42

S Y S T E M I N T E G R AT I O N

IntroductionAdhesive wafer bonding is an impor-tant packaging process in severalfields, for example • Micro Optics• Traditional IC Packaging• Temporary wafer bonding – thin

wafer handling / layer transfer• Lab on Chip• Microfluidics

However, in order to address theseapplications there are some specificrequirements and several technicalchallenges that need to be over-come.

Requirements of Adhesive WaferBonding:• Bonding environment – vacuum /

air / inert gas• Layer control – bondline thickness

/ width; total thickness variation(TTV)

• Accurate Alignment of the wafers(<3μm)

• Fast, efficient curing of the adhe-sive

• Adhesive bonding may be a simpleprocess, but high performance re-quires high specification tooling.

Adhesive Wafer Bonding Challenges:• Wafer size 200mm now, 300mm in

the future• Handling of non-standard wafer

thickness (e.g. multi wafer assem-blies may be several mm thick)

• Adhesives act as a lubricant! = dif-ficult to align wafer to requiredaccuracy and apply a uniformclamping force (required for TTV /layer thickness control) withoutloss of alignment accuracy

• Cure challenge:- it is best to cure“in-situ” to guarantee alignmentaccuracy is maintained.

Alignment IssuesHigh accuracy alignment can beproblematic when using many adhe-sives. The problems mostly stem fromthe adhesive acting as a lubricant be-tween the wafers, and also from theadhesive actively pulling the wafersout of alignment due to capillaryforces.

In many wafer bonding systems theparts are aligned with high accuracyin a mask aligner, and then trans-ferred to a wafer bonding tool. Inthe wafer bonding tool the wafersare brought into contact and aclamping force is applied, followedby a thermal cycle. Unfortunately, itis often reported that the high accu-racy alignment is lost somewhere be-tween these two steps, and the oper-ator is not aware of the problem un-til the wafer bond is complete.The AML system (Figure 1) aligns thewafers and performs the bondingstep in the same chamber, meaningthe wafer alignment, contact, andcuring steps can be observed “in-situ” throughout the process. Thesein-situ tools include wafer edgeclamping, and wafer contact controlmechanisms which prevent move-ment on wafer contact, and stopwafer movement during the bondprocess (this is especially importantfor thermal cure epoxies where theviscosity may reduce dramaticallyduring the “heat up” phase of thecure cycle). Additionally, the align-ment accuracy can be monitored,and in the event of a problem aris-ing, the process can be halted and itmay be possible to reclaim valuablewafers before the adhesive has beencured.

The In-situ nature of the process alsoallows direct observation of thebonding layer, such that the processcan be developed using direct anddynamic feedback – e.g. this allowsthe bonding force to be controlledto give a precise bond line widthwithout requiring a full parametricstudy.

Application and Metrology of Adhesive layersThere are several ways of applyingthe adhesives to the wafer surface.For many of the results reportedhere, the layers were applied at theAML Bondcentre using either screenprinting (Reprint Services Semi-auto-matic Model R29-V) or roller coating(heavy metal roller on flat plate).Other results were obtained usingcustomer, real application wafers.The layer thickness and uniformitywere measured using a combinationof Tencor P15 surface profiler, and aSigmatech ST9600 TTV measurementtool. Alignment accuracy was mea-sured using standard optical mi-croscopy.

Thermal Cure adhesive BondingThe system set-up for thermal curingis shown schematically in Figure 2. IfUV curing is required, then a UV LEDarray replaces one of the platenheaters – allowing both UV and ther-mal cure without any toolingchange.

This set-up includes the followingkey features:

Large Gap Between Wafers – This isparticularly beneficial for ensuringthorough out-gassing of the wafersurfaces prior to bonding and there-by preventing air bubbles forming inthe adhesive layer. It also enables afaster pump down to the requiredbonding pressure – increasingthroughput.

Wafer Edge Clamp – This clamp,which is compatible with a standardSEMI “C” edge wafer, can reliablyhold a wafer upside down in the pro-cess chamber without touching the

1μm Alignment Accuracy Adhesive Wafer Bonding –Thermal and UV cureNick Aitken and Tony Rogers

Figure 1: AML wafer bonding platform – align-ment and bonding in same chamber

N. Aitken

T. Rogers

mstnews409:mstnews 04.11.2009 14:54 Seite 42

S Y S T E M I N T E G R AT I O N

43

wafer surface. This is important foradhesive bonding as any mechanismon the wafer surface may becomecontaminated with adhesive, whichcan result in time consuming clean-ing steps between bonds, or tooldowntime.

In-situ X, Y, Ө & Z Manipulation –Once the process temperature and /or the required vacuum level havebeen reached, then the wafers –which can already have been alignedduring the pump down / heatingphase – can be brought into contact.If any alignment shift occurs at thisstage, then the operator has the op-portunity to address this, as it is pos-sible to move the lower wafer bysmall amounts, in X, Y & Ө, aftercontact, but before curing. However,this post-contact alignment compen-sation is not usually needed if thespring-pin set-up (Figure 3) isutilised.

Spring Pin – This ensures that the ini-tial contact is always at the centre ofthe wafer and the bond then propa-gates from the centre outwards. Thiscontrol is achieved by using the pinto impart a controlled bow into theupper wafer. Once the requiredbonding parameters have beenachieved (chamber pressure, upperand lower wafer temperature, align-ment), then the Z-drive is used tobring the wafers into contact andthen apply the required bonding

force (up to 15kN,although most ad-hesive bond pro-cesses require <2KN force). As thebonding force is in-creased from zero,it gradually over-comes the springpin force, andhence removes thebow from the up-per wafer andclamps the wafers

flat – in precise alignment.

In-situ Cooling – Post-bond cooling isachieved by directing jets of nitrogengas, from inwardly facing holes in asupply pipe that surrounds theplatens, onto the platens. Becausethe thermal mass of the in-chambertooling is restricted to just the upperand lower platens and their respec-tive heaters (ie no transfer jigs for in-terfacing with a separate aligner)then the cooling is fast and efficient.A typical thermally cured adhesiveprocess on 200mm wafers will typi-cally take 15 minutes – includingalignment – see Figure 4 belowwhich shows the platen temperatureand chamber pressure over the 15-minute cycle for an Epotek 353NDadhesive bond. For this application itwas necessary to cool the wafers to <40°C before they could be removedfrom the chamber; however in manycases the wafers may be removed atthe peak cure temperature (normally<200°C).

Alignment Results for Thermal Adhesive Curing1 m alignment accuracy wafer bond-ing has been demonstrated on300 μm thick, 200mm diameterwafers in a “worst case scenario”,where thick layer Epotek 353ND ther-mal cure epoxy was the bondingmedium. The alignment accuracy wasverified using a vernier scale that waspatterned onto the two wafers. Theresulting overlay of the two patternsenables direct reading of the align-ment accuracy. An example is shownin Figure 5. This 1 m alignment wasachieved using Epotek 353ND adhe-sive, of 18 μm to 20 μm thickness.

For even faster throughput, the in-situ concept of the aligner-bonderhas been extended to include a UVLED array. This array is located insidethe bond chamber (see Figure 2), andthe UV light is directed to the bondinterface via a transparent quartzlower platen that replaces the metalplaten used for thermal curing . Hav-ing loaded the wafers, pumpeddown the chamber, and aligned the

Figure 2: Schematic of Tooling Solution: Thermal Adhesive Bonding: In-situ High Accuracy Alignment and Bonding

Figure 3: Spring Pin and Edge Clamp ToolingShowing Controlled Bow Imparted to Wafer

Figure 4: Example of Process Parameters for a Thermal Adhesive Bond

Figure 5: Vernier System Showing <1μm Align-ment Measurement for the Bonding of TwoGlass Wafers using 20 Micron Thick AdhesiveUV Cured Adhesive Bonding.

mstnews409:mstnews 04.11.2009 14:54 Seite 43

S Y S T E M I N T E G R AT I O N

44

wafers, they are then brought intocontact, exposed to UV for the re-quired time (a few minutes depend-ing on the UV adhesive type andthickness, for example Permabond675 UV adhesive cure time = 7~10seconds using 395nm centred LEDsource). The chamber is then ventedand the bonded pair removed. Thetotal cycle time can be as short as 5minutes, making it a very attractiveoption for manufacturing. DifferentLED cure heads are available to han-dle different adhesive types. It is alsoworth noting that the vacuum bond-ing environment can be important inUV cure applications – the presenceof oxygen at the UV adhesive surfacecan prevent complete curing of thematerial – and for thin bond layerspresence of oxygen may inhibit thebond altogether.

With the LED source the UV light isconcentrated in a relatively narrowbandwidth as shown in Figure 6. Thepurple curve shows the intensity vs.wavelength distribution for an LEDarray in comparison with that for aconventional UV source – shown inred. The AML machines currentlyhave two options for the LED wave-length. These are:

UV LED (395nm ±20nm or 365nm±10nm). The power levels are 4W cm-2

and 1.5W cm2 respectively. The LEDarrays include water cooling and socan be run for indefinite periods ifrequired.

The following data (Figure 7) showsthe results for a real industrial appli-cation of aligned UV adhesive bond-ing using a 30 μm thick layer. Thealigner bonders were originally de-signed for use with face-to-facealignment marks on the bonding sur-faces of the wafers. However the re-sults shown below were obtained forwafer for which the alignment markswere on the back surface of the

wafers and because the wafers were1mm and 2mm thick, the alignmentmarks were widely separated (3mm)during the alignment process. In suchcircumstances an image capture solu-tion needs to be used. The targetalignment accuracy for the applica-tion (micro-optics, high alignmentaccuracy wafer level packaging) was5 microns. An example of an accu-rately aligned UV adhesive bond isshown in Figure 8. Data taken form areal industrial application using200mm wafers up to 3mm stackheight.

Real Time Alignment MonitoringIn addition to providing highthroughput aligned bonding, theability to observe the bond interfacethroughout the process means thatthe in-situ aligner bonders offer fur-ther benefits. Some examples are asfollows:• Adhesive Flow & Alignment Shift

can be Monitored – See Below• Enables Alignment Correction or

Wafer Recycling• Software Monitors Pre- & Post-

Contact Alignment & AppliesPass/Fail Criteria

The advantages of observing thebond interface through the viewingapertures are demonstrated in the

following example. This is demon-strated below in the images shownin Figure 9. These screen shots showan area of a machined silicon waferbeing adhesively bonded to a glasscover wafer. The silicon machiningincludes overspill channels for theadhesive to run into and this processwas observed in real time in order tooptimize the end point of the bond-ing process. This real time viewingenables rapid process optimizationwithout the need to run a large ma-trix of experiments.

Conclusions• In-situ aligner-bonder system

offers 1μm accuracy for thick (>20 μm) adhesives.

• System proven for 100, 150,200mm wafers of thickness300 μm - 2100μm.

• Handles up to 100μm wafer bow /warp.

• Ideal for micro-optical & high ac-curacy WLP solutions.

• Backside alignment capability forup to 3mm height separation.

• Visible or IR transparent sub-strates.

• Tooling solutions for thermal orUV cure.

• High throughput (5 min cycle timefor aligned UV adhesive bonding).

Contact:Nick Aitken & Tony RogersApplied Microengineering Ltd, Oxfordshire, UK E-Mail: aml@aml.co.uk

Figure 6: Wavelength Distribution for UV LEDArray (purple) in comparison with a Conven-tional UV Source (Red)

Figure 7: Alignment Accuracy Results for an Industrial Application of Aligned Bonding of UV CuredAdhesive for Micro-optics WLP

Figure 8: 30 m thickness UV adhesive alignment

Figure 9: Screen Shots from an Aligner-Bonder Showing the Ability to Monitor Adhesive Flow andCuring in Real Time

mstnews409:mstnews 04.11.2009 14:54 Seite 44