Contact lithography

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Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.

Contents

History

The first integrated circuits had features of 200 micrometres which were printed using contact lithography. This technique was popular in the 1960s until it was substituted by proximity printing, where a gap is introduced between the photomask and the substrate. Proximity printing had poorer resolution than contact printing (due to the gap allowing more diffraction to occur) but generated far less defects. The resolution was sufficient for down to 2 micrometre production. In 1978, the step-and-repeat projection system appeared. [1] The platform gained wide acceptance due to the reduction of the mask image and is still in use today.

Contact lithography is still commonly practiced today, mainly in applications requiring thick photoresist and/or double-sided alignment and exposure. Advanced 3D packaging, optical devices, and micro-electromechanical systems (MEMS) applications fall into this category. In addition, the contact platform is the same as used in imprint processes.

Recently, two developments have given contact lithography potential for comeback in semiconductor lithography. First, surface plasmon resonance enhancements including the use of silver films as lenses have been demonstrated to give resolution of less than 50 and even 22 nm using wavelengths of 365 and 436 nm. [2] [3] [4] The exotic dispersion relation of surface plasmon has led to the extremely short wavelength, which helps to break the diffraction limit. [2] Second, nanoimprint lithography has already gained popularity outside the semiconductor sector (e.g., hard-drive, biotechnology) and is a candidate for sub-45 nm semiconductor lithography, driving defect reduction practices and uniformity improvement for masks in contact with the substrate. Step-and-flash imprint lithography (SFIL), a popular form of nanoimprint lithography which involves UV curing of the imprint film, essentially uses the same setup as contact lithography.

Operating principle

Generally, a photomask is created, which consists of opaque chromium patterns on a transparent glass plate. The substrate is coated with a thin film of UV-sensitive photoresist. The substrate is then placed underneath the photomask, and pressed into contact with it. The sample is then exposed, during which UV light is shone from the top side of the photomask. Photoresist beneath transparent glass is exposed, and becomes able to be dissolved by a developer, while photoresist under chrome does not receive any UV exposure and will remain intact after developing. The result is the original pattern replicated in the form of photoresist. The pattern may then be permanently transferred into the substrate via any number of microfabrication processes, such as etching or lift-off. A single photomask may be used many times to repeatably reproduce a pattern onto different substrates. A "contact aligner" [5] is generally used to perform this operation, so that previous patterns on a substrate may be aligned to the pattern one wants to expose.

Upon exiting the photomask-photoresist interface, the image-forming light is subject to near-field diffraction as it propagates through the photoresist. Diffraction causes the image to lose contrast with increasing depth into the photoresist. This can be explained by the rapid decay of the highest-order evanescent waves with increasing distance from the photomask-photoresist interface. This effect can be partly mitigated by using thinner photoresist. Contrast enhancements based on plasmon resonances and lensing films have recently been disclosed. [3] The chief advantage of contact lithography is the elimination of the need for complex projection optics between object and image. The resolution limit in today's projection optical systems originates from the finite size of the final imaging lens and its distance from the image plane. More specifically, the projection optics can only capture a limited spatial frequency spectrum from the object (photomask). Contact printing has no such resolution limit but is sensitive to the presence of defects on the mask or on the substrate.

Types of contact masks

There are several types of contact lithography masks.

The standard binary intensity amplitude mask defines dark and light areas where light is blocked or transmitted, respectively. The dark areas are patterned films consisting of chromium or other metal.

The light coupling mask has a corrugated dielectric surface. Each protrusion acts as a localized waveguide. [6] Light is transmitted primarily through the protrusions as a result of this localized guiding effect. Since less contact area is needed, there is less potential for defects.

A hybrid nanoimprint-contact mask utilizes both contact imaging and mechanical imprinting, [7] and has been proposed to optimize imaging of both large and small features simultaneously by eliminating imprint residual layer issues.

Contact masks have traditionally been fairly large (>100 mm), but it is possible that alignment tolerances may require smaller mask sizes to allow stepping between exposures.

As in nanoimprint lithography, the mask needs to have roughly the same feature size as the desired image. Contact masks can be formed directly from other contact masks, or by direct writing (e.g., electron beam lithography).

Resolution enhancements

As noted above, thinner photoresist can help improve image contrast. Reflections from the layer underlying the photoresist also have to be taken into account when absorption and evanescent wave decay are reduced.

The resolution of contact lithography has been predicted to surpass λ/20 periodicity. [8]

The pitch resolution of contact lithography can be readily enhanced by multiple exposures generating feature images between previously exposed features. This is suitable for nested array features, as in memory layouts.

Surface plasmons are collective oscillations of free electrons confined to metal surfaces. They couple strongly to light, forming surface plasmon polaritons. Such excitations effectively behave as waves with very short wavelength (approaching the x-ray regime). [2] By exciting such oscillations under the right conditions, multiple features can appear in between a pair of grooves in the contact mask. [9] The resolution achievable by surface plasmon polariton standing waves on a thin metallic film is <10 nm with a wavelength in the 380-390 nm range using a <20 nm silver film. [2] In addition, deep narrow slits in metallic transmission gratings have been shown to allow resonances that amplify light passing through the slits. [10]

A layer of metal film, has been proposed to act as a 'perfect lens' for amplifying the evanescent waves, resulting in enhanced image contrast. This requires tuning the permittivity to have a negative real part, e.g., silver at 436 nm wavelength. [11] The use of such a lens allows imaging to be achieved with a wide tolerance of distance between mask and photoresist, while achieving extreme resolution enhancement by use of surface plasmon interference, e.g., a half-pitch of 25 nm with 436 nm wavelength. [11] The perfect lens effect is only effective for certain conditions, but allows a resolution roughly equal to the layer thickness. [12] Hence a sub-10 nm resolution appears feasible with this approach as well.

The use of surface plasmon interference gives an edge over other lithography techniques, as the number of mask features can be much less than the number of features in the desired image, making the mask easier to fabricate and inspect. [2] [13] While silver is the most commonly used metal for demonstrating surface plasmons for lithography, aluminum has also been used at 365 nm wavelength. [14]

While these resolution enhancement techniques allow 10 nm features to be contemplated, other factors must be considered for practical implementation. The most fundamental limitation appears to be photoresist roughness, which becomes predominant for shorter sub-wavelength periods where only the zeroth diffraction order is expected to propagate. [3] All the pattern details are in this case conveyed by the evanescent waves, which decay more rapidly for finer resolution. As a result, the photoresist's inherent roughness following development can become more significant than the pattern.

Defect and contamination issues

As with any technology that relies on surface contact, defects are a strong concern. Defects are particularly detrimental to contact lithography in two respects. First, a hard defect can widen the gap between the mask and the substrate. This can easily cause images based on evanescent waves or surface plasmon interference to disappear. Second, smaller, softer defects attached to the metal surface of the mask may not disturb the gap but can still alter the evanescent wave distribution or destroy the surface plasmon interference condition.

Oxidation of the metal surface [15] also destroys plasmon resonance conditions (as the oxide surface is not a metal).

Related Research Articles

In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, ultraviolet light is used to transfer a geometric design from an optical mask to a light-sensitive chemical (photoresist) coated on the substrate. The photoresist either breaks down or hardens where it is exposed to light. The patterned film is then created by removing the softer parts of the coating with appropriate solvents.

<span class="mw-page-title-main">Photoresist</span> Light-sensitive material used in making electronics

A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.

<span class="mw-page-title-main">Photomask</span>

A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography and the production of integrated circuits in particular. Masks are used to produce a pattern on a substrate, normally a thin slice of silicon known as a wafer in the case of chip manufacturing. Several masks are used in turn, each one reproducing a layer of the completed design, and together they are known as a mask set.

<span class="mw-page-title-main">Immersion lithography</span> Photolithography technique where there is a layer of water between a lens and a microchip

Immersion lithography is a photolithography resolution enhancement technique for manufacturing integrated circuits (ICs) that replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The resolution is increased by a factor equal to the refractive index of the liquid. Current immersion lithography tools use highly purified water for this liquid, achieving feature sizes below 45 nanometers. ASML and Nikon are currently the only manufacturers of immersion lithography systems.

<span class="mw-page-title-main">Electron-beam lithography</span> Lithographic technique that uses a scanning beam of electrons

Electron-beam lithography is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing). The purpose, as with photolithography, is to create very small structures in the resist that can subsequently be transferred to the substrate material, often by etching.

Masklesslithography (MPL) is a photomask-less photolithography-like technology used to project or focal-spot write the image pattern onto a chemical resist-coated substrate by means of UV radiation or electron beam.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Extreme ultraviolet lithography</span> Lithographic technique using an extreme ultraviolet wavelength, usually 13.5 nm

Extreme ultraviolet lithography is an optical lithography technology used in steppers, machines that make integrated circuits (ICs) for computers and other electronic devices. It uses a range of extreme ultraviolet (EUV) wavelengths, roughly spanning a 2% FWHM bandwidth about 13.5 nm, to produce a pattern by exposing reflective photomask to UV light which gets reflected onto a substrate covered by photoresist. It is widely applied in semiconductor device fabrication process.

Next-generation lithography or NGL is a term used in integrated circuit manufacturing to describe the lithography technologies in development which are intended to replace current techniques. The term applies to any lithography method which uses a shorter-wavelength light or beam type than the current state of the art, such as X-ray lithography, electron beam lithography, focused ion beam lithography, and nanoimprint lithography. The term may also be used to describe techniques which achieve finer resolution features from an existing light wavelength.

<span class="mw-page-title-main">Stepper</span>

A stepper is a device used in the manufacture of integrated circuits (ICs) that is similar in operation to a slide projector or a photographic enlarger. Stepper is short for step-and-repeat camera. Steppers are an essential part of the complex process, called photolithography, which creates millions of microscopic circuit elements on the surface of chips of silicon. These chips form the heart of ICs such as computer processors, memory chips, and many other devices.

<span class="mw-page-title-main">Nanoimprint lithography</span> Method of fabricating nanometer scale patterns using a special stamp

Nanoimprint lithography (NIL) is a method of fabricating nanometer scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer or polymer formulation that is cured by heat or UV light during the imprinting. Adhesion between the resist and the template is controlled to allow proper release.

A superlens, or super lens, is a lens which uses metamaterials to go beyond the diffraction limit. For example, in 1995, Guerra combined a transparent grating having 50nm lines and spaces with a conventional microscope immersion objective. The resulting "superlens" resolved a silicon sample also having 50nm lines and spaces, far beyond the classical diffraction limit imposed by the illumination having 650nm wavelength in air. The diffraction limit is a feature of conventional lenses and microscopes that limits the fineness of their resolution depending on the illumination wavelength and the numerical aperture NA of the objective lens. Many lens designs have been proposed that go beyond the diffraction limit in some way, but constraints and obstacles face each of them.

<span class="mw-page-title-main">Optical proximity correction</span> Photolithography enhancement technique

Optical proximity correction (OPC) is a photolithography enhancement technique commonly used to compensate for image errors due to diffraction or process effects. The need for OPC is seen mainly in the making of semiconductor devices and is due to the limitations of light to maintain the edge placement integrity of the original design, after processing, into the etched image on the silicon wafer. These projected images appear with irregularities such as line widths that are narrower or wider than designed, these are amenable to compensation by changing the pattern on the photomask used for imaging. Other distortions such as rounded corners are driven by the resolution of the optical imaging tool and are harder to compensate for. Such distortions, if not corrected for, may significantly alter the electrical properties of what was being fabricated. Optical proximity correction corrects these errors by moving edges or adding extra polygons to the pattern written on the photomask. This may be driven by pre-computed look-up tables based on width and spacing between features or by using compact models to dynamically simulate the final pattern and thereby drive the movement of edges, typically broken into sections, to find the best solution,. The objective is to reproduce on the semiconductor wafer, as well as possible, the original layout drawn by the designer.

Resolution enhancement technologies are methods used to modify the photomasks in the lithographic processes used to make integrated circuits to compensate for limitations in the optical resolution of the projection systems. These processes allow the creation of features well beyond the limit that would normally apply due to the Rayleigh criterion. Modern technologies allow the creation of features on the order of 5 nanometers (nm), far below the normal resolution possible using deep ultraviolet (DUV) light.

Interference lithography is a technique for patterning regular arrays of fine features, without the use of complex optical systems or photomasks.

Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.

Plasmonic nanolithography is a nanolithographic process that utilizes surface plasmon excitations such as surface plasmon polaritons (SPPs) to fabricate nanoscale structures. SPPs, which are surface waves that propagate in between planar dielectric-metal layers in the optical regime, can bypass the diffraction limit on the optical resolution that acts as a bottleneck for conventional photolithography.

Computational lithography is the set of mathematical and algorithmic approaches designed to improve the resolution attainable through photolithography. Computational lithography has come to the forefront of photolithography in 2008 as the semiconductor industry grappled with the challenges associated with the transition to 22 nanometer CMOS fabrication process technology and beyond.

<span class="mw-page-title-main">X-ray lithography</span> Lithographic technique that uses X-rays instead of light

X-ray lithography is a process used in semiconductor device fabrication industry to selectively remove parts of a thin film of photoresist. It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist, or simply "resist," on the substrate to reach extremely small topological size of a feature. A series of chemical treatments then engraves the produced pattern into the material underneath the photoresist.

Surface plasmon resonance microscopy (SPRM), also called surface plasmon resonance imaging (SPRI), is a label free analytical tool that combines the surface plasmon resonance of metallic surfaces with imaging of the metallic surface. The heterogeneity of the refractive index of the metallic surface imparts high contrast images, caused by the shift in the resonance angle. SPRM can achieve a sub-nanometer thickness sensitivity and lateral resolution achieves values of micrometer scale. SPRM is used to characterize surfaces such as self-assembled monolayers, multilayer films, metal nanoparticles, oligonucleotide arrays, and binding and reduction reactions. Surface plasmon polaritons are surface electromagnetic waves coupled to oscillating free electrons of a metallic surface that propagate along a metal/dielectric interface. Since polaritons are highly sensitive to small changes in the refractive index of the metallic material, it can be used as a biosensing tool that does not require labeling. SPRM measurements can be made in real-time, such as measuring binding kinetics of membrane proteins in single cells, or dna hybridization.

References

  1. ↑ Su, Frederic (1997-02-01). "Microlithography: from contact printing to projection systems". SPIE Newsroom. SPIE-Intl Soc Optical Eng. doi:10.1117/2.6199702.0001. ISSN   1818-2259.
  2. 1 2 3 4 5 Luo, Xiangang; Ishihara, Teruya (2004-06-07). "Surface plasmon resonant interference nanolithography technique". Applied Physics Letters. AIP Publishing. 84 (23): 4780–4782. Bibcode:2004ApPhL..84.4780L. doi:10.1063/1.1760221. ISSN   0003-6951.
  3. 1 2 3 Melville, David O. S.; Blaikie, Richard J. (2005). "Super-resolution imaging through a planar silver layer". Optics Express. The Optical Society. 13 (6): 2127–2134. Bibcode:2005OExpr..13.2127M. doi: 10.1364/opex.13.002127 . ISSN   1094-4087. PMID   19495100.
  4. ↑ Gao, Ping; Yao, Na; Wang, Changtao; Zhao, Zeyu; Luo, Yunfei; et al. (2015-03-02). "Enhancing aspect profile of half-pitch 32 nm and 22 nm lithography with plasmonic cavity lens". Applied Physics Letters. AIP Publishing. 106 (9): 093110. Bibcode:2015ApPhL.106i3110G. doi:10.1063/1.4914000. ISSN   0003-6951.
  5. ↑ "Contact Lithography". www.nanotech.ucsb.edu. Archived from the original on 2010-06-26.
  6. ↑ Martin, Olivier J. F.; Piller, Nicolas B.; Schmid, Heinz; Biebuyck, Hans; Michel, Bruno (1998-09-28). "Energy flow in light-coupling masks for lensless optical lithography". Optics Express. The Optical Society. 3 (7): 280–285. Bibcode:1998OExpr...3..280M. doi: 10.1364/oe.3.000280 . ISSN   1094-4087. PMID   19384370.
  7. ↑ Cheng, Xing; Jay Guo, L. (2004). "A combined-nanoimprint-and-photolithography patterning technique". Microelectronic Engineering. Elsevier BV. 71 (3–4): 277–282. doi:10.1016/j.mee.2004.01.041. ISSN   0167-9317.
  8. ↑ McNab, Sharee J.; Blaikie, Richard J. (2000-01-01). "Contrast in the evanescent near field of λ/20 period gratings for photolithography". Applied Optics. The Optical Society. 39 (1): 20–25. Bibcode:2000ApOpt..39...20M. doi:10.1364/ao.39.000020. ISSN   0003-6935. PMID   18337865.
  9. ↑ Luo, Xiangang; Ishihara, Teruya (2004). "Subwavelength photolithography based on surface-plasmon polariton resonance". Optics Express. The Optical Society. 12 (14): 3055–3065. Bibcode:2004OExpr..12.3055L. doi: 10.1364/opex.12.003055 . ISSN   1094-4087. PMID   19483824.
  10. ↑ Porto, J. A.; García-Vidal, F. J.; Pendry, J. B. (1999-10-04). "Transmission Resonances on Metallic Gratings with Very Narrow Slits". Physical Review Letters. American Physical Society (APS). 83 (14): 2845–2848. arXiv: cond-mat/9904365 . Bibcode:1999PhRvL..83.2845P. doi:10.1103/physrevlett.83.2845. ISSN   0031-9007. S2CID   27576694.
  11. 1 2 X. Jiao et al., Progress in Electromagnetics Research Symposium 2005, pp. 1-5 (2005)
  12. ↑ Smith, David R.; Schurig, David; Rosenbluth, Marshall; Schultz, Sheldon; Ramakrishna, S. Anantha; Pendry, John B. (2003-03-10). "Limitations on subdiffraction imaging with a negative refractive index slab". Applied Physics Letters. 82 (10): 1506–1508. arXiv: cond-mat/0206568 . Bibcode:2003ApPhL..82.1506S. doi:10.1063/1.1554779. ISSN   0003-6951. S2CID   39687616.
  13. ↑ Salomon, Laurent; Grillot, Frédéric; Zayats, Anatoly V.; de Fornel, Frédérique (2001-02-05). "Near-Field Distribution of Optical Transmission of Periodic Subwavelength Holes in a Metal Film". Physical Review Letters. American Physical Society (APS). 86 (6): 1110–1113. Bibcode:2001PhRvL..86.1110S. doi:10.1103/physrevlett.86.1110. ISSN   0031-9007. PMID   11178022.
  14. ↑ Srituravanich, Werayut; Fang, Nicholas; Sun, Cheng; Luo, Qi; Zhang, Xiang (2004). "Plasmonic Nanolithography". Nano Letters. American Chemical Society (ACS). 4 (6): 1085–1088. Bibcode:2004NanoL...4.1085S. doi:10.1021/nl049573q. ISSN   1530-6984.
  15. ↑ E.g., W. Cai et al., Appl. Phys. Lett. vol. 83, pp. 1705-1710 (1998)
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