Versatile Multilevel Soft Lithography Method with ... - ACS Publications

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Versatile Multilevel Soft Lithography Method with Micrometer Alignment Using All-Flexible Rubber Stamps and Moiré Fringe Technique Omar Fakhr,* Khaled Karrai, and Paolo Lugli Attocube Systems AG Königinstr. 11a RGB, 80539, Munich (Germany), Lehrstuhl für Nanoelektronik Technische Universität München Arcisstrasse 21, 80333, Munich (Germany) S Supporting Information *

ABSTRACT: Soft lithography has gathered wide interest for the fabrication of unconventional micrometer and nanometersized structures and devices. Nevertheless, accurate alignment is essential to achieve multilevel soft lithography. Because of the soft nature of the stamp materials, such as soft polydimethylsiloxane, they are susceptible to mechanical distortions, which lower the registration accuracy. To reduce the distortions we backed the stamp with a polymer foil and minimized the overall forces applied to the stamp. We furthermore employed an alignment method using additive type moiré fringe technique that is easy to implement and does not require extensive processing steps. The alignment results show less than 1 μm misalignment when the stamp is brought again onto a previously structured rigid template. When performing two consecutive lithography steps by transfer printing of thin gold films, we were able to obtain average registration accuracy of 1.3 μm over an area of 400 mm2. This method is versatile and can be used for several soft lithography techniques. Better results can be obtained with smaller moiré gratings and the use of harder materials.

1. INTRODUCTION In recent years, interest in unconventional nanopatterning techniques has risen exponentially.1 Nanoimprint lithography and soft lithography are two of such techniques. Having the advantage of reduced cost and high throughput fabrication in the micrometer and nanometer range, they are regarded as complementary or even replacement technologies to photolithography. Specifically, soft lithography has the advantage of being the least expensive as it uses soft rubber stamps, usually polydimethylsiloxane (PDMS), for structuring. The technology was introduced by Kumar et al. in 1993 by printing selfassembled monolayers on gold covered substrates.2 Henceforth, different structuring methods and several proof-of-concept studies were conducted. Soft lithography has shown the ability to fabricate 2D and 3D3 structures on both flat and uneven substrates.4 Soft lithography has been applied in biotechnology (e.g., structuring proteins, DNA or Antigen arrays,5−7 fabricating microfluidic channels8), or in organic electronics (e.g., laminating organic semiconductor devices,9 structuring organic semiconductor polymers10) optical lithography,11 or even molecular scale imprinting.12 However, the main limitation of soft lithography, and especially micro- and nanocontact printing, is the absence of accurate multilayer structuring capabilities.13,14 In order to complement or even replace photolithogrpahy,1 multilevel structuring with high registration accuracy is needed. © 2012 American Chemical Society

Commonly used stamp material for soft lithography is soft PDMS (sPDMS), a soft rubberlike material with a very low elasticity modulus E of approximately 2 MPa. This low elasticity makes the stamp easily distorted by the least mechanical forces, typically needed for the printing process. PDMS also suffers from intrinsic distortions, which occur during the fabrication of the stamp.15,16 These distortions limit the fidelity of printing micrometer structures and also the registration accuracy, which is the ability to align several structures on the stamps to these on a substrate. Several studies were conducted to characterize or decrease the distortions of soft stamps. One alternative is substituting sPDMS with harder materials, like as hPDMS (E = 8.2 MPa) or rigiflex molds (E = ca. 1 GPa).17−19 With higher elasticity, or harder stamp material, better alignment accuracy and reduced distortions can be obtained.20 Nevertheless, these materials have the disadvantage of being too brittle for several applications. 21 sPDMS is thus still used due to its conformability and ease of use. The ability to fabricate multilayered devices using soft stamps requires the development of ways to minimize these distortions, before and during printing. Additionally, a reliable alignment method is needed. This is a challenge, because no further adjustments to the alignment can be made, once the contact Received: November 8, 2011 Revised: January 25, 2012 Published: February 8, 2012 4024

dx.doi.org/10.1021/la204377g | Langmuir 2012, 28, 4024−4029

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Figure 1. a) 3D illustration of the device setup: The all-flexible PDMS stamp is mounted on a thin line of sealing lubricant to avoid friction, pressure p is applied on the stamp forcing it to bend downward toward the substrate, which is mounted on positioner stages. The yellow rays symbolize the incident light from the microscope. b) Alignment Microscope Image showing the aligned stamp in contact with a prestructured substrate. The moiré overlapping is visible, indicating a slight misalignment in the y-direction, which cannot be identified by looking at the middle structures. c) An optical microscope image showing a Stamp in contact with the narrow lubricant area. Here, the thickness was about 162 μm wide. and hence the alignment capability. For our purposes, we use transparent polymer foils of 100 μm thickness to support our stamps. These foils have an elasticity modulus of approximately 1.5 GPa. Our stamps were fabricated from a template made by photolithography. We coated silicon wafers with SU-8 resist. The thickness of the film was approximately 1.6 μm. After exposure and developing, we obtained structures with positive slopes of about 15°. The sPDMS stamp is fabricated as follows: i) Base and Curing agent were mixed in a ratio of 10:1 and degassed for 1 h, ii) PDMS was spin coated over the master, iii) the plasma treated backing foils were gently brought into contact to the PDMS to avoid the entrapment of air bubbles, iv) PDMS was cured at temperatures ranging from 50 to 100 °C. Afterward, the flexible stamp was easily peeled from the master. The spin coating step of the sPDMS on the substrate at 1500 rpm for 10 s resulted in a layer thickness of 60 μm. We chose low thicknesses to avoid extended distortions caused by heat during the curing step.15 The result is a sufficiently bendable stamp, which provides enhanced accuracy in alignment. 2.2. Methods for Minimizing Distortions. We were able to minimize mechanically induced distortions by minimizing both normal and friction forces applied to the stamp. We mounted the stamp on a specially designed fixture. Figure 1 shows the concept of mounting the circular stamp. Between stamp and fixture, a thin and narrow line of sealing lubricant was applied. This lubricant has a width of approximately 200 μm along the edge of the stamp. Because of the very small contact area between stamp and fixture, and also because of the low friction caused by the lubricant, shear and friction forces applied to the stamp were held to a minimum. It also ensured easy mounting of the stamp without causing distortions. The total friction forces applied on our stamps were less than 0.25N. In addition, the lubricant used, acts as a good sealing agent when pressure is applied. The normal forces applied to the stamp were purely pneumatic, which is much more homogeneous than applying normal forces mechanically. The pressure distribution over the printing area was uniform. A typical pressure of 20 mbar was applied to the stamp before contact with the substrate was established causing the stamp to deform like a segment of a sphere. The deflection of the stamp was between 0.5 and 1 mm. In spite of the bending, we did not observe deformations to the stamp, which is due to the thin lubricant film and the low amount of pressure applied. When the substrate is slowly moved toward the deflected stamp, contact is initiated at a single point. We approached the substrate at speeds between 5 and 10 μm/ sec. By further approaching, contact spreads uniformly throughout the whole printing area. Thereby, air voids between stamp and substrate are eliminated, ensuring homogeneous printing and better registration.

between stamp and substrate is established. Furthermore, rubber stamps can have uneven surface, partially caused by heat induced shrinking while curing.15 This could aggravate the alignment results, when parts of the stamp get unintentionally in contact with the substrate. State-of-the-art equipment was reported to give misalignment values of sPDMS in the order of 4.95 μm over an area of 250 μm2.20 Recently, a self-alignment concept was introduced, which enables a layer-to-layer misalignment of 10 μm over 5 mm area.22 This concept, however, demands extensive fabrication steps, involving prestructuring of the target substrate to allow correct alignment. In this article, we introduce a concept of handling and aligning PDMS stamps. This method combines ease of use and versatility, making it applicable for a variety of structuring techniques. Our system is able to i) establish conformal contact without causing air voids, ii) facilitate and control the peeling of the stamp from the substrate, iii) have control on printing and peeling speeds, which is necessary for high-speed contact printing23 and for kinetic control of adhesion,24 iv) apply controlled pressures, v) while having the capability to reproducible alignment results and high registration accuracy. Because of its ease of fabrication, our approach guarantees the cost-effectiveness of soft lithography.

2. EXPERIMENTAL DETAILS 2.1. Fabrication of Flexible PDMS Stamps with BackSupport. The most common method for avoiding distortions of soft stamps is by backing it with a glass plate, widely called the platen press model.25 Here, the substrate is approached to the glass-backed stamp and pressed against it. The method has proven to have several disadvantages: i) the glass backing prevents structuring of uneven substrates, or ii) it requires relatively high pressures (around 1 bar) to establish overall contact, which could distort high aspect ratio structures, iii) it has no control of contact spreading or the speed of propagation and peeling, iv) it cannot avoid having air voids upon contact; v) it requires sizable separation force to remove the stamp from the substrates or the original master.26 In our approach we use an all-flexible stamp; using backing polymer foils to backbone the soft stamp. Polymer backing has the advantage of requiring less pressure for stamping and the ability to structure uneven substrates. Polymer backing sheets like Kapton has been used in previous studies.27 The low transparency of Kapton hinders the detection of alignment marks, 4025


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+ p2)/2(p1 − p2). In the case of the two beat structures I1 and I2, the shift occurs in opposite directions, magnifying thereby the lateral shift by another factor of 2, as seen in part b of Figure 2. This method was first used by Li et al. to align NIL stamps and resulted into a misalignment of 150 nm on a 1inch area die.29 For that study, the moiré patterns were created on a mask from fused silica. The gratings on the mask were made from a 100 nm silicon−nitride and an additional 10 nm of chrome. To further increase contrast of overlapping moiré fringes, the structures on the substrate were also made out of 20 nm of chrome. The periods p1 and p2 were set to 1 and 0.95 μm. For the ease of fabrication, our stamps are made purely out of sPDMS, with a polymer backing. The stamp is a replica of a master made out of SU-8 polymer on silicon substrates with relief height of approximately 1.6 μm. Thereby, the contrast of the patterns is enhanced by the increased relief height of the structures. To be able to work with photolithography for the master fabrication, p1 and p2 were set to 10 and 11 μm, respectively. These dimensions have proven to magnify the misalignment by a factor of 21: a 1micrometer shift of the substrate toward the stamp results in a comparative shift of 21 μm of the beat structures. A misalignment of about 5.5 μm would thereby lead to repetition of the beat structures; that is the moiré patterns I1 and I2 would be again in synchrony as in the case of perfect alignment. To avoid this, course alignment marks containing of crosses and brackets were employed.

Additionally, this method makes certain that contact is always initiated in the middle of the stamp and eliminates unintentional contact. The use of low pressures not only enhances the registration accuracy, it minimizes the distortions applied to stamp structures, like roof collapsing or buckling.16 The substrate was mounted on a stack of four nanopositioners through a vacuum chuck. A positioner in z-direction allows the substrate to move toward or away from the stamp. The other three positioners allow x, y, and rotational movements. The positioners have a step resolution of approximately 50 nm. Both step size and movement speeds can be controlled electrically. Varying the approaching and withdrawing speed of the z-positioner toward and from the substrate allows control of both printing and peeling speeds. Our propagation velocity can get down to 0.2 mm/sec and up to 3 mm/sec depending on the approaching and reproaching speed of the z-positioner as well as the degree of deflection of the stamp. 2.3. Moiré Fringe Alignment Technique. Moiré fringe technique is an attractive method for alignment and was used in previous studies to characterize distortions of soft lithography stamps.15,28 The moiré patterns are usually generated by a superimposition of two gratings with different periods (p1 and p2). When these two gratings are brought on top of each other, and illuminated, a resulting beat pattern appears with a period P = p1 × p2/(p1 − p2). The contrast of the resulting pattern can be enhanced for example by using coherent light sources, increasing the contrast of the patterns or decreasing the distance between both gratings. When p1 and p2 are slightly shifted from each other, the shift of the resulting pattern is magnified. For alignment purposes, the gratings are placed on both the mask (or stamp) and on the to be structured substrate. To detect the perfect alignment, a reference needs to be present. We thereby used the alignment gratings method described by Li et al.29 Two gratings, also referred two as intensity distributions, i1stamp and i2stamp were placed alongside on the stamp; the complementary gratings (i2subsrate and i1substrate) were prestructured on the substrate. Here i1stamp and i1substrate have the period p1, i2stamp and i2subsrate have the period p2. By superimposition, two intensity distributions with the periods I1 and I2 are created, whereas I1 = i1stamp + i2subsrate and I2= i2stamp +i1substrate. Both moiré patterns I1 and I2 have the same period P. In the case of perfect alignment, these patterns are in synchrony or aligned, as seen in part a

3. RESULTS AND DISCUSSION 3.1. Detection of Moiré Fringes. We investigated the detection of the moiré overlapping on two differently produced

Figure 3. Optical microscope images of a stamp brought into contact with the master. Perfect alignment allows the protruded structures to fall into the trenches. The wetting apparent in the trenches appears brighter compared to unwetted regions, (emphasized partially by the arrows). Although the wetting did not occur in all structures, no misalignment could be detected. The wetting area increases through time due to adhesion forces. Figure 2. Comparison in the substrate shift and the corresponding change in the moiré overlapping: a) A microscope image of two moiré columns while aligning, the moiré structures in both columns are in synchrony indicating alignment in the y axis, the moiré fringes are a result from superimposition of gratings produced on gold thin films and the grating in the stamp; b) An estimate 500 nm shift in y direction leads to a moiré shift between the two columns, the moiré pattern in the left columns moves downward, whereas the other shifts upward. The white arrows point to the area of maximum intensity oscillation of the moiré overlapping.

gratings on the substrate. One was made with photolithography using SU8 polymer and the other was produced by nanoTransfer Printing (nTP) of thin gold films, with a thickness of 15−20 nm onto silicon substrates. The nanotransfer printing was also performed using sPDMS stamps.31,32,30 The alignment marks were placed in the middle, or the lowest point of the bended stamp and also the initial contact point. The substrate containing the corresponding grating was brought near to the stamp using the z-stage positioner until the moiré patterns were visible, using thereby a 3.1× (NA = 0.095) incident light optical microscope with a CCD camera. The magnification of the microscope is very low compared to other alignment studies,

of Figure 2. Deviating from the perfect alignment in the direction of the grating leads to a shift of the moiré patterns by the factor Δx = (p1 4026


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magnifications. Although the numerical aperture hardly allows the identification of 5 μm structures, the resulting moiré patterns were clearly visible. Figure 2 shows optical microscope images taken while aligning the stamp. The right image shows a slight shift of the substrate in y-direction, which results in a bigger shift to the resulting moiré pattern. The degree of contrast did not differ from substrates using SU-8 or transferred gold, which is typical for additive type moiré fringes. Further enhancement of the contrast can be achieved by signal processing of the captured image, or using detectors with nonlinear response.33 After aligning the stamp in x, y and rotational directions, the substrate was brought nearer to the stamp at slow speeds. Assuming there is no rotational shift while further approaching, we adjusted slight shifts in x and y directions in real time, whereas the substrate was drawn nearer to the stamp until contact was established. 3.2. Aligning Stamp to Master. To characterize the distortion of the whole system, we aligned and printed the stamp again onto its master, to allow the structures to fall again into each other. It was previously stated that this was not possible using sPDMS.20 Nevertheless, we were able to bring the sPDMS stamp into perfect alignment; the structures of both master and stamp falling into each other in the middle region. This was assisted by the 15° sidewall slope of the structures on both stamp and substrate (0° indicating thereby a vertical slope). We believe that this slope compensates for a misalignment value of about ±500 μm. The structures did not align further away from the center due to heat-induced shrinkage of PDMS. We observed the entwining of the structures on both stamp and substrate by a certain wetting characteristic, which implyies conformal contact in these areas. This wetting can be seen in Figure 3 as brighter parts within the structure. Although certain areas did not show this wetting characteristic, no misalignment or distortions of these structures were observed, apart from heat induced shrinkage further away from the center. Control points at a distance of 11,3 mm away from the center had misplacement of 7 μm with a misplacement vector pointing toward the center indicating thereby an isotropic shrinkage of 0.06%. A quantitative analysis of misalignments using this method was not possible while the stamp was still in contact with the substrate due to the limited focusing capabilities of optical microscopes with high numerical aperture. 3.3. Two-Step Lithography via Transfer Printing of Thin Gold Films. We performed a two-step printing process in order to quantitatively characterize misalignment. Two soft lithography steps imply an accumulation of the distortions of two different stamps while printing. We performed our lithography steps using nanotransfer printing of gold films.34 Our gold films were evaporated thermally, which leads to further distortions of the stamp, caused by radiant heating from the evaporation source, especially at long evaporation times.27,35 Because of such distortions, good alignment results are considerably harder to obtain than when other techniques are used; for example, microContact Printing (μCP). Thus the two-step procedure using nanoTransfer Printing is a formidable test for our method. Thin layer of gold (15−20 nm) was evaporated onto the stamps at a rate of 0.5 Å/sec. A titanium layer of 1.5 nm was subsequently evaporated at a rate of 0.1 Å/sec. Titanium serves as an adhesion promoter of the thin metal film. In the first lithography step, the whole area of 400 mm2 is transferred including the alignment marks located in the center onto a

Figure 4. a) Mapping of the alignment control points for each sample; C indicates the center, where the alignment marks were located. Eighteen control points at different radiuses from the center are evaluated. The furthest control points are located at 11.3 radius. b) AFM micrograph of control point 1,1a (1 mm from center); the two gold pads were transferred on top of each other via cold welding of gold; the misalignment between the two pads is about 300 nm in y direction; c) AFM micrograph of control point at 11.3 mm distance from center (9,9a), inner and outer crosses have a gold layer thickness of 15 nm and 20 nm respectively to demonstrate different alignment processes. The misalignment is 1 μm in both x and y directions. Scale bar is 1 μm.

but has the advantage of having a large depth of focus of about 50 μm. This is necessary to be able to detect both substrate and stamp at the same time, and thereby the resulting moiré overlapping. In addition the stamp has a large relief height of 1.6 μm, making it difficult to focus using lenses with higher 4027


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Table 1. Misalignment Values for 54 Control Points 1 radius 1

2

3

dx /μm dy/μm dia/μm dx/μm dy/μm dia/μm dx/μm dy/μm dia/μm

1a 1 mm

0.9 0.2 0.9 0.1 0.2 0.3 0.2 0.2 0.3

0.5 0.8 1.0 0.0 0.3 0.3 0.4 0.7 0.8

2

2a 2 mm

0.8 0.4 0.9 0.4 0.5 0.6 0.5 0.2 0.6

0.9 0.9 1.3 0.4 0.3 0.5 0.4 0.6 0.7

3

3a 5 mm

0.2 0.1 0.2 1.2 0.5 1.3 0.2 0.2 0.3

0.1 1.2 1.1 1.6 0.9 1.8 0.6 1.7 1.8

4

4a

5

6 mm 0.2 0.4 0.4 1.5 0.4 1.6 0.2 0.2 0.3

0.3 1.4 1.4 1.5 1.1 1.9 0.4 1.3 1.4

5a 7 mm

0.2 0.6 0.6 1.8 0.9 2.0 0.4 0.6 0.7

0.1 1.5 1.5 1.7 1.9 2.6 0.1 1.5 1.5

6

6a 8 mm

0.1 0.6 0.6 2.5 0.9 2.6 0.1 0.5 0.5

0.0 1.5 1.5 1.6 1.6 2.3 0.1 1.6 1.6

7

7a 9 mm

0.5 1.2 1.2 2.0 0.4 2.1 0.1 0.5 0.5

0.4 1.4 1.4 2.1 1.5 2.5 0.1 1.9 1.9

8

8a

10 mm 0.9 1.0 1.3 1.8 1.3 2.2 0.0 0.7 0.7

0.7 2.0 2.1 2.4 0.3 2.4 0.5 1.8 1.8

9

9a

av.

11.3 mm 1.2 2.3 2.6 1.5 0.4 1.6 0.3 0.9 0.9

1.0 2.5 2.7 2.0 1.4 2.5 0.5 2.1 2.1

0.5 1.1 1.3 1.5 0.8 1.7 0.3 0.9 1.0

located at 1 mm radius and the furthest at a 11.3 mm (9 and 9a), as can be seen in part a of Figure 4. Parts b and c of Figure 4 show AFM micrographs of two control points 1 and 9, respectively. Table 1 shows misalignment values of three different two-step lithography processes, using thermal evaporation and transfer of gold films. The average diagonal misalignment of a total of 54 control points is 1.3 μm. We found difficulties in perfectly adjusting the rotational misalignment, which was of about 0.01°. Further decreasing these values would demand smaller moiré periods. Figure 5 shows statistics of misalignment values. The diagram indicates that several misalignment values fall in the sub-1 μm range with the maximum misplacement value of 2.8 μm. These values not only represent the misalignment limitations, but they represent the overall distortions of two consecutive prints. We estimate the distortions to cause an overall misplacement of less than 1 μm at any given point. Evaluation of misalignment for a two-step lithography process in 18 control points for three different samples; the second row shows the distances of the control points from the center. dx and dy indicate misalignment in x and y directions respectively, dia indicates the diagonal misalignment value, which is calculated from dx and dy.

4. CONCLUSIONS In conclusion, we demonstrate a method for handling and aligning sPDMS stamps, which could open the door for multilayer device fabrication. This was achieved by i) using flexible stamps, ii) reducing normal and friction forces, and iii) employing moiré fringe alignment technique. Our approach has the advantage of exploiting the benefits of a polymer backed stamp and a versatile printing and alignment system, while maintaining low cost fabrication and ease of use. We further demonstrated the possibility of using moiré fringe technique in aligning PDMS stamps to a substrate, without any extensive preparation of the grating structures. A two-step lithography process using transfer printing of gold films was performed, with average misalignment values of 1.3 μm over an area of 400 mm2, with several misalignment values below the submicrometer range. Further enhancing of these values could be achieved using finer moiré patterns. This method could be also used for printing other soft stamps such as hPDMS and rigiflex molds. Because of their stiffer character, these materials might even show better results. Printing of larger areas would require thicker polymer backing and better rotational sensitivity of the moiré patterns.

Figure 5. Statistics of misalignment values: a) misalignment values in both x and y direction. The statistical peak is located around 0.4 μm. b) diagonal misalignment values with a peak at 0.6 μm.

silicon substrate. Apart from containing alignment marks, the stamp contains control points throughout the area, which are also transferred. In the second step, the alignment marks with the corresponding grating was shadow masked during evaporation, while the remaining areas were exposed to the gold flux. This shadow masking was needed to make transparent window at the alignment marks, whereas other areas of the stamp were covered with gold. This stamp had also control points at the same distances as the first stamp. By moiré aligning and printing the second stamp, the control points should therefore fall into each other. The misalignment was subsequently characterized by an optical microscope with a magnification of 100×. The value of the misalignment was determined by measuring the shift in both x and y directions of the two printed structures in 18 control points throughout the substrate. These control points are located at different distances from the center, with the nearest control point (1 and 1a) is 4028


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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Omar Fakhr would like to thank Dr. Marc Hennemeyer and Dr. Ran Ji from Suss MicroTec for the revision of the manuscript and the constructive dicusssions.

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