A 4-in.-based single-step UV-NIL tool using a low vacuum environment and additive air pressure

Ultraviolet nanoimprint lithography (UV-NIL) is a promising technology for the fabrication of sub-10-nm features. Research has focused on employing a large-area stamp to improve UV-NIL throughput, but a large-area stamp makes it difficult to obtain an acceptable uniform residual layer thickness and/or avoid defects such as air entrapment. This paper presents the development of a single-step UV-NIL tool in which a 4-in. Pyrex stamp is first used to imprint coated resin against a 4-in. Si wafer in a low vacuum environment. Pressurized N₂ is subsequently applied to the wafer bottom to improve the quality of imprint results. This UV-NIL tool was used to successfully imprint a 4-in. stamp with recessed patterns engraved over the entire stamp areas onto a 4-in. Si wafer.     

A step-and-repeat UV-nanoimprint lithography process using an elementwise patterned stamp

Ultraviolet-nanoimprint lithography (UV-NIL) is a promising cost-effective method for defining nanoscale structures at room temperature and low pressure. To apply a large-area stamp to a high throughput step-and-repeat process at atmospheric conditions, we proposed a new UV-NIL process that uses an elementwise patterned stamp (EPS), which consists of elements separated by channels, and additive gas pressurization. The proposed UV-NIL process required just four imprints to press an 8-in. wafer. EPS features measuring 50-80 nm were successfully transferred onto the wafers. The experiments demonstrated that a 5 × 5-in.² EPS could be used with a step-and-repeat UV-NIL process to imprint 8-in. wafers under atmospheric conditions.     

Fabrication of low-cost mold and nanoimprint lithography using polystyrene nanosphere

The periodic arrays of nanostructure were successfully patterned on Si wafers by ultraviolet nanoimprint lithography (UV-NIL) using nanosphere lithography (NSL). Two-dimensional (2D) well ordered self-assembled arrays were obtained on Si wafer by using nanosphere and the tilted-drain method. We tried to combine two techniques and hard mold of Si mold for NIL and polymer mold of acrylate-based polymer were fabricated by NSL. The Si master mold and polymer mold were formed by Cr lift-off and ICP-RIE process. The surface has a low surface energy at the interface with 1H, 1H, 2H, 2H-perfluorooctyl-trichlorosilane (FOTS) vapor-coating, which can eliminate the problem of the adherence to the surface of the mold during demolding. Finally, nanopatterns were formed by UV-NIL, where the residual layer was not observed.     

A novel loading and demoulding process control in UV nanoimprint lithography

In UV nanoimprint lithography (NIL) with elastic mould, a novel multi-step loading and demoulding process, called distortion reduction by pressure releasing (DRPR) and two-step curing method for demoulding, is developed. This novel imprint process is continuous, the pressure releasing method, used to optimize the loading process, can reduce the distortions of imprint mould and wafer stage, while obtain better cavity filling and thin and uniform residual layer; through two-step curing method instead of traditional simple demoulding, the curing degree of resist can be controlled, which is helpful to decrease the demoulding force and avoid residual layer pulled-up while ensure replicated protrusions not collapse. It is a novel and robust process with high fidelity of pattern replication in micro/nano structures fabrication, and the replication error caused by distortions and “blind” demoulding can be reduced effectively.     

Fabrication of a two-step Ni stamp for blind via hole application on PWB

This study examined imprint lithography with a two-step Ni stamp to solve the laser process problems and simultaneously form a blind via and layer pattern. The Ni stamp was fabricated by electroplating on a dry-etched Si mold, made from a SOI (silicon on insulator) wafer, and pattern replication. For the pattern transfer of the Ni stamp, hot embossing was performed on SU8-coated BT and Si wafer substrates. The residual layer was of a uniform thickness with an embossed shape of acceptable squareness.     

Applications of UV-nanoimprint soft stamps in fabrication of single-frequency diode lasers

We show how to use a modified poly-dimethyl-siloxane (PDMS) soft stamp to reduce pattern deformation and residual layer thickness in soft UV-nanoimprint lithography. A soft stamp thinned with toluene reduces the residual layer of a resist by as much as 50% compared to an unthinned stamp. We apply the soft UV-nanoimprint to prepare nanopatterned waveguides for a single-frequency diode laser. This laser operates with a side-mode suppression ratio of 50 dB, which indicates that the patterns are precise and uniform over the whole imprint field. To the best of our knowledge, this is the first single-frequency laser fabricated by soft UV-nanoimprint technology.     

Filling behavior of UV nanoimprint resin observed by using a midair structure mold

The filling behavior of resin during UV nanoimprint lithography (UV-NIL) was observed by using a “midair structure mold” and by changing the imprint pressure. The midair structure molds were fabricated by electron beam lithography (EBL) using hydrogen silsesquioxane (HSQ) as a negative tone resist. After the fabrication of midair structure mold, two types of surface treatment molds, which were with or without release coating, were prepared. Using these molds, the filling behavior of a UV curable resin was investigated at various pressures. The results indicate that a pressure of approximately 1.2 MPa is necessary for complete filling in the case of molds treated with a release agent. This method demonstrates effect of a release coating for UV-NIL.     

p型4H-SiC同质外延有效层厚度

通过化学气相沉积法,采用不同生长工艺在4°偏角4H-SiC衬底上制备p型4H-SiC同质外延片。提出了p型4H-SiC同质外延中有效层厚度的概念,研究发现导致外延有效层厚度减少的直接原因是自掺杂效应的存在。采用傅里叶红外光谱仪(FT-IR)、汞探针电容电压(Hg-CV)和表面缺陷测试仪对p型4H-SiC同质外延片进行表征,讨论了不同工艺对外延有效层厚度的影响。结果表明,采用隔离法和阻挡层法均能提高外延有效层厚度,且掺杂浓度随距表面深度变化斜率值由1.323减小到0.073。然而,阻挡层法斜率值能进一步优化至0.050,是由于有效抑制了外延中固相和气相自掺杂。对比于优化前工艺,采用阻挡层法制备的p型4H-SiC同质外延片厚度不均匀性和表面总缺陷数量处于同一水平,掺杂浓度不均匀性由2.95%改善到2.67%。综上,采用阻挡层法能够制备出高有效层厚度、高一致性和高质量的p型4H-SiC同质外延片。     

紫外纳米压印用PMMA转移层膜的制备及性能

以苯甲醚为溶剂,采用旋涂法制备PMMA(聚甲基丙烯酸甲酯)转移层膜。当PMMA的质量分数为5%、旋涂速度为2000～6000r/min时,转移层膜的厚度为90～150nm,粗糙度为0.3nm,可满足纳米压印要求。采用接触角测量仪测试计算出PMMA、PS转移层膜的表面能,并通过转移层膜与压印胶之间的粘附功和界面张力的计算,评价了PMMA、PS和Si片对压印胶的润湿和粘附性能。结果表明,PMMA膜可改善压印胶在基片上的润湿铺展性能和粘附性能,而PS膜虽能改善基片的润湿铺展性能,却不利于压印胶的粘附。     

UV enhanced substrate conformal imprint lithography (UV-SCIL) technique for photonic crystals patterning in LED manufacturing

The global LED (light emitting diode) market reached 5 billion dollors in 2008 and will be driven towards 9 billion dollors by 2011 [1]. The current applications are dominated by portable device backlighting, e.g. cell phones, PDAs, GPS, laptop etc. In order to open the general lighting market doors the luminous efficiency needs to be improved significantly. Photonic crystal (PhC) structures in LEDs have been demonstrated to enhance light extraction efficiency on the wafer level by researchers [2]. However, there is still a great challenge to fabricate PhC structures on LED wafers cost-effectively. Nanoimprint lithography (NIL) [3] has attracted considerable attentions in this field due to its high resolution, high throughput and low cost of ownership (CoO). However, the current NIL techniques with rigid stamps rely strongly on the substrate flatness and the production atmosphere. Those factors hinder the integration of NIL into high volume production lines. UV-NIL with flexible stamps [4], e.g. PDMS stamps, allows the large-area imprint in a single step and is less-sensitive to the production atmosphere. However, the resolution is normally limited due to stamp distortion caused by imprint pressure.A novel NIL technique developed by Philips Research and Süss MicroTec, substrate conformal imprint lithography (SCIL), bridges the gap between UV-NIL with rigid stamp for best resolution and soft stamp for large-area patterning. Based on a cost-effective upgrade on Süss mask aligner, the capability can be enhanced to nanoimprint with resolution of down to sub-10 nm on an up to 6 inch area without affecting the established conventional optical lithographic processes on the machine. Benefit from the exposure unit on the mask aligners, the SCIL process is now extended with UV-curing option, which can help to improve the throughput dramatically. In this paper, the fabrication of photonic crystal structures with SCIL technique on Süss MA6 mask aligner is demonstrated. In addition, the industrialization considerations of UV-SCIL process in high volume manufacturing are briefly discussed.     

Simulation of fluid flow in the step and flash imprint lithography process

Step and flash imprint lithography (SFIL) is a photolithography process in which the photoresist is dispensed onto the wafer in its liquid monomer form and then imprinted and cured into a desired pattern instead of using traditional optical systems. The mask used in the SFIL process is a template of the desired features that is made using electron beam writing. Several variable sized drops of monomer are dispensed onto the wafer for imprinting. The base layer thickness at the end of the imprinting process is typically about 50 nm, with an approximate imprint area of 1 in². This disparate length scale allows simulation of the fluid movement between the template and wafer by solving governing equations of fluid mechanics simplified by lubrication theory. Capillary forces are also an important factor governing fluid movement; a dimensionless capillary number describes the relative importance of these forces to the viscous forces in the fluid. This paper presents a simulation to model the flow and coalescence of the multiple fluid drops and the effect the number of drops dispensed has on imprint time. The imprint time is shown to decrease with increasing numbers of drops or with an applied force on the template. Appropriate filling of features on the template is an important issue in SFIL, which is presented in this study by simulating the interface movement into and around the feature by a modified boundary condition on the governing equations. It is found that above a critical aspect ratio, features do not fill and fluid does not spread outside the mask edge. The simulation provides a predictive tool for understanding and optimizing fluid management in SFIL.     

Evaluation of filling behavior on UV nanoimprint lithography using release coating

Ultra violet nanoimprint lithography (UV-NIL), which is able to obtain the nano-scale pattern effectively and quickly, is strongly desired for the next-generation lithography technology. However, it is well known that the higher viscosity UV-curable resin with UV-NIL tends to be the shorter obtained pattern without the sufficient transfer pressure. This phenomenon is caused by the filling behavior of UV-curable resin into the UV-NIL mold, thus, the investigation of the filling behavior is very important. In this study, the filling behavior in UV-NIL was observed by using a “midair structure mold”, which is able to eliminate the bubble defect. As a result, it is clear that the filling behavior with low transfer pressure was depended on the capillary force in the mold pattern, which is described by the mold aperture size, the mold surface condition and the resin property.     

A novel overlay process for imprint lithography using load release and alignment error pre-compensation method

As a low-cost and high-efficiency alternative to conventional photolithography, imprint lithography has been paid serious attention for it can realize the pattern transfer easily and repeatedly. Even so, there is still a long way to go before this technique can be used to manufacture ICs since the layer-to-layer alignment is a main obstacle. In this paper, the pattern distortion and position shift induced by loading force and friction force are investigated and an optimized process is proposed to realize high precision overlay, in which an optimal loading force and a load release and an alignment error pre-compensation (LRAEPC) method are adopted. The optimal loading force is acquired by comparing the variety trends of the residual layer and position shift with the loading force. Using the optimal loading force, transferred patterns with proper resist thickness and the minimal distortion are achieved. In LRAEPC method, load release is used to correct the pattern distortion and to alleviate the position shift. To avoid subsequent position adjustment in the fluid after load release, the pre-compensation alignment is performed before the mold is in contact with the wafer. This process does not locate correct alignment position and there is an alignment compensation value determined by the statistical data of alignment position shift after load release. Using this pre-compensation alignment method, the correct alignment can be obtained after load release.     

Development of a very large-area ultraviolet imprint lithography process

We propose a very large-area ultraviolet imprint lithography process as a promising alternative to expensive conventional optical lithography for the production of display panels. This process uses a large-area hard stamp in a low vacuum environment. The hard quartz stamp is used to achieve high overlay accuracy, and the vacuum environment is required to ensure that air bubble defects do not occur during imprinting. We demonstrate that the quartz stamp with microscale patterns can be used for imprinting 18-in. diagonal substrates via single-step UV imprint in a low vacuum environment to obtain a practical residual layer thickness (RLT) for micro pattern transfer to the substrate. Numerical analysis is performed to clarify the physical phenomena underlying imprint process.     

Squeeze time investigations for step and flash imprint lithography

In order to study the dependence of squeeze time on residual layer thickness and feature size and distribution we have experimentally investigated the squeeze time for step and flash imprint lithography for both structured and unstructured templates. It appeared that there are distinct differences between blank and structured templates as far as squeeze timing is concerned. Especially below residual layer thicknesses of 20–25 nm the squeeze time goes up considerably.     

纳米压印光刻技术--下一代批量生产的光刻技术

纳米压印光刻技术已被证实是纳米尺寸大面积结构复制的最有前途的下一代技术之一。这种速度快、成本低的方法成为生物化学、μ级流化学、μ-TAS和通信器件制造以及纳米尺寸范围内广泛应用的一种日渐重要的方法,如生物医学、纳米流体学、纳米光学应用、数据存储等领域。由于标准光刻系统的波长限制、巨大的开发工作量、以及高昂的工艺和设备成本,纳米压印光刻技术可能成为主流IC产业中一种真正富有竞争性方法。对细小到亚10nm范围内的极小复制结构,纳米压印技术没有物理极限。从几种纳米压印光刻技术中选择两种前景广阔的方法——热压印光刻(HEL)和紫外压印光刻(UV-NIL)技术给予介绍。两种技术对各种各样的材料以及全部作图的衬底大批量生产提供了快速印制。重点介绍了HEL和UV-NIL两种技术的结果。全片压印尺寸达200mm直径,图形分辨力高,拓展到纳米尺寸范围。     

紫外纳米压印技术的图形转移层工艺研究

研究了紫外纳米压印技术的图形转移层工艺,通过改变膜厚进行压印对比实验,将转速控制在3000～4000r/min,成功地得到了50nm光栅结构的高保真图形,复型精度可以达到93.75%。同时阐明,纳米压印技术由于具有超高分辨率、低成本、高产量等显著特点,将成为下一代光刻技术(NGL)的主要候选者之一。     

Distortion reduction by load release for imprint lithography

Due to the light source limitation and prohibitive cost inherent in conventional photolithography, various nontraditional patterning technologies, such as imprint lithography, electron beam or X-ray lithography have been attempted over the past 10 years. In this paper, a UV imprint lithography process is introduced for patterning sub-micrometer structures by using a soft PDMS mould, and an imprint experimental device with a loading mechanism driven by PZT for generating a time-variant load is described. As shown experimentally, an increased pressing load will reduce the thickness of the resist layer, leading to a reliable etching-through of the resist. It is found, however, that the mechanical pressing can generate geometrical distortion on the patterned resist mainly due to the elasticity. Incorporated with the use of a low viscosity photo-curable resist, a loading process with a load release step is proposed to reduce the geometrical distortion on the resist patterns. In the loading process, the loading force is partially released after the press peak but before the resist curing. Such a loading process can reduce the elastic distortion while attaining a thin remained resist layer. It is shown that this loading process, called Distortion Reduction by Load Release or DRLR simply can be combined with an imprint process for different patterning areas and feature sizes.     

Press and release imprint: Control of the flexible mold deformation and the local variation of residual layer thickness in soft UV-NIL

We report on a two step soft UV nanoimprint process termed “Press and Release Imprint (PRI) that enables the reduction of both the mold deformation and the local variation of the residual layer thickness, thereby allowing high fidelity pattern replication with a uniform local residual layer thickness. The effect of imprint pressure on the mold deformation, local variation of residual layer thickness as well as required mold release time has been investigated for microscale patterns in the range of 10-100 μm. The potentials of PRI are demonstrated by high fidelity replication of micro-patterns with a uniform residual layer of minimum thickness.     

Compact LED based nanoimprinter for UV-NIL

UV-based nanoimprint lithography (UV-NIL) is a cheap and fast way to imprint patterns ranging from nanometres to micrometres. However, commonly used equipment can be expensive and require a clean room infrastructure. Here we present the design and testing of a simple UV-NIL system based on a light emitting diode. The current design permits imprints of 10 × 10 mm² in size using a 25 × 25 mm² master. This printer can be used in a semi-clean environment such as a laminar flow bench. The imprinter was used to imprint photoresists as well as UV sensitised hydrogels. The best results were obtained using SU-8 photoresist with features down to 50 nm in size, only limited by the imprint master. Patterns in SU-8 resist were also transferred into silicon substrates by reactive ion etching demonstrating its full potential as a lithographic tool.