Image Reversal Process Research Articles

Challenges in Lift-Off Process Using CAMP Negative Photoresist in III–V IC Fabrication

In III–V IC processing, lift-off patterning is universally used. Historically used negative resist has its limitations; hence the image reverse process using positive tone resist has been utilized as the main work horse in lift-off applications. Metal lift-off processes using a specially designed chemically amplified (CAMP) negative tone resist have been evaluated for thin and thick metal layers to have a varying degree of success. Lift-off using CAMP photoresists has made it possible to reduce cycle time (~57% reduction in $1\times$ theoretical cycle time per layer) and reduce cost (due to elimination of steps) as compared to the image reverse process. However, achieving consistent lift-off and critical dimension (CD) control presents challenges, which are discussed in this paper.

Correction to “Evolution, Challenges and Attributes of Near Micron Sized TaN Resistors for Mixed Signal IC Applications From a Lithography Perspective” [Nov 18 460-466

The abstract in [1] is incorrect. The word “photograph” was used in error rather than the more correct, “photo.” The abstract should read: In this paper, the evolution of various lithographic processes aimed at fabricating robust tantalum nitride (TaN) resistors in GaAs-based BiFET and BiHEMT technologies are discussed in detail. Such approaches include the common image reverse photo process (via amine poisoning) using a positive tone resist, the use of a chemically amplified negative tone resist, and also straight patterning of a positive tone resist. The aforementioned approaches were found to have their own unique challenges as it applied to printing robust TaN-based resistors approaching nearly a micron in width which is needed for high density mixed signal designs. We will explore many of these challenges in detail for the various lithography schemes.

Evolution, Challenges and Attributes of Near Micron Sized TaN Resistors for Mixed Signal IC Applications From a Lithography Perspective

The abstract in [1] is incorrect. The word “photograph” was used in error rather than the more correct, “photo.” The abstract should read: In this paper, the evolution of various lithographic processes aimed at fabricating robust tantalum nitride (TaN) resistors in GaAs-based BiFET and BiHEMT technologies are discussed in detail. Such approaches include the common image reverse photo process (via amine poisoning) using a positive tone resist, the use of a chemically amplified negative tone resist, and also straight patterning of a positive tone resist. The aforementioned approaches were found to have their own unique challenges as it applied to printing robust TaN-based resistors approaching nearly a micron in width which is needed for high density mixed signal designs. We will explore many of these challenges in detail for the various lithography schemes.

Nitride etching with hydrofluorocarbons. II. Evaluation of C4H9F for tight pitch Si3N4 patterning applications

A novel etch chemistry, C4H9F was evaluated for highly selective patterning of SiN using both organic and SiOx masks for line/space and cylindrical geometries. Patterning of 80 nm of SiN using 65 nm of the organic planarization layer (OPL) mask with a C4H9F:O2:CF4 admixture yielded line edge roughness (LER) and line width roughness (LWR) values of 1.9 and 2.8 nm on sub-25 nm critical dimension (CD) SiN lines at 50 nm pitch, with >50 nm mask retention up to 40% overetch values. The LER and LWR values were 60% and 66% lower than features patterned using a CF4:CHF3 admixture, which also exhibited complete OPL mask loss and nitride erosion of 27 nm even at the endpointed etch condition. Using 20 nm of SiOx as a mask and removing CF4 from the chemistry admixture, LER and LWR values of 2.7 and 3.3 nm were achieved with nearly full SiOx retention for features of identical dimensions. At smaller mask pitches (21 and 24 nm) formed by directed self-assembly of high-χ block copolymers, the highly selective nature of C4H9F to organic masks facilitated SiN pattern transfer where CF4 based admixtures failed, achieving line CDs of ∼12 nm with LWR and LER of 1.6 and 2.2 nm. Applied to alternate geometry, hole patterning in SiN at ∼30 nm CD and 60 nm pitch was demonstrated. A subsequent image reversal process yielded pillars by filling the hole pattern with OPL and etching back the surrounding SiN layer, reaffirming the extremely high selectivity of C4H9F to organic masks.

An integrated Mach-Zehnder interferometric sensor fabricated by using Cr mask extension lithography

An integrated Mach-Zehnder interferometric chip operating at 632.8 nm was designed and fabricated using planar rib waveguide technology for optical sensor applications. A rib waveguide with a silicon-oxynitride core layer and silicon-oxide clad layers was geometrically designed to have a singlemode operation and a high surface sensitivity by using an effective index method. A chromium mask layer was employed as an etch stop to protect the core layer of the rib waveguide and was patterned using an image reversal process with a photo mask applied to the sensor zone. The image reversal process was modified in order to obtain a longer chromium mask layer against the sensor zone. We have, therefore, established a fabrication method for an integrated Mach-Zehnder interferometric sensor without the addition of a new photo mask. The optical measurement with this device for a mixture of deionized water (DIW) and phosphate buffer solution (PBS) finally showed a sensitivity of about π/(6.25 × 10−4).

Fabrication of Fine-Line Pattern with Image Reversal Process

This paper describes a simple and effective lift-off method that relies upon a single layer of positive photoresist and lithography technology. We have succeeded in patterning narrow lines in a photoresist film by image reversal process. Image reversal with AZ 5214-E resist is characterized by contact lithography. A process of patterning different line widths was developed based on image reversal technology, using AZ5214-E, followed by pre-bake, exposure, reversal bake, flood exposure and development. We could obtain very neat patterns with 2-5μm dimensions and their relative features have been supported by scanning electron microscope (SEM) pictures. The application of the proposed process is suitable for the electrode fabrication in MEMS SAW devices.

Gold Stripe Optical Waveguides Fabricated by a Novel Double-Layered Liftoff Process

To fabricate uniform and reliable thin gold stripes that provide low-loss optical waveguides, we developed a novel liftoff process placing an additional SiNx layer under conventional photoresists. By patterning a photoresist and over-etching the SiNx, the photoresist patterns become free-standing structures on a lower-cladding. This leads to uniform metal stripes with good reproducibility and effectively removes parasitic structures on the edge of the metal stripe in the image reversal photolithography process. By applying the newly developed process to polymer-based gold stripe waveguide fabrication, we improved the propagation losses about two times compared with that incurred by the conventional image-reversal process.

Spatial-frequency multiplication with multilevel interference lithography

The authors present a large-area spatial-frequency multiplication fabrication process for patterning one-dimensional periodic structures using multilevel interference lithography. In this process, multiple grating levels with different phase offsets are overlaid by aligning to a reference grating. Each grating level is pattern transfered into a single hard mask layer, effectively reducing the grating period. The linewidth of the grating lines is controlled with nanometer repeatability by plasma etching and an image-reversal process. The authors demonstrate overlay accuracy of 0.6±1.9nm over 16×12mm2 for two levels of 200nm period gratings. Using this process, a subdiffraction-limited resolution grating with 100nm period is fabricated using light with λ=351.1nm. This process can also be used to fabricate more complex periodic geometries.

Novel nonlithographic quantum wire array fabrication

We have developed a novel nonlithographic technique for the fabrication of large arrays of quantum wires with complex layered structures that will find applications in many fields. The technique is based on an image reversal process to form an array of tantalum oxide nano-pillars that are used as masks for the etching of epitaxially grown semiconductor layers on an arbitrary substrate. This technique can be used to form quantum wires of complex layered structures that are essential for most practical applications. We have developed the technology to create quantum wires of silicon, however, the technique can be extended to any other material system.

Microstructured UV-sensitive luminescent sol–gel layers

Abstract Carboxylic acid complexes of lanthanide ions may show an excellent sensitivity to UV radiation and often a luminescence quantum yield near unity. By incorporating them into an inert matrix such as a sol–gel layer, it is possible to prevent degradation of the complexes under atmospheric conditions or by UV irradiation. This work focuses on the lateral structuring of thin sol–gel layers which have been doped with two different luminescent complexes, namely terbium picolinate and europium picolinate. It is demonstrated that structuring of the doped layers with standard photolithographic processes is possible, without harming the luminescent properties of the incorporated complexes. Best results are achieved in an image reversal process, while the lift-off approach with a thick layer of photoresist leads to inhomogenities in the layer. Smallest features of 40 μm were reproduced with excellent edge quality.

Imprint/Photo Hybrid Lithography Using Conventional Contact Aligner

Fine patterns were fabricated by a combination of imprint lithography and photo-lithography using a conventional contact aligner. A resist film was pressed at an atmospheric pressure. Since it was difficult to press the mold mask onto the resist film completely, the residual resist was removed by a photolithography process. An image reversal process is used. It was important that the press was carried out on an unbaked resist. Patterns were exposed to a mercury lamp without a monochromatic filter during the atmospheric pressure press. After the photolithography no residual resist could be found. Good patterns can be obtained up to approximately 0.5 µm. A 0.35 µm line and 0.65 µm space pattern can also be fabricated, but the resist thickness of the fabricated pattern is less than the initial resist thickness.

Arrays of holes fabricated by electron-beam lithography combined with image reversal process using nickel pulse reversal plating

A critical issue in fabricating arrays of holes is to achieve high-aspect-ratio structures. Formation of ordered arrays of nanoholes in silicon nitride was investigated by the use of ultrathin hard etch mask formed by nickel pulse reversal plating to invert the tonality of a dry e-beam resist patterned by e-beam lithography. Ni plating was carried out using a commercial plating solution based on nickel sulfamate salt without organic additives. Reactive ion etching using SF6/CH4 was found to be very effective for pattern transfer to silicon nitride. Holes array of 100 nm diam, 270 nm period, and 400 nm depth was fabricated on a 5×5 mm2 area.

Fabrication Process for Surface Micromachine with Small Sticking Problem by Use of Image Reversal Resist as Sacrificial Layer

A new process is developed for a surface micromachine fabrication with a small sticking problem. Electroplated Ni film is used as maicromachine material and a resist for the image reversal process is used as a sacrificial layer. The processes for the image reversal are carried out before making movable structures. The resist is developed after making movable structures. Movable structures are fixed by small resist columns when the development of the sacrificial layer resist is finished. Finally, the small columns are removed by O2 plasma. Cantilevers with long and wide beam are fabricated by the new process. The cantilever with 1000μm length and 100μm width beam can be fabricated without the sticking problem. The mechanical properties of the electroplated Ni film are examined. It is found that the mechanical properties of the electroplated Ni film is almost same as those of a bulk Ni.

Semiconductor Nanostructures defined by self-organizing Polymers

AbstractIn the near future it will be more and more important to produce real nanometer-sized structures for semiconductor devices (e.g., quantum dot lasers) but also for nano-biomechanical applications like the so-called total analysis system implemented on one chip.We describe here a technique to create nanometer-sized structures in semiconductors and metals by the use of self-assembling diblock copolymers as nano-lithographic masks. Semiconductor quantum structures with very high aspect ratio of 1:10 were fabricated from III-V semiconductor heterostructures by anisotropic dry etching. In a first step, so-called diblock copolymer micelles were generated in a toluene solution. These micelles were loaded by a noblemetal salt. With a “Langmuir Blodgett” technique we can decorate complete wafers with a monolayer of highly ordered micelles, covering almost the complete surface. After treatment in a hydrogen plasma all of the organic components are removed and only crystalline metal clusters of ~12 nm size remain. This metal cluster mask can be used directly in a highly anisotropic chlorine dry etching process to etch cylinders in GaAs and its In and Al alloys. It is also possible to etch through a quantum well layer underneath the surface in order to produce quantum dots.By evaporating metals and applying a wet chemical image reversal process, we can invert the etched structure and generate a gauzy gold film with nano-holes inside. It is thinkable to use this porous gold film as a nano-filter in upcoming nano-biotechnology applications.

Fabrication of a low-operating voltage diamond thin film metal–semiconductor–metal photodetector by laser writing lithography

An ultraviolet-sensitive photodetector based on the metal-semiconductor-metal structure has been fabricated on a chemical-vapor-deposited diamond thin film grown on a silicon substrate. Device processing techniques employed include maskless laser writing lithography, image reversal processing and sacrificial layer inclusion, which resulted in devices with an electrode finger separation of only 2.5 μm. This allows for a low operating voltage of only 5 V, which is very much lower than what has been achieved so far.

Methods for fabricating arrays of holes using interference lithography

Optical interference lithography provides a robust patterning technology capable of achieving deep submicron resolution over extremely large field sizes (∼1 m2). Here, we compare two approaches for fabricating arrays of holes using interferometric techniques. We first show that, by applying an image reversal process to standard two-beam interference lithography, arrays of high aspect ratio holes (2:1) can be generated. This process scales well to submicron periods and allows holes as small as 0.1 μm to be patterned. Next, we present an analysis of the multiple-beam approach for patterning holes. This technique offers a potentially higher throughput process compared to other techniques. We demonstrate that, while the formation of higher contrast intensity profiles is possible by interfering four or more beams, the shape and modulation depth of such profiles are sensitive to relative phase variations. This dependence complicates the application of multiple-beam techniques for patterning large uniform arrays of resist structures.

Sub-10-nm Si Lines Fabricated Using Shifted Mask Patterns Controlled with Electron Beam Lithography and KOH Anisotropic Etching

We developed a new method for Si nanofabrication that provides sub-10-nm resolution using electron beam lithography combined with KOH anisotropic etching of a Si(1\\overline10) substrate. The method, called shifted mask pattern method, uses two shifted mask patterns to reduce the Si linewidth. The two shifted mask patterns delineated in the <11\\overline2> direction are used as a KOH etching mask where the masks are shifted vertically to each other in the <11\\overline2> direction. During KOH anisotropic etching, Si linewidth under the shifted part of the masks is reduced due to the etching's smoothing resulting from the low etching rate of the Si(111) plane. The linewidth can be controlled by changing the mask shift. An image reversal process using electron cyclotron resonance (ECR) plasma oxidation was used to form a plasma oxide mask for KOH etching. This mask was obtained by ECR oxygen plasma irradiation using a ZEP520 resist mask pattern. Highly accurate position and linewidth c ontrol was achieved with a 70-kV electron beam nanolithography system that has a minimum deflection step of 2 nm. The variation of pattern position and linewidth was about 1 nm. Using this method, we have been able to fabricate lines as narrow as 2 nm.

Simulation of the image reversal submicron process in integrated circuit fabrication

An algorithm for the simulation of the image reversal process based on Cellular Automata is presented for the first time. The development resist profiles produced by this algorithm are of the same geometry and shape as those obtained by experiments. Furthermore, the dependence of the slope of a resist-line-edge on the bake time has also been successfully reproduced using this algorithm.

An Electron Beam Nanolithography System and its Application to Si Nanofabrication

We present an electron beam nanolithography system which features sub-10-nm beam size over a large 480×480 µ m2 field and a high 70 kV acceleration voltage with a Zr/O/W thermal field emitter tip. A beam can be deflected at 100 MHz in 2-nm steps, which allows the use of highly sensitive resist. The system is equipped with a highly sensitive YAG detector for electrons backscattered from a registration mark as well as a C/W multilayer knife edge for beam size measurement and focusing. These techniques achieve a beam size of about 6 nm. A 10-nm-scale resist pattern was obtained using ZEP520 resist with this system. Furthermore, Si nanostructures have been obtained by using an image reversal process with ECR plasma oxidation. Photoluminescence was observed from the Si nanowires fabricated with this system.

Si nanostructures fabricated by electron beam lithography combined with image reversal process using electron cyclotron resonance plasma oxidation

A new image reversal process has been developed for Si nanodevice fabrication that uses electron beam lithography and electron cyclotron resonance (ECR) plasma techniques. This process is based on Si oxidation with an ECR oxygen plasma through the openings in resist mask patterns. Si on SiO2 is selectively etched by either Cl2-based ECR plasma etching or KOH anisotropic etching by using a plasma oxide mask. ECR plasma formed silicon oxide with a thickness of 2–3 nm was found to be an excellent etch mask for these etching techniques. Highly directional ECR oxygen plasma keeps the change in the resist linewidth and edge roughness small enough for nanofabrication. Furthermore, the linewidth of reversed Si patterns can be reduced by SF6 addition to Cl2 in ECR plasma etching. This image reversal process successfully achieves 10-nm-scale Si wires and pillars.