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CIRP Annals - Manufacturing Technology 61 (2012) 303–306

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Dual mode control of the rotational grinding process

E. Ahearne (2)*, D. Logan, G. Byrne (1)

The Advanced Manufacturing Science (AMS) Research Centre, School of Mechanical and Materials Engineering, University College Dublin, Dublin, Ireland

1. Introduction

The rotational grinding process is widely used in the productionof silicon substrates for semiconductor integrated circuits. Thefundamental configuration is regarded as optimum for producinglarge diameter substrates that meet industry specifications fortotal thickness variation (TTV) and site flatness (SFQR) [1,2]. Theprocess is also used for ‘‘backgrinding’’ after microfabricationwhere the substrate thickness is reduced to a practicable minimumto conform to critical end-use design requirements [3,4]. In both‘‘back-end’’ and ‘‘front end’’ variants of the process, minimisationof surface roughness and subsurface damage is always a criticalobjective in order to maximise the structural integrity of thesubstrate and minimise the significant costs of subsequentprocessing [1,5].

The process engenders by definition [1] a very specific machineand tool configuration, shown schematically in Fig. 1. It comprisesnominally parallel but offset axes of rotation of tool and work withunilateral engagement of the work surface as shown. In the mostcommon mode of operation, the infeed (f) and infeed rate (df/dt)are controlled to remove a preset depth of cut and impart aspecified surface finish. The simple geometry and kinematicsensure that flat surfaces are generated independently of the toolsurface profile depending only on the alignment of the axes ofrotation and assuming infinite machine rigidity [6].

However, the simple configuration is characterised by inher-ently varying local kinematics as clearly the workspeed (vw) variesas a function of radial distance, r, from the centre of rotation of thework. The effect of this has been modelled and simulated to predictsurface roughness and chip-scale parameters [7–9] while, experi-

of non-infinite stiffness where elastic deflections are present atend of the machine cycle [9,12].

The effects of the varying local kinematics may be substantiattributable to the normal force variation noting that, per H[13], the ‘‘true input to the grinding process is the normal forHahn also added that the normal force is ‘‘uncontrolledconventional grinding machines’’. Of course, research into

fundamental mechanisms in grinding semiconductor and otbrittle materials has shown that the normal force affects, not omaterial removal rates, surface roughness and subsurface deptdamage, but also the critical transitions from ductile to brmechanisms and between brittle crack modes [14–16].

These considerations have provided the motivation for

present work which describes an approach to controlling the lnormal force in rotational grinding in particular with a viewassessing the potential to improve surface finish and integritythe approach is based on control of such a fundamental parameit may be regarded as an alternative development strategyprecise and predictable ‘‘position control’’ and applicable to loprecision class machines.

2. System description

The fundamental objective of the system design now descriis the realisation of a dual mode of operation in rotational grinda normal ‘‘infeed mode’’ and a ‘‘local normal force control’’ moThe objective and primary functional requirement in the secmode of operation is repeatable high resolution, and hfrequency response, control of the normal force temporally

spatially (as a function of radial distance from the centre of rota

A R T I C L E I N F O

Keywords:

Grinding

Control

Silicon

A B S T R A C T

The rotational grinding process enables the production of substrates to meet the submicron plana

specifications required for micro-fabrication of semiconductor integrated circuits. Improvement

process capability, with respect to both form and finish, have been generally realised by the developm

of machine tools and systems based on a principle of precise and predictable ‘‘position’’ control

alternative principle for optimisation is demonstrated here comprising a dual mode control sys

where a ‘‘finishing mode’’ is based on local normal force control. Test results show significant rela

improvements in levels of surface roughness and a reduction in the normal spatial variation.

� 2012 C

Contents lists available at SciVerse ScienceDirect

CIRP Annals - Manufacturing Technology

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forset-ion

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mentally, local normal forces have been measured by integrating aminiature piezoelectric force in the force flux of a single grindingsegment [9,10]. Varying levels of surface roughness and subsurfacedepth of damage have also been confirmed experimentally [9,11]while variations in TTV and planarity are also possible on machines

outivesThe* Corresponding author.

0007-8506/$ – see front matter � 2012 CIRP.

http://dx.doi.org/10.1016/j.cirp.2012.03.095

of the work). For ‘‘proof-of-concept’’, it was designed

integration and testing on the bespoke rotational grinding

up shown in Fig. 2 realised by modification of an ultraprecisturning centre (Hembrug).

The end-of-spindle assembly (1) is mounted on a Gprecision grinding spindle (2), with an axial and radial run-<1 mm. A 4 kW AC induction motor under inverter control drthe spindle (3) and provides a speed range of 1200–5000 rpm.

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E. Ahearne et al. / CIRP Annals - Manufacturing Technology 61 (2012) 303–306304

dle and motor are rigidly mounted to a base providingpendent adjustment of axis alignment (a and b) (4). A 200 mmeter porous ceramic vacuum chuck (5) provides rigid fixing of

silicon substrates and a work speed range of 1–500 rpm.ant is supplied with a variable flow rate to the inside andide of the tool (6). Connection to the force sensor and actuatorscilitated through a multi-conductor slip ring unit (7). Thebrug turning centre provides high resolution CNC infeed and

ed rate control over a wide range.n terms of realisation of the design objectives and specifictional requirements, the identification of a basic technology to

ise dynamic closed loop force control was critical. The adoptedtion is shown in Fig. 3 and comprises a preloaded piezo-ator driving a flexure plate with affixed abrasive segment indegree of freedom. In order to establish ‘‘proof of principle’’,uate the core micro-actuator technology, and enable modularlopment, the configuration shown in Fig. 4 was realised. Itprises two micro-actuator drive units; a ‘‘lead actuator’’ per3 and, mounted diametrically opposite, a ‘‘lag’’ actuator, whichherwise similar but for the force sensor. The other segmentsn are fixed and, when the two micro-actuator segments arected, the tool is driven in the normal single ‘‘position control’’e involving infeed under the machine CNC control followed byeset ‘‘sparkout’’ time.

dual mode (DM) machine cycle is shown in Fig. 5. The ‘‘forcerol or finishing mode’’ is implemented after a ‘‘normal’’ single

mode sequence under machine CNC control or, more specifically,after ‘‘sparkout’’ and retraction of the tool by some microns. Themicro-actuator driven segments are then extended and controlledas a function of tool angular rotation, determined by timing withreference to a proximity sensor, to generate the programmednormal force; providing also for the lag between the two engagingsegments. In view of the reduced number of engaging segments inthis mode, the speed of rotation of the work is reduced pro rata torealise geometrically similar ‘‘swept areas’’ by the segments. Thecontrol software for the system was based on an empirical modelrelating the required normal force to the piezo-actuator controllerinput voltage but depending on the tool speed, the work speed andthe monitored radial distance from the chuck centre of rotation.Due to constraints on controller processing speed, a low tool speed(1250 rpm) was found to be necessary to ensure repeatable forcecontrol.

3. Experimental programme

3.1. Process capability/single mode cycle

The baseline for assessment of the dual mode system is theperformance, or process capability, of the standard single mode ofoperation. The process conditions that apply are shown in Table 1based mainly on preliminary optimisation and the tool manu-facturers recommendations for backgrinding silicon. The testsinvolved monitoring force profiles at intervals during infeed andmeasuring surface roughness (Ra) on samples produced underthese conditions. The reproducibility of these results was alsoassessed to determine in particular if the tool ‘‘sharpness’’ wasreasonably constant.

3.2. System performance/capability

The objective here is to assess the DM ‘‘system capability’’ bydetermining if the normal force can be controlled, as a function ofthe radial position, to programmed levels, with a level of variationthat is significantly less than the range of variation in a ‘‘standard’’(SM) machine cycle. For the purpose of this research the objective

Fig. 1. The rotational grinding configuration.

Fig. 2. Rotational grinding set-up/machine platform.

Fig. 4. Dual-mode control ‘‘intelligent tool’’.

Fig. 3. Dual mode control module: lead actuator configuration. Fig. 5. Dual mode control machine cycle.

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E. Ahearne et al. / CIRP Annals - Manufacturing Technology 61 (2012) 303–306 305

was to show that a constant local normal force could be maintainedduring the finishing cycle. In this case, force levels of 1 and 4 Nwere to be maintained.

3.3. Dual mode comparative tests

A series of randomised comparative tests were implementedalternating between two dual modes of operation; DM, the dualmode shown in Fig. 5 involving ‘‘constant force’’ finishing at 1 N fora fixed time interval (shown to remove >10 mm depth of cut) and,DF, a dual mode cycle but a standard single mode cycle is followedby an equivalent constant low infeed rate for an equivalent interval(an infeed rate of 0.16 mm s�1 with Nw at 100 rpm to realise asimilar depth of cut per rotation, ae, for the same number ofrotations).

As indicated, the immediate interest is in the potential forimproved control of surface finish. The surface roughness wasmeasured on all samples after an ultrasonic cleaning procedure.Both stylus (Taylor Hobson Talysurf 2) and white light inter-ferometry (Veeco) instruments were used to measure surfaceroughness at 20 mm intervals from the substrate centre and in afixed crystallographic direction (relative to the notch). Three stylusmeasurements were taken in a ‘‘tangential’’ direction.

4. Results

4.1. Process capability/single mode cycle

Fig. 6 shows three normal force (FN) profiles sampled at about10, 20 and 30 mm infeed under ‘‘standard’’ grinding conditions asdefined in Table 1 and inset in the figure. There is no significantdifference between profile samplings, noting also that the levelsshown were arrived at after 3–4 mm infeed and reduced to near

zero by 4–5 s into sparkout. Also, the profiles shown were founbe repeatable and reproducible thus exhibiting constant ‘‘shaness’’ and supporting the manufacturers claim that the tool isdressing for grinding silicon.

Fig. 7 (SM) shows average surface roughness levels (roughnaverage, Ra) at the indicated radial distances based on 5 samproduced under the same ‘‘standard’’ conditions in repeat machcycles (blue line). These results are a baseline for subsequent te

4.2. System performance/capability

The system performance is indicated by conformance to

primary functional requirement. This is demonstrated in Fig. 8normal force measurements as a function of radial distance, r.

average and standard deviation shown is based on 10 profisampled at equal intervals in the ‘‘finishing’’ mode with 1 andpresets.

4.3. Dual mode comparative tests

Fig. 9 shows the surface roughness results from the randomtests to compare the two modes of operation being: DM,

‘‘constant force’’ finishing mode at 1 N and, DF, the low consinfeed rate finishing mode. It is shown that there is a significdifference between the two sets of results with levels of surroughness produced in the DM mode reduced and less absovariation with radial distance. These results should be conside

Fig. 7. Comparison of surface roughness profiles produced under standard (s

mode, SM) conditions, low constant infeed (DF) and constant force (DM) condi

vs. depth of cut.

Table 1Optimised process parameters.

Fig. 6. Normal force profiles during a ‘‘standard’’ machine cycle.

Tool specification Type 2A2, F200 mm, D46, C75,

proprietary metal bond,

22 segments, 2 mm � 8 mm

Work material F200 mm, h1 0 0i orientation,

Epi grade silicon, ground finish

(<1 mm Ra)

Infeed, f 30 mm

Infeed rate, df/dt 1 mm s�1

Sparkout time 30 s

Work speed, Nw 100 rpm

Tool speed, Nc 1250 rpm

Coolant Process water, 4 L min�1/nozzleFig. 8. Control of normal force during finishing in a dual mode cycle.

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E. Ahearne et al. / CIRP Annals - Manufacturing Technology 61 (2012) 303–306306

e context of the actual depth of cut during the finishing phasech for the DM mode ranged from 10 to 12 mm and, for the DFe, ranged from 5 to 11 mm.

n order to investigate further, the change in Ra during a DMhing cycle was found as a function of the (measured) depth ofThe results are shown in Fig. 7.

iscussion

he improvement in surface roughness levels shown in Fig. 9 supported by visual comparison of areal surface micrographs.as concluded from these observations that the level of brittleture on samples produced under constant (1 N) force isificantly reduced implying also an improvement in surfacegrity. The underlying mechanisms that result in consistent andificantly higher levels of surface roughness under infeedrol, may be elucidated by observations on the normal force

ation during infeed. It was observed that the normal forceeased up to a level of about 2 N and then reduced rapidly. It isulated that this is at a level where brittle fracture is moregetically favourable.

6. Conclusion

A novel dual mode control system for rotational grinding hasbeen described. It enables operation on the basis of (normal) infeedcontrol and or on the basis of local normal force control. It has beenshown that levels of surface roughness can be significantlyimproved by following a normal machine cycle with a low normalforce finishing mode.

Acknowledgements

We would like to thank our sponsors, Enterprise Ireland, andcollaborators, Kistler GmbH and Atlantic Diamond, for theirsupport.

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. Comparison of surface roughness profiles on samples produced by constant

(DM) and low constant infeed (DF) finishing.