therefore the local state of blade stiffness is also a function of the blade texture. In order to assure the desired truth of rotation at the inner blade diameter, higher load stresses are required in the rolling direction. The dependence from the mate- rial texture was also confirmed by strain measure- ments (fig. 6 ) .

1 2 I D - b l a d e 5 3 5 B - 4 2 2 153-029-015-01

lo'+ struin control with st ram gauges I n i--_- ' - I

'1 012 O! 0'6 Oi8 1'0 1 ' 2 1'' mm /6 ID-elongation Ad

Fig. 6: Loading strains dependant from rolling direction

The largest stretching of the blade material can be confirmed in rolling direction. For that important reason the stiffness has its highest amount in this area. Adequate to the value of the Young.8 modulus, which was in advance determined in tensile tests, the radial strain for 9 =45O and 9 =90° to the rolling direction is lower. With Hooke's law the reference tension in radial and tangential direction can be calculated for various ID-elongations (fig. 7 ) .

I Od 0; d k 06 /8 1 0 I \ 1'4 mm h 10-elongation Ad

Fig. 7: Stress calculation from strain gauge measurement

The comparison with the results from the FEM-calcu- lation shows differences especially deviating from the direction of rolling, because the Younq ' 8 mo- dulus had a constant value of 2.1*105 N/mm in the assumption. Principally the theoretical calculation by the finite element method is confirmed.

3 . 3 Determination of blade stresses by electromagnetic methods

Up till now the static stiffness of an ID-blade and the state of stress are only controlled in case of clamping and tensioning a new blade or when the wafer quality is no more acceptable. This is done by mea- suring the axial blade deflection under constant load as described before. For improving and controlling the process it is however desirable, to measure the stresses during the cutting-off process, too. This cannot be achieved by using deflection methods and neither by measurements using strain gauges.

For these reasons, non-destructive electromagnetic methods were applied and tested. The stress-sensitive qantities which have been used, were the magnetic Barkhausen noise Mm and the coercivity force H /5,6/. There is used a non-destructive testing pro5 cedure which is called 3MA (micromagnetic - multiparameter microstructure and stress analysis). The testing time for one measuring point is about 0 . 5 8 , the local resolution depends mainly on the sensor-geometry and is about 2 mm in diameter.

An essential pre-supposition for the use of electro- magnetic methods is an exact calibration of the mea- suring device in respect of the blade material. Especially the anisotropy of the material has to be known. How this anisotropy influences the magnetic properties H and t+lmax in an unloaded blade can be seen in fig.%.

10 b l a d e r - 76mm, r,-Zllrnm mat CrNi $tee( c o l d rolled load stress 0.0 Nlmrn: ,"- 3 M A - o n o l y s e r

Fig. 8 : Influence of texture on

While the coercivity, which can be correlated to the hardness or dislocation density of a material, is almost independant from the dfrection of measurement, the magnetic Barkhausen no188 amplitude shows a strong dependency from the rolling direction.

For the calibration under load, test specimens with different orientations to the rolling direction were cut out of the blades and loaded in a tensile testing machine. The results for one specimen ( I p = O o l are plotted in fig. 9.

magnetic properties

I T 500 lu10 1500 N/mm2 2000

lood stress 8

Fig. 9: Load stresses and magnetic properties

At first it is obvious that the measured dependencies are not of linear nature. MmaX shows a typical beha- viour of magnetic hard steel qualities with a kind of saturation effect for higher loads. The coercivity is less sensitive to load alterations and shows even an unusual reaction in the high tensile region because the H -values are incresing with higher stresses. Becauss of this countercurrent response, a product

(H 8 M ) was calculated. A linear dependency be- twgen TRTs product and the load stresses was the re- sult, which could not be expected and must not be assigned to other materials. In this case, however, this linear dependency was also found out for speci- mens with different orientations to the rolling di- rection of the material (fig. 10).

iood stress 6

Fig. 10: Product of magnetic quantities versus load stresses

The results from the tensile testing have been obtained for an uniaxial loading. A tensioned ID- blade has, however, a two-dimensional state of stress with a change of the relation between radial and tangential stress components from point to point. Consequently the magnetic quantities have no linear dependency from the load of analyzed tensioned blades (fig. 11).

Nevertheless the information is sufficient to tension the blade up to a desired state of stress. The de- termination of the magnetic quantities on a rotating tool will be the next step which has to be develo- ped.

I D - elongation Ad

Fig. 11: Magnetic quantities when tensioning an ID-blade

4. Process behaviour during cutting-off

The following results from ID cut-off grinding were obtained from slicing 100-nun silicon single crystals with 27"-blades (ID=235mm, OD=690m). Such blades produce an everage kerf loss of about 0.3 mm. The wafer thickness was adjusted to be 0 . 5 mm. These investigations were realized with an ID cutting ma- chine (IDS 3 3 ) from the Georg Killer Nurnberg company -GMN-, Germany. In the ID cutting-off process the tangential forces are much lower than the normal forces. Fig. 12 shows how the maximum forces are in- fluenced by the radial feed speed and the cutting speed.

Increasing feed speed leads to higher forces. It can be noticed, however, that a further increase beyond a feed speed of 60 mm/min results in a progressive in- crease of the forces. This effect is drawn back to the influence of the coolant and the chip removal. High removal rates lead to a higher volume of silicon chips which have to be conveyed out of the cutting gap. A large amount of chips can prevent the coolant

221

I 0 ~ b lode S 35 B ~ 690- 235 -0 29 -015 -015

vlr = 60 rnmlmin , v( - 20 mls LL-

r06i01 feed speed vI,

Fig. 12: Maximum process forces in ID cutting-of f

from penetrating into the contact zone between cut- ting edge and workpiece. This leads to higher forces and temperatures and to lower wafer qualities.

The influence of the cutting speed on the forces is similar to conventional grinding operations. Higher cutting speeds lead to lower forces, because the active grain count is constant while the chip-thick- ness increases. The upper cutting speed limit is gi- ven by the temperature in the contact zone and con- cludes in damages of the diamond grit.

Due to the small cutting gap the supply and discharge of coolant and the removal of produced silicon chips is essential for an undisturbed work cycle. The coo- lant supply is achieved by a small coolant nozzle close to the cutting edge before entering the cutting gap. The fluid which is usually pure water, is car- ried forward by the rotating cutting edge into the contact zone (fig. 13).

cool on t and silicon:

h o t on t Fig. 13: Coolant supply

The silicon chips are taken o u t of the gap by the coolant with the aid of flywheel forces. If the interaction between coolant and chip removal is not optimized, a disturbance of the process and wafer vibrations can occur. The faults can be recognized by monitoring the tangential forces and the axial blade deflection (fig. 14)

The left figure shows a static-free cutting-off with an increase of forces till the maximum contact length is reached and subsequently a decrease of force. In the right figure vibrations were noticed. They lead to a rejected wafer with severe damages. The vibra- tions can additionally be observed further down in fig. 14 in the plot of the axial blade deflection near the cut outlet. The reason for these vibrations could be drawn back to an insufficient removal of silicon chips out of the gap and an adhesive layer of silicon at the blade surface (fig. 15).

The adhesive silicon layer diminished the gap width too much. This leads to an accumulation of water and chips in the remaining gap. If the pressure between the wafer and the blade and thus the tangential for- ces exceed a certain value, the chips are able to get out of the gap. This reduces the forces momentary, but immediately this cycle starts again. An important result from this investigation was to keep the blade clean from silicon chips. This could be achieved by

- 0 %

50 mm 100 0 r a d i a l f e e d f

Fig 14: Tangential forces and blade deflection a) static-free b) with wafer vibrations

Fig. 15: Coolant and silicon chips in the gap adding a cleaning agent to the water coolant which dissolves the chips from the blade. With such proce- dure, vibrations of the wafers could be avoided.

5 . Cotrclusions

In order to make the ID cut-off grinding process more predictable and to improve the workpiece quality, the blade stresses, the forces and the axial blade de- flections are important quantities which have to be measured and controlled. For the experimental deter- mination of blade stresses a new electromagnetic de- termination can be applied. The monitoring of pro- cess forces gives valuable informations about the admissible machining parameters and the occurence of vibrations. These vibrations can be avoided by clea- ning the blade core from chips and by assuring a sufficient cutting gap width. The use of the ID cut- off grinding for other brittle materials, like cera- mics, seems t.o be promising.

References

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/2/ Buttner. A., 1985, ID sawing-diameters increase,

/3/ Brandt, G., 1985, Slizing and grinding semicon- ductor materials, Industrial Diamond Review,

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45.2: 86-90

/4/ Meis. F.U.. Struth. W.. 1985, InnenlochsXgen, Geometrie und Kinematik des Werkzeugeingriffs, Industrie-Anzeiger 107,98: 21-24

/ 5 / Brinksmeier, E., Schneider, E., Theiner, W.A., Tonshoff, H.K., 1984, Nondestructive testing for evaluating surface integrity, Ann. of CIRP, 33,2:

/ 6 / Brinksmeier, E., 1987, Trennschleifen sprodharter Werkstoffe. Technische Mitteilungen, Haus der Technik, 80.1: 11-16

489-509

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