Microelectronic Engineering 88 (2011) 2056–2058

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Mechanistic study of lift-off for continuous metal layers after T-NIL

Andre Mayer ⇑, Saskia Möllenbeck, Khalid Dhima, Si Wang, Hella-Christin ScheerMicrostructure Engineering, Faculty of Electrical, Information and Media Engineering, University of Wuppertal, Wuppertal D-42119, Germany

a r t i c l e i n f o

Article history:Available online 16 February 2011

Keywords:Thermal nanoimprint lithographyLift-offSputtering

0167-9317/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.mee.2011.02.020

⇑ Corresponding author.E-mail address: amayer@uni-wuppertal.de (A. Ma

a b s t r a c t

Thermal nanoimprint and sputtering, two low-cost techniques, are combined to define metallic patternsby means of lift-off. The typically positive sidewall angle of imprinted structures in combination withsputtering for layer deposition leads to a continuous metal layer which has to be ruptured at a definiteposition, at the best along the respective pattern base, so that lift-off can work. This issue is investigatedtaking polystyrene (PS) as the imprint polymer and toluene as the lift-off solvent. The impact of thegeometry factors and the impact of soaking and the power of ultrasonic agitation are addressed bothexperimentally and analytically to derive the conditions for the successful lift-off of 250 nm wide linesover large areas.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Lift-off is a simple and common technique to prepare structuredmetal layers. In general, undercut or recessed resist structures witha non-continuous metal layer on top are used for the lift-off. Anundercut structure can be achieved with a 2-layer-system wherethe bottom layer, the lift-off layer, can easily be undercut by a sol-vent; alternatively, the recess required may be achieved by anoverexposure of the resin during optical lithography or, in case ofelectron beam lithography, by the naturally divergent shape ofthe dissipation volume.

We are using thermal nanoimprint as lithography technique(T-NIL). Typically, imprinted structures are characterized bypositive sidewall angles. Under such conditions lift-off asks for a2-layer-process [1] – imprint of the top layer and undercutting ofthe bottom layer, e.g. a typical lift-off resist, by a solvent. Our pur-pose is to define a single layer lift-off process with T-NIL. The basisis an imprint in a thin initial layer under partial cavity filling con-ditions [2,3] resulting in imprinted structures with ultra thin oreven negligible residual layers. The offered amount of polymerfor the imprint (as provided by the initial spin coated layer) is solow that the cavity filling remains incomplete and the elevatedstamp structures are imprinted through. We found that with suchsamples a lift-off can be performed without previous residual layerremoval [4]. Additionally, the intent is to exchange the conven-tional evaporation process used for lift-off by sputter coating. Sput-tering, due to the higher working pressure and the simplerpumping system required, is more cost-efficient than evaporation,in particular for large area deposition. Together with imprint it

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yer).

provides a promising, low-cost, large area preparation scheme,e.g. for the definition of sensors with only one metal layer, withouttopography.

Lift-off by imprint and subsequent sputtering, in addition to theadverse slope angle of the polymer structures, is challenging, assputtering provides an improved conformity of layer depositioncompared to evaporation. Therefore the metal layers on top ofthe imprinted structures can expected to be continuous. Earlier re-sults have shown that even thick layers – up to 200 nm – of Cr orCr/Au [5] are still porous enough to allow solvent to penetrate [6]and to swell the underlying polymer. Successful lift-off then re-quires a well directed ripping of the continuous layer, preferablyalong the pattern base.

This contribution addresses the lifting of patterns defined by T-NIL and covered by continuous metal layers deposited by sputter-ing, without previous residual layer removal. The mechanics ofthe lifting process is investigated experimentally and analytically,and the impact of geometrical factors and lifting parameters is dis-cussed. In view of the preparation of a typical sensor the focus is seton the successful lift-off for large fields of lines of about 250 nm inwidth.

2. Experimental

For our experiments polystyrene (350 kg/mol, Sigma–Aldrich)was deposited by spin coating on Si substrates. The layer thicknessranged from 190 to 240 nm. For the imprint a silicon stamp(2 cm � 2 cm) with an anti-sticking treatment was used. The stampis fully patterned with test structures; for lift-off we concentratedon a field (6 mm � 6 mm) of 250 nm lines and 550 nm spaces.The stamp height was 360 nm. For a complete filling of the stampcavities within this line field without a remaining residual layer a

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polymer height of hfill = 240 nm was calculated. Imprint wasperformed at a temperature of 160 �C, a force of 4 kN and a timeof 5 min. After imprint the samples were sputtered with 10 nm Cras an adhesive layer and 50 nm Au (SCD 040, Balzers Union). Forlift-off the samples were soaked in toluene for up to 15 min andthen lifted under ultrasonic agitation (up to 30% power, 30 min,room temperature). Inspection was done by optical and electronmicroscopy (SEM).

Fig. 2. SEM image of the replica of two stamps. (a) Suitable stamp for lift off:sidewall angle a � 90�, low sidewall roughness, sharp edges along the base. (b) Lesssuitable stamp for lift-off: sidewall angle a � 80�, high sidewall roughness androunded edges along the base.

3. Results

Fig. 1 shows a lift-off result for the 250 nm lines as obtained byT-NIL and sputtering without residual layer removal; about 95%were lifted successfully as estimated from visual inspection ofthe entire 36 mm2 field in an optical microscope. The line edgesindicate that the continuous metal layer was ripped off along thepattern base. The line edge roughness due to rip-off is increasedcompared to the line edge roughness of the imprinted line as de-fined by the stamp. This will limit the process to line widths ofabout 100–150 nm. For the 250 nm lines shown here the result isacceptable as the line edge roughness remains small compared tothe line width itself. To obtain this result a set of conditions hadto be fulfilled which will be discussed in detail.

3.1. Stamp

The stamp profile has to be well defined. This is pointed out inFig. 2 by the replica of two different stamps obtained by imprint un-der the same conditions (T = 190 �C, F = 4 kN, t = 5 min) into thickpolymeric layers (PMMA 75 kg/mol). They reflect the invertedstamp profile, the imprinted 250 nm lines and the completely filled550 nm wide stamp cavities. Fig. 2a refers to a stamp which is wellsuitable for lift-off. The sidewalls are almost vertical with lowroughness. The stamp replicated in Fig. 2b is less suitable for lift-off. The sidewalls are less steep, the sidewall angle is only about80�. Due to an improved sidewall coverage, a sidewall angle smallerthan 90� is much more critical for lift-off with sputtered layers thanwith the conventional evaporated layers. The roughness of the side-walls is high. This will increase the line roughness after lift-off. Mostcritical is the fact that the top edges of the elevated structures of thestamp – the trenches in Fig. 2b – are rounded. For lift-off in generalthis results in a decrease of the contact area between the lifted linesand the substrate that might reduce their stability during lift-offand during further processing or use as well; the convex, undercutline shape is prone to damage. For lift-off with sputtering, where

Fig. 1. Microscope image of a lift-off result for 250 nm lines with continuous metallayers (10 nm Cr, 50 nm Au). 95% of the entire field, 6 mm � 6 mm, was lifted. SEMimage insert: The lines are well defined; the line edges are indicating the rupture ofthe metal layer.

continuous layers have to be broken in a defined way, the decreasedcontact area and thus the decreased overall adhesion force may leadto a complete rip-off of the metal layer. Moreover the roundededges impede a well-defined layer scission and may lead to CDchanges along the lines. For the result shown in Fig. 1 the stampwith the profile shown in Fig. 2a was used. The stamp height wasconfirmed to be H = 360 nm, the most important parameter todefine the initial layer thickness suitable for a specific amount offilling of the stamp cavities.

3.2. Initial layer thickness

To assure a residual free imprint [2,3] the initial layer thickness,h0, has to meet partial cavity filling conditions, that is h0 < hfill. Wefound that the best lift-off results are obtained when the stamp cav-ities are almost filled. In order to achieve this the initial layer thick-ness h0 has to be chosen so that the local polymer volume availablefor filling the cavities is adequate to fill the smallest ones, which fillfirst. This adequate value h0 depends on the structure density of thestamp, in case of linear geometries on the ratio between the ele-vated stamp structure width s and the cavity width w. As a conse-quence wider cavities remain more or less unfilled. Unfortunately,physical self-assembly may occur with unfilled cavities; the poly-mer locally accumulates within the cavity, leaving an empty spacebeside. This is fatal for any lift-off as shown in Fig. 3a. Though thelift-off itself was almost successful, the defects resulting fromself-assembly represent short-circuits between the 250 nm widemetal lines. To avoid physical self-assembly the imprint tempera-ture and/or the imprint time have to be decreased, which may re-quire compensation by an increased imprint pressure. The resultshown in Fig. 1 was obtained by imprint for 5 min at a temperatureof 160 �C into an initial layer thickness of h0 = 230 nm.

3.3. Soaking time

In contrast to lift-off with evaporated layers, lift-off with sput-tered layers which are continuous requires a soaking time in a suit-able solvent for swelling, here toluene for PS. Only the swollenpolymer provides the necessary inner pressure to crack the metallayer along the pattern base. Without the soaking step a large part

Fig. 3. Defective lift-off result: (a) Short-circuits caused by physical self assembly;(b) incomplete rupture of the metal layer along one side only. The rupture is welldefined at the base of the structures.

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of the line field remained un-lifted, whereas in other parts thecomplete layer was ripped-off. Only within small random areasthe lines were lifted successfully. For the result shown in Fig. 1the soaking time was 15 min.

3.4. Ultrasonic power

Unless with evaporated layers where ultra sonic power merelyassists the lifting process, ultimately agitation is required for suc-cessful lift-off of the continuous, sputtered layer. We found thatthe pattern size determines the optimum level of ultra sonicpower. Structures comprising a large polymer volume require asmaller ultra sonic power than structures comprising a small poly-mer volume. An ultra sonic power too high (P30%) leads to a com-plete detachment of the Au from the Cr adhesive layer. A treatmenttime too long has the same effect, although much less pronounced.Admittedly, power and treatment time can be optimized with alimited range of pattern sizes only. Structures comprising a largerpolymer volume than the ones for which the process was opti-mized may suffer from detachment. Structures comprising a smal-ler polymer volume may remain un-lifted. The result shown inFig. 1 was obtained within a time of 30 min at an ultra sonic powerof 20%.

4. Discussions

Lift-off with continuous layers requires the swelling of the poly-mer first followed by a controlled rupture of the metal layer. Thepressure due to swelling, p, results in a tensile stress, rt, in the me-tal layer. When the tensile stress is higher than the stress to breakrB (rt > rB), the metal layer ruptures. With a constant layer thick-ness, t, rupture occurs at locations of maximum tensile stress.Otherwise locations of reduced layer thickness are most prone torupture (see Fig. 3b).

The pressure due to swelling is proportional to the relativechange of the initial polymer volume, p � DV0 � V0 = w � H, withV0 being the volume per length of linear structures with height Hand width w. The tensile stress in the metal layer results from anequilibrium of forces. In the metal layer the force per length, FML,is proportional to the layer thickness, FML = rt � t. The counter-force, FC, is given by the pressure acting across the respectivecross-section. For the metal layer on top of the structure this isthe structure height, FC,Top = p � H, for the metal layer on both side-walls this is the structure half width, FC,SW = p �w/2. Therefore thetensile stress is given by rt = FC/t.

Depending on whether the stamp height or the structure halfwidth is longer a metal layer of constant thickness ruptures onthe top of the structure or along its side under similar internalpressure; preferential rapture along the sidewalls occurs withH 6 w/2. When considering the geometry dependence of the inter-nal pressure for a constant structure height H, as typical of thestamps used for imprint, the top stress increases linearly withthe structure size (rt,top � w) whereas the sidewall stress increasesquadratically (rt,top � w2).

The consequence for lift-off with sputtered layers is two-fold;large structures (w > 2H) will be lifted without problems, thesteepness of the sidewalls is not critical. For lifting of small struc-tures a flaw is ultimately required to ensure sidewall breakage, themore the smaller the structure width becomes. Fortunately sput-tering over vertical sidewalls will always provide a flaw at the baseof the sidewalls. Thus successful lift-off of small structures withcontinuous layers asks for almost vertical sidewalls.

5. Summary and Conclusion

Lift-off with sputtered layers on top of residual-free imprintedstructures was investigated. The results show that larger patternscan always be lifted, but with smaller patterns the sidewall steep-ness is critical, as a flaw in the sputtered layer is required to lift themetal layer in a controlled way. Furthermore, soaking in solvent isrequired to swell the polymer before lifting the metal under ultra-sonic agitation. Large fields of 250 nm wide lines with 800 nmpitch were successfully lifted within 30 min at an ultrasonic powerof 20% after 15 min of soaking.

Acknowledgment

Partial funding by Deutsche Forschungsgemeinschaft DFG ishighly acknowledged.

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