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Precision Engineering 35 (2011) 294–301

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Precision Engineering

journa l homepage: www.e lsev ier .com/ locate /prec is ion

Experimental and theoretical investigation of needle contact behavior of waferlevel probing

Hao-Yuan Changa, Wen-Fung Pana, Shueei-Muh Linb,∗

a Department of Engineering Science, National Cheng Kung University, Tainan 701, Taiwan, ROCb Department of Mechanical Engineering, Kun Shan University, 949 DaWan Road, Tainan 710-03, Taiwan, ROC

a r t i c l e i n f o

Article history:Received 18 February 2009Received in revised form 1 June 2010Accepted 13 November 2010Available online 1 December 2010

Keywords:Wafer testingProbe cardTheoretical simulation

a b s t r a c t

Wafer probing technique is important for testing integrated circuit devices. For the safety of measuredchips, the probe card must be examined to verify the performance before online testing as it must avoidthe defect of the pad edge excursion and the excessive scrub length. In addition, during wafer testingthe oxidized layer of pad has to be pierced to gain the stable testing by the needle, which should havesufficient force. Therefore, a well-designed probing system is significantly important for wafer testing.In this paper, the simple and accurate theoretical method is established, which benefits to design anappropriate probing system rapidly. Moreover, the finite element method is also developed for analyz-ing the contact behavior between the needle and the aluminum pad during wafer testing. The results,which are determined by using the theoretical analysis, the finite element method and the experiment,are consistent. Finally, the parameters of a well-designed probing system for testing smaller chips areinvestigated.

© 2010 Elsevier Inc. All rights reserved.

1. Introduction

The probe card, which provides the interface between the testequipment and IC (integrated circuit) devices, tests the devices andcontacts with the bonding pads of each chip on the wafer as shownin Fig. 1. In the meantime, the signal from the tester system is trans-mitted to the chip. When the chips are tested, a map of good andbad chip locations is created. It is used to determine which chipsare the good ones and these chips should be assembled in pack-age. Indeed, any yield loss of wafer is expensive as the package andassembly processes are expensive. Hence, it is important to packageonly good chips to save the cost of IC manufacturing.

The geometry and movement of a probe influences the qualityof the contact behavior. It also affects the shape of the scrub mark.Moreover, the quality of scrub marks influences the feedback oftesting which provides information to engineer for troubleshootingof electrical problems. Through the analysis of probe scrub marks,the testing engineer can monitor and enhance the testing perfor-mance to gain higher yield of devices. In general, the entire scrubmark must be within the passivation opening of the pad. However,if the probe needle is not aligned properly, the needle could pene-trate the edge of the passivation layer and causes the defect of edgeexcursion as shown in Fig. 2. The pad edge excursion will add extra

∗ Corresponding author.E-mail addresses: smlin45@gmail.com, smlin@mail.ksu.edu.tw (S.-M. Lin).

cost of die loss and excessive scrub length, which may damage thedie pad and influence the quality of wire bonding during packaging.Thus the good quality of the probe mark is significantly concernedduring wafer testing.

Some literatures have been devoted to the needle contactbehavior with pads. Maekawa et al. [1] investigated the contactbehavior due to the adhesive pick-up of aluminum on the probetip. A contact mechanism and an adhesion phenomenon are exam-ined to improve the unstable contact. They found that a stablecontact resistance could be achieved using a probe with a tipradius R = 15 �m, which effectively produces a clean aluminum sur-face. Varnau [2] performed the flip chip reliability of the probebefore-bump process using different overdrives and contact times.Hotchkiss et al. [3] derived the relationship between scrub markdamages ranging of the total pad area and the package reliabil-ity of wire bonding. The experimental data shows that bonding topads with >20% probe damage will produce a higher incidence oflifted balls during production. Tan et al. [4] studied the feasibility ofprobe-before-bump technology. It was found that probe-on-Ni/Auleft almost undetectable probe marks on the UBM and probe-on-aluminum could result in heavy probe mark when a high overdrivewas used.

The overdrive is the distance which the chuck upwards fromthe initial probe–pad contact position to the setting height of thecheck. Indeed, a significant concern about wafer testing is that thesetting of proper probing recipes avoids the issue of pad damageand increases the successful rate of wafer testing to reduce the

0141-6359/$ – see front matter © 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.precisioneng.2010.11.003

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Fig. 1. Probe card configuration.

Fig. 2. The phenomenon of pad edge excursion.

testing time and costs. Nevertheless, up to now, little research hasbeen conducted the theoretical investigation for the wafer prob-ing process. In this paper, the three-dimensional finite-elementmethod, the experiment and the analytical method are establishedand compared. Furthermore, the analytical method is very ben-eficial to investigate the relation between the overdrive and thescrub mark and it is much easier to derive the relations among sev-eral parameters of wafer probing by using the analytical methodthan the finite-element method [5] and match with experimen-tal results. Finally, the parameters of the well-designed probe fortesting smaller pads of chips are derived.

2. Finite element analysis

In this paper, the finite element analysis, which is used toobserve the contact behavior of wafer probing, is compared withthe analytical method. The dimensions of three commercial probesare chosen as shown in Table 1 for investigating the probing behav-ior. In the meanwhile, the geometric definitions of the probe areshown in Fig. 3.

This paper refers to a micro-force tensile test which is performedto calculate the stress–strain curve for the tungsten needles usedin the probe card [5,6] and material properties are listed in Table 2.

Table 1Dimensions of probing needles.

Probe Probe diameterDp (�m)

Beam length LB

(�m)Knee diameterDk (�m)

Tip diameterDT (�m)

Shooting angle�1 (◦)

Bending angle� (◦)

Approachingangle �3 (◦)

Tip length LT

(�m)Taper lengthLTP (�m)

1 100 1778 45 18 6 102 6 254 12702 125 2032 50 18 6 102 6 254 14483 150 2286 53 20 6 102 6 254 1651

Advanced Semiconductor Engineering Inc. (ASE Inc.).

Fig. 3. Geometry and coordinate of probing needle.

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Table 2Material properties.

Part Density(g/cm3)

Elasticmodulus (GPa)

Poisson’sration

Yield stress(MPa)

Tungsten needle 19.32 290.85 0.28 1770.24Aluminum pad 8.96 70 0.22 124Silicon substrate 2.33 131 0.23

Fig. 4. Finite element model for probing needle and pad.

Read and Dally [7] found that with respect to the aluminum pad, thebulk material properties of aluminum might be unsuitable for thestudy as the manufacturing process of aluminum pad is different tothe conventional one and the yield strength of aluminum film wasfound to be nearly 124 MPa. Table 2 lists all the material propertiesof the tungsten needle, the aluminum pad and the silicon substrate[6].

Table 3Comparison of theoretical and simulated scrub length with the experimental onesfor different probes.

ıv (�m) Dp (�m) ıH (�m) Discrepancy (%)FEM/theory

Experimental FEM Theory

30 100 14.4 15.0 14.7 4.2/2.1125 15.6 16.5 16.6 5.5/6.4150 17.5 16.9 17.6 3.7/0.1

50 100 21.5 22.4 20.2 4.2/0.1125 22.8 24.6 24.5 7.9/7.5150 23.6 25.3 25.2 7.2/6.8

The probing test is analyzed by using ANSYS finite element codehere [8] and the finite element model simulates the movement ofthe needle along the specific trajectory. In order to ensure that theneedle moves on the X–Y plane, a bit of the vertical displacementon the X–Z plane is shown in Fig. 4. Thus boundary conditions allof degrees of freedom on the base of the needle are constrainedand the aluminum pad moves up to contact with the needle. Fig. 5shows profiles of scrub marks by using three kinds of needleswith different probe diameters {100 �m, 125 �m, and 150 �m}at the overdrive of 30 �m. In addition, Table 3 shows the resultsthrough the finite element method, the experiment and the ana-lytical method. The results further point out that the discrepancybetween numerical and experimental results of the scrub marklength ıH is less than approximately 8%. Hence, the reliability ofthe finite element model is validated.

Through the finite element method, the von-Mises stress distri-bution on the probe during the wafer probing is shown in Fig. 6. Itis observed that the maximum effective stress is located in the ele-ments which close to the angle of the probe. Fig. 7 shows relationsbetween the maximum von-Mises stress �vm,max and the overdriveıv of the three probes listed in Table 1. It is important to note that the

Fig. 5. The scrub mark profiles by using the finite element method with the overdrive 30 �m and probe diameters Dp: (a) 100 �m, (b) 125 �m, and (c) 150 �m. [Thecorresponding parameters are listed in Table 1.]

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Fig. 6. The von-Mises stress distribution on the probing needle with the probediameter Dp = 125 �m.

Fig. 7. The relation between the maximum von-Mises stress �vm,max and the over-drive ıv.

Fig. 9. The relation between the tip horizontal shift ıTH and the scrub mark lengthıH.

maximum von-Mises stress �vm increases with the overdrive ıv andthe probe diameter Dp. However, even if the overdrive is increasedto 60 �m, the maximum von-Mises stress is about 660 MPa whichis less than the yield strength (1899 MPa [6]) of the tungsten probe.Therefore, no permanent deformation occurs to the needle.

Fig. 8. Experimental scrub mark profiles of the pad (58 �m × 80 �m) with the probe diameter Dp = 125 �m and three overdrives ıv: (a) 20 �m (b) 30 �m, (c) 50 �m and (d)the measurement method of scrub mark length ıH.

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Fig. 10. The relation between the vertical contact force Fv and the overdrive ıv with different probe diameters Dp: (a) 100 �m, (b) 125 �m, and (c) 150 �m.

3. Experimental analysis

The TEL prober (Tokyo Electron P12XLn) is used for wafers prob-ing and testing [9]. Fig. 8 shows profiles of the scrub mark by usingDp of 125 �m listed in Table 1, at overdrive of 20 �m, 30 �m and50 �m respectively and the measurement method of the scrubmark length ıH. In addition, the dimension of the pad in this exper-iment is 58 �m × 80 �m. During wafer testing, the chuck movesupwards to ensure that the pads on the wafer are in contact withthe needles and the needles scrub over pads until the setting heightof the chuck. It means that the definition of overdrive ıv is the ver-tical distance from the start to the end of the chuck. In addition, italso should be noted that the horizontal sliding of a probe tip on thesurface of the metal pads is very important for wafer testing as thesurface of each metal pad is covered by a very thin layer of an oxidematerial which could affect the electrical performance of devices.In order to contact with the aluminum pad to gain the stable test-ing, the thin oxide must be pierced. When the horizontal sliding of

the probe tip occurs, there exists the friction between the tip andthe surface of the metal pad and friction factor � = 0.3 is consideredin the paper. Therefore, if the friction is enough, the oxide layer canbe pierced to enhance the stability of the testing.

4. Theoretical analysis

4.1. Theory formulation

Although doing the wafer probing experiments is direct and reli-able, it is expensive and laborious. Thus the numerical method isdeveloped to predict the scrub mark, for example, the finite ele-ment method is usually adopted. On the other hand, the simpleand accurate analytical method, which is determined the relationsamong the overdrive, the scrub remark length, the vertical contactforce and the horizontal friction force, is significantly beneficial forthe design of a proper needle for wafer testing.

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In this section, the Castigliano’s 2nd theorem is used to derivethe analytical solution of the mechanism for wafer probing and theelementary beam theory, commonly known as the Euler–Bernoullibeam theory, is considered. The theory stipulates that the rotatoryinertia and shear deformation effects are negligible and this theoryis valid if the slender ratio of the length of the beam to its thick-ness is relatively large, approximately more than 10, and if the ratioof the deformation on the length of the beam is relatively small,approximately less than 0.1. In short, large slender ratio and smalldeformation are the assumptions of the Euler–Bernoulli theory. Aslisted in Table 1, the slender ratios of the three probes are about18. Based on the Castigliano’s 2nd theorem, the tip vertical andhorizontal displacements are expressed as:

ıV = ∂U

∂Fv= ∂

∂Fv

∫l

(M2

2EI

)ds =

∫l

M(s)EI

∂M(s)∂Fv

ds (1)

ıTH = ∂U

∂Fh= ∂

∂Fh

∫l

(M2

2EI

)ds =

∫l

M(s)EI

∂M(s)∂Fh

ds (2)

where U is the strain energy, M is the moment of beam, E is theYoung’s modulus, I is the area moment of inertia and s is the coor-dinate along the beam. Fh and Fv are the horizontal friction andvertical contact forces, respectively. In addition, the horizontal fric-tion force Fh = ıFv where ı is the friction factor at the contact surface.ıH and ıv are the tip horizontal and vertical displacements, respec-tively. Moreover, the vertical displacement ıv is conventionallycalled as the overdrive. The tip vertical displacement ıTH is the dis-placement of the tip geometry center and L is the overall lengthof the needle. In general, the geometry of a probing needle is non-uniform, as shown in Fig. 3. The bending moment M and the areamoment of inertia I are expressed as:

M(s) =

⎧⎪⎨⎪⎩FvX1(s) − FhY1(s), 0 < s < L1FvX2(s) − FhY2(s), L1 < s < L1 + L2FvX3(s) − FhY3(s), L1 + L2 < s < L1 + L2 + L3FvX4(s) − FhY4(s), L1 + L2 + L3 < s < L1 + L2 + L3 + L4

(3)

I(s) =

⎧⎪⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎪⎩

�r41

4, 0 < s < L1

�

4

[r1 − (s2 − L1)(r1 − r2)

L2

]4

, L1 < s < L1 + L2

�

4

[r2 − (s3 − (L1 + L2))(r2 − r3)

L3

]4

, L1 + L2 < s < L1 + L2 + L3

�

4

[r3 − (s4 − (L1 + L2 + L3))(r3 − r4)

L4

]4

, L1 + L2 + L3

< s < L1 + L2 + L3 + L4

(4)

where

X1 = Xd − s1 cos �1, Y1 = Yd − s1 cos �1X2 = Xd − Xa − (s2 − L1) cos �1, Y2 = Yd − Ya − (s2 − L1) sin �1X3 = Xd − X0 − R cos [�/2 − �1 − (s3 − L1 − L2)/R]Y3 = Yd − Y0 + R sin [�/2 − �1 − (s3 − L1 − L2)/R]X4 = [(L4 − (s4 − L1 − L2 − L3)] cos �3Y4 = [(L4 − (s4 − L1 − L2 − L3)] sin �3

(5)

Substituting Eqs. (3)–(5) into Eqs. (1) and (2), the tip horizontal andvertical displacements {ıTH, ıv} are easily derived.

4.2. Numerical results and discussion

The real contact area of a probe needle is significantly smallerthan the observed scrub mark area [8]. Based on the results byusing the finite element method, the scrub mark length ıH is almostlarger than the tip horizontal displacement ıTH about 25% of the tipdiameter DT, as shown in Fig. 9. Furthermore, Table 3 shows the

Fig. 11. Theoretical relations between the vertical contact force Fv and the overdriveıv with different probe diameters Dp.

comparison of the experimental, numerical and theoretical scrubmark lengths ıH and it is found that the theoretical and experi-mental results are consistent. Besides, Fig. 10 shows the relationbetween the overdrive ıv and the vertical contact force Fv. It hasfound that vertical contact force Fv increases with the overdriveıv. In addition, Fig. 11 shows the theoretical relations between thevertical contact force Fv and the overdrive ıv with different probediameters Dp. In addition, Fig. 12 shows the relation between thevertical contact force Fv and the scrub mark length ıH. It has gainedthat the scrub mark length ıH increases with the vertical contactforce Fv. Finally, the relation between the scrub mark length ıH andthe overdrive ıv is shown in Fig. 13. It is shown that the theoreticaland experimental results are very consistent as well.

Fig. 12. The relation between the scrub mark length ıH and the vertical contact forceFv for the case with the probe diameters Dp = 150 �m and the friction factor � = 0.3.

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Fig. 13. Comparison of the relation between the scrub mark length ıH and the over-drive ıv obtained through the theory and the experiment with the probe diameterDp = 100 �m.

Protecting against the pad edge excursion and the excessivescrub length, the residual distance (RD) between the scrub lengthand the edge of the bond pad opening is usually at least 12 �min four sides and one should consider another practical factorthat the error of needle alignment is about ±7 �m. Generally,the dimension of an integrated circuit and the pad size has beenbecoming smaller to cause the water testing which becomes moredifficult as the probe mark is hard controlled within the bondpad opening. For example, the pad size is 40 �m × 40 �m and thecorresponding safety contact region is 16 �m × 16 �m. In otherwords, it means that the allowed scrub mark length must be lessthan 16 �m. The shooting angle �1, which plays an importantrole to influence the scrub length ıH, is investigated and shownin Fig. 14. It is found that increasing the shooting angle �1 candecrease the scrub mark length ıH.

Fig. 14. The influence of the shooting angle �1 on the relation between the scrubmark length ıH and the overdrive ıv with the probe diameter Dp = 100 �m.

Fig. 15. The influence of the beam length LB on the relation between the scrub marklength ıH and the overdrive ıv with the probe diameter Dp = 100 �m.

Protecting from the damage and deleting the oxidized layer ofthe aluminum pad, the gradient of the vertical contact force dFv/dıv

is generally in the domain [0.7, 1.1 gw/mil]. This criterion assuresthat there is enough friction to pierce the oxidized layer of the pad.Moreover, the maximum overdrive ıv is usually less than 75 �m[10,11] as too much overdrive ıv could cause the longer scrubmark and more particles to accumulate at the tip of the needleto increase the contact resistance. In addition, it could wear needlelength rapidly. Fig. 15 shows the influence of the beam length LB onthe relations between the scrub mark length ıH and the overdrive ıv

with the probe diameter Dp = 100 �m. Furthermore, Fig. 16 showsthe influence of the beam diameter Dp on the relation between thevertical contact force Fv and the overdrive ıv. It is found that twoprobes with Dp of 80 �m and 75 �m meet this criterion. Therefore,

Fig. 16. The influence of the beam diameter Dp on the relation between the ver-tical contact force Fv and the overdrive ıv [in spite of the probe diameter, all theparameters of the probe with the probe diameter Dp = 100 �m, listed in Table 1, areconsidered].

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the smaller the probe diameter Dp is, the much smaller the gradientof the vertical contact force dFv/dıv will be.

5. Conclusion

In this paper, the simple and accurate theoretical analysis forinvestigating the behavior of wafer probing, which are established,compare with the finite element method and the experimentalmethod. Furthermore, the results show that the relations of wafertesting among the vertical contact forces, the horizontal frictionforces, the overdrive and the scrub mark length can be easily andaccurately derived from the theoretical analysis. It has been gen-erally accepted that the pad size of the integrated circuit has beenbecoming smaller and it also means that challenges of wafer test-ing will become tough gradually as more difficult to control theprobe mark. This paper has presented that the theoretical analysisis an effective method which benefits to easily predict the contactbehavior of the probing needle and avoid occurring the defect of paddamage for wafer testing. Finally, based on the experimental andsimulating results, the following important conclusions are drawn:

(1) At the same overdrive ıv, the scrub mark length ıH and themaximum von-Mises stress increase with the probe diameterDp.

(2) The more the overdrive ıv is, the larger the scrub mark lengthıH and the maximum von-Mises stress will be.

(3) Increasing the shooting angle �1 decreases the scrub marklength ıH.

(4) At the same length of beam, the smaller the probe diameter Dp

is, the much smaller the gradient of the vertical contact forcedFv/dıv will be.

(5) Protecting from the damage of the measured pads(40 �m × 40 �m) considers the vertical contact force topierce the oxidized layer of pad. Thus two probes with Dp of80 �m and 75 �m are recommended.

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[2] Varnau MJ. Impact of wafer probe damage on flip chip yields and reliability. In:Proceedings of IEEE/CPMT-IEMT. 1996. p. 293–7.

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