BPN a new thick negative photoresist with high aspect ratio for MEMS applications

The BPN is a negative photoresist, sensitive in the UV at 365 nm and was previously dedicated for Wafer Level Packaging (WLP) applications. This photoresist offer the advantage of forming thick layers, however, it suffers from low aspect ratio (2:1 declared by the supplier). This work reports the optimization of BPN’s technological process enabling forming 30–160 μm thick molds for electroplating purposes. Our results revealed an aspect ratio as high as 16:1 while having vertical sidewalls using conventional photolithography.     

Novel positive-tone thick photoresist for high aspect ratio microsystem technology

 A methacrylate copolymer combining chemically amplified concept and casting technique was developed as a novel thick photoresist for the UV-LIGA process. Photoresist layers up to 500 μm in thickness can be fabricated easily. Microstructures fabricated by the novel thick photoresist were demonstrated. At present, the ring-shape microstructures with 150 μm tall and 15 μm wide have been realized and the calculated aspect ratio is 10. Received: 10 August 2001/Accepted: 24 September 2001     

Releasing high aspect ratio SU-8 microstructures using AZ photoresist as a sacrificial layer on metallized Si substrates

This paper presents a successful method for releasing high aspect ratio SU-8 micro-structures by the use of positive photoresist (AZ 4620) as sacrificial layer. The AZ 4620 photoresist sacrificial layer was dissolved by the SU-8 developer (propylene glycol monomethyl ether acetate). Thus, this process reduces the need for complex microfabrication steps and equipments which are otherwise required in traditional methods using metal sacrificial layers. The current method is both cost-effective and time-effective because no additional releasing method or material is needed to remove the fabricated SU-8 structures. Further, the influence of surface energy on the adhesion between Si and SU-8 was demonstrated and metallic thin layer coating on Si was employed to further reduce the lift-off duration. The results obtained showed that the duration for lift-off of SU-8 structures from metal (Al) coated Si substrate is much lower (approximately 90 % time saving) and the surface morphology of the released structures has lesser micropore concentration compared to the process employing bare Si as the substrate. In both processes AZ 4620 was the sacrificial layer whereas the metalized Si substrate could be re-used.     

A novel method to overcome photoresist collapse with high aspect ratio structures

A novel method based on electromagnetic wave (EW) was firstly proposed to dry photoresist with high aspect ratio after electron beam lithography. This method used EW to penetrate photoresist and heat the water stored between resist patterns directly, then the water evaporated with absorbing energy of EW. An array of 15,625 pillars and lines with 14.9 nm width of aspect ratio 13 and 17 respectively were dried successfully. By analyzing the heating mechanism, we demonstrated that the EW can decrease surface tension of water effectively and may be applicable for cleaning via hole and the inner wall of carbon nanotubes.     

Aqueous base developable: easy stripping, high aspect ratio negative photoresist for optical and proton beam lithography

A variety of different photo resists are used for the fabrication of polymer and metal high aspect ratio structures. Among them SU-8, a chemically amplified negative tone photoresist is the mostly used. However, after processing the finished resist pattern (SU-8) is hardly removed from the substrate. In the present work the formulation and process optimization of a negative tone chemically amplified photoresist (TADEP) is presented. TADEP resist owns two advantages: the dissolution of the uncrosslinked areas in IC standard aqueous developers and the easy stripping in acetone by the assistance of ultrasonic bath. The TADEP resist is successfully applied for the fabrication of polymer and metal structures, after electroplating and stripping and also in the case of Proton Beam Writing.     

A new removable resist for high aspect ratio applications

A variety of different photo resists are used for fabrication of MEMS. Presently good results were reported for SU-8, a chemically amplified negative tone photoresist. But SU-8 has a disadvantage for some applications in LIGA technique, especially in the X-ray mask fabrication. After processing the finished resist pattern are hardly soluble from the substrate. This paper will briefly describe the current status of the development of the new negative tone photoresist CAR 44 whose big advantage is the easy removableness of the cross linked pattern. This work widely uses the contents of the presentation “A New Removable Resist for High Aspect Ratio Applications” to the High Aspect Ratio Micro Structure Technology workshop HARMST 2005 held in Gyeongyu (Republic of Korea), June 10–13, 2005.     

Developing new markets for high aspect ratio micro-machined devices

 This paper reports on the commercialization strategy being developed by MEMStek Products, LLC (formerly CONCIS) to startup a component business using sacrificial LIGA (S-LIGA) as the initial manufacturing technology. S-LIGA provides the ability to fabricate fluidic, mechanical, and optical components and modules in the mesoscopic scale that exists between conventional silicon micro-machining and precision machining tools such as molding, stamping, and embossing. S-LIGA technology commercialization has some unique challenges. The S-LIGA technology must be developed from its current state into a viable manufacturing process. Since nearly every device fabricated sees x-ray exposure at least once, this creates the need for a production oriented synchrotron facility with the capacity for handling substantial wafer volume. Reliably electro-plating into 0.01 mm feature size becomes a major technical challenge to overcome. Finally, targeting the initial applications for the market requires a careful selection of initial customers and partners. Received: 25 August 1997/23 November 1997     

Micro-ECM for production of microsystems with a high aspect ratio

The most used processes for production of microsytem components are basically from the semiconductor technology. The material properties of used silicon often dont achieve the demands of for example: micro-surgery, biotechnology life science, fluidics or high temperature environment. For microstructuring of highly stressed metals, like stainless and heat resisting steels, cold work tool steels, hot work tool steels and nickel-base-alloys and a variety of metals there is no manufacturing-process. An interesting possibility for structuring this type of materials are the electrochemical machining processes (ECM). Some new developed ECM-sinking-processes are working with oscillating tool-electrode, to improve shape accuracy.     

Electropolishing as a method for deburring high aspect ratio nickel RF MEMS

High aspect ratio nickel radio frequency microelectromechanical systems (RF MEMS) were fabricated by X-ray lithography and electroplating. Control of growth during electroforming of micro components is in general a problem in terms of achieving homogeneous thickness due to a non-uniform current distribution across a layout. It is necessary to level the deposited layer by means of a lapping process which results in burr formation. To ensure the functional properties of the devices these burrs have to be removed. Electropolishing with a current density of 80 A/dm² is used for burr removal from nickel micro components containing small width (~10 μm) structural elements. The investigated metal removal rates range from 0.2 to 1.8 μm/s depending on burr formation, presence or absence of resist and device position in the layout during electropolishing. Furthermore, edge rounding, a common electropolishing effect, is only observed when electropolishing in the absence of resist.     

Challenges with high aspect ratio nanoimprint

When nanoimprint is not used for lithography purposes (NIL), but for the direct patterning of polymeric layers, high aspect ratio patterns may be of interest for a number of applications. The definition of such patterns in a nanoimprint process deals with two aspects, a successful filling of the high aspect ratio cavities of the stamp used, followed by a successful separation of the high aspect ratio structures defined in the polymeric layer on the substrate, from the stamp. These two aspects are addressed by shedding light to the impact of capillary effects during the filling of high aspect ratio cavities, and to the deformation processes involved in the separation of the stamp from the polymeric structures, where adhesional energies have to be overcome without cohesional failure. Both aspects are discussed in terms of the geometries involved, the stamp geometries as well as the polymeric layer thickness, and correlations with thermally-assisted (T-NIL) and UV-assisted (UV-NIL) processing are deduced. The aspects discussed are typical of a nanoimprint situation with thin polymeric layers on hard substrates.     

A new micromachined sensor system for tactile measurements of high aspect ratio microstructures

A new tactile sensor with piezoresistive read-out is presented. The sensor is designed for measurements of high aspect ratio structures with a resolution of some 10 nm and a measuring range of hundreds of micrometer. Possible applications of the sensor are suggested. The silicon micromachining fabrication process is shown in detail next to the finite element simulations we performed. First measurements and a calibration process are described and the results are shown. The implementation into a measuring system is indicated.     

Metrology of high aspect ratio MEMS

Lack of metrology tools for inspecting high aspect ratio MEMS severely limits the degree to which tolerances of a given part can be examined. Tools such as SEMs, AFMs, vision-based systems, and profilometers are good at examining two-dimensional entities of a part or for calculating surface roughness characteristics. None of these tools, however, can extract full three-dimensional data sets of high aspect ratio MEMS for part inspection. The hardware is either limited by the steep sidewalls of the part or by the simple fact that the acquisition method only collects two-dimensional data. This research proposes a method for extracting three-dimensional information from a part using multiple two-dimensional pointclouds. A fiducial setup is proposed which would allow for the registration of multiple pointclouds. A computer-aided inspection (CAI) software platform has been developed to handle the multiple data sets. The software platform implements a least-squares localization routine to compute the best-fit deviations from the nominal CAD geometry, as well as algorithms to determine the correct alignment between two pointclouds. With this software platform, both form errors of a single pointcloud and geometric errors with respect to multiple pointclouds can be calculated.This work was partially funded by Sandia National Laboratories and the National Science Foundation under Grant Number DMI-9988664. The government has certain rights in this material. Any opinions, findings and conclusions or recommendations are those of the authors and do not necessarily reflect the views of the Sandia National Laboratories or National Science Foundation.This paper was presented at HARMST 2003 in June 2003.     

The micromilling process for high aspect ratio microstructures

High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.) The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures.     

The micromilling process for high aspect ratio microstructures

 High aspect ratio microstructures are currently created by several processes which include lithography (X-ray, deep ultraviolet, etc.) and mechanical machining (diamond machining, microdrilling, etc.). The lithographic processes require more extensive processing equipment such as an energy source, mask/mask holder/mask aligner, photoresist and substrate, and chemical development capacity. In addition, these processes are serial in nature and each adds to the tolerances of the finished structure. The current mechanical processes provide for the direct removal of the substrate material in a single step but are more limited in the geometric patterns which can be created. In conventional machining, the process which provides the most versatility in geometric patterns is milling. The micromilling process has two basic components. The first is the fabrication of small milling cutters with very sharp cutting edges. The second is the actual removal of the workpiece material with a very precise and repeatable machine tool. Several basic cutter designs have been fabricated using focused ion beam micromachining and are undergoing testing. The cutter diameters are nominally 100 micrometers and 22 micrometers. Results have been obtained which show that this process can be very effective for the rapid fabrication of molds and mask structures. Received: 30 October 1995 / Accepted: 4 March 1996     

New development strategies for high aspect ratio microstructures

 In the first step of the LIGA process a resist layer, typically PMMA (polymethylmethacrylate), is pattered by deep X-ray lithography. The exposed parts are subsequently dissolved by an organic developer. The quality and the achievable height of the microstructure is decisively determined by the development process. In order to increase the aspect ratio and maintain the quality of the microstructures the parameters influencing the development process were investigated. In the case of dip development and ultrasound development a strong dependency of the development rate on the temperature, dose value and depth of deposition has been noticed. The development rate increases with increasing dose value and temperature and decreases with increasing depth of deposition. In case of dip development the development course can be described by a phenomenological equation which considers the three mentioned parameters. In the case of ultrasound further parameters have to be taken into account: the geometry and the dimensions of the strucutres. Received: 25 August 1997/Accepted: 3 September 1997     

Polysilicon sacrificial layer etching using ClF3 for thin film encapsulation of silicon acceleration sensors with high aspect ratio

We present a new thin film encapsulation technique for surface micromachined sensors using a polysilicon multilayer process. The main feature of the encapsulation process is that both the sacrificial layer above the silicon sensor structure and the cap layer consist of epitaxial polysilicon. The sacrificial layer is removed by chlorine trifluoride (ClF₃) plasmaless gas-phase etching through vents within the cap layer. The perforated cap membrane is sealed by a nonconformal oxide deposition. The method has been applied to a silicon surface micromachined acceleration sensor with high aspect ratio structures, but is broadly applicable. Capacitance–voltage measurements have been performed to show the electrical functionality of the accelerometer.     

Manufacturing of microstructures with high aspect ratio by micromachining

The development of micromachining plays an important role in miniaturization of microsystems. Micromachining is a very flexible and compared to EDM or ECM a very fast machining process with a high material removal rate. A wide range of materials like polymers, metals and alloys as well as some sorts of ceramics can be machined. Also 3D-structures can be easily manufactured. Additionally, big advances have been made concerning the realization of high aspect ratios for small diameter end mills. Whereas end mills below 100 μm diameter are limited to an aspect ratio of 1.5, end mills of 100 μm diameter are available up to an aspect ration of ten now. A few years ago, end mills in this diameter range were uncoated. Nowadays, there was a big progress in the coating technology so that these end mills can be coated with layers as thin as 0.5 μm.     

Micromechanical machining of high aspect ratio prototypes

 Micromechanical machining uses physical cutting tools in high precision machines to fabricate parts with micrometers features and sub-micrometer tolerances. An advantage of this process is the ability to use any machinable material, quick process planning and material removal, and three-dimensional geometry only limited by the machine and tools used. Disadvantages are that forces are placed on micro cutting tools causing deflection and possible breaking. Deflection reduces process precision and tool breakage results in repeated set up, slower production, and poorer tolerances. Nevertheless, these processes have created many diverse prototypes ranging from biomedical to space applications. Received: 10 August 2001/Accepted: 24 September 2001     

Fabrication of a high aspect ratio nanoporous array on silicon

In this study, a simple method for the fabrication of high aspect ratio silicon nanoporous arrays is developed. A N-type silicon wafer is used as the substrate material. A micro-scale pattern of the desired porous array is transferred to the front surface of the silicon wafer by photolithography after which the wafer is placed in a home-made fixture to efficiently expel the etching generated air and promptly hold the back-side illumination light. A halogen lamp is used as the light source for backside illumination to enhance the electron–hole pair generation. An anodization process is then carried out using a new etchant consisting of hydrofluoric acid and mixed EtOH and EMSO surfactant to effectively polish the pore surfaces and sharpen the tips of the etched pores. A nanochannel array with a nano-tip of 61.4?nm is obtained.     

Ultraviolet embossing for patterning high aspect ratio polymeric microstructures

In ultraviolet (UV) embossing, a substrate with a coating of liquid or semi-solid UV curable resin mix is pressed against a patterned embossing mold. The resin mix is irradiated with UV before demolding of the hardened microstructures. UV embossing can be done at room temperature and low pressure. However, demolding of UV embossed high aspect ratio microstructures from a metallic mold is typically difficult since there is no differential thermal contraction between the mold and the embossing. Several factors have been identified to influence demolding of UV embossed microstructures: (1) Roughness of mold, (2) Taper angle of microstructures of mold, (3) Chemical interaction between mold and embossing, (4) Tensile and crosslinking shrinkage properties of the irradiated resin and (5) Uniformity of crosslinking process through the thickness of the molded microstructures. By controlling these five parameters, a microarray with an aspect ratio of 5 was demonstrated using a Formulation containing epoxy acrylate, Irgacure® 651, silicone acrylate and other acrylates. The embossed microstructures replicated the features of the mold very well. It was also shown that by controlling the amount of irradiation, the tensile modulus of the UV formulation increased whilst the elongation decreased. An optimum irradiation is needed for clean demolding from the mold without microcracking.This research was supported by a Strategic Development Scheme fund (SDS 15/2001) from the Nanyang Technological University. The authors also acknowledge the kind contributions of chemicals by UCB Chemicals, Sartomer, Henkel (Singapore), Dupont (Singapore) and Ciba Chemicals and a microstructured mold by Dr R. C. Liang of SiPix Imaging (CA, USA). The second author acknowledges the financial support of Nanyang Technological University through a Research Scholarship.