Laser direct write system for fabricating seamless roll-to-roll lithography tools

Implementations of roll to roll contact lithography require new approaches towards manufacturing tooling, including stamps for roll to roll nanoimprint lithography (NIL) and soft lithography. Suitable roll based tools must have seamless micro- or nano-scale patterns and must be scalable to roll widths of one meter. The authors have developed a new centrifugal stamp casting process that can produce uniform cylindrical polymer stamps in a scalable manner. The pattern on the resulting polymer tool is replicated against a corresponding master pattern on the inner diameter of a centrifuge drum. This master pattern is created in photoresist using a UV laser direct write system. This paper discusses the design and implementation of a laser direct write system targeting the internal diameter of a rotating drum. The design uses flying optics to focus a laser beam along the axis of the centrifuge drum and to redirect the beam towards the drum surface. Experimental patterning results show uniform coatings of negative photoresist in the centrifuge drum that are effectively patterned with a 405 nm laser diode. Seamless patterns are shown to be replicated in a 50 mm diameter, 60 mm long cylindrical stamp made from polydimethylsiloxane (PDMS). Direct write results show gratings with line widths of 10 microns in negative photoresist. Using an FPGA, the laser can be accurately timed against the centrifuge encoder to create complex patterns.Center for Clean Water and Clean Energy at MIT and KFUP

A Lab Scale Polymer Micro-Embossing Machine for Process Control Research

Micro embossing is the process of fabricating micron-sized features by plastic deformation of a work piece by means of a shaped tool or die. In our work we are concerned with hot embossing of PMMA to produce features in the 1-100 µm range, suited for micro fluidic and photonic product requirements. As with any deformation process, hot micro embossing yields products with dimensional variation. In ideal situations, several products can be manufactured repeatedly with the same machine settings and be expected to be exactly the same in every aspect with each other. In reality, final products from the same process might have variations in dimensions and material properties even if the machine settings are kept constant. These can be natural variations of the process or variations caused by external disturbances. Variations can decrease the yield of a manufacturing process and can also restrict the ultimate level of product precision attainable. Manufacturing process control entails the study of the origins of process variations and the use of this knowledge to reduce this variation under production conditions. Manufacturing process control is thus vital in establishing the ultimate cost, quality, rate and flexibility of any manufacturing process. Thus, we are interested in controlling inputs that have a direct effect on the work piece’s final shape and thus the productivity. For hot embossing of PMMA, we are interested in controlling: (1) the temperature of the die and work piece during the entire forming process (forming & cooling), (2) the rate of cooling of the die and substrate, (3) die and substrate platen displacement and displacement rate, (4) force applied on the platen and distribution of this force. The temperature needs to be controlled as it affects the thermo-mechanical behavior of the PMMA. The forming temperature determines the forming properties of the material, the rate of cooling will affect the amount of shrinkage and thermal stresses that the polymer work piece undergoes. The displacement will affect the flow of the material into the die and this will affect the feature size and depths to be embossed. The displacement rate will affect the non-Newtonian polymeric material flow rate into the cavity. It is imperative to carefully control those inputs to minimize variations in forming behavior and final product dimension and properties. An experimental lab scale micro embossing machine to address the needs was designed and fabricated at the Manufacturing Process Control Lab (MPCL) at MIT. An Instron Model 5869 Table mounted materials testing system of capacity 50KN (11250lb) was chosen as the platform for the apparatus and it was modified to accept the forming platens with temperature controllers which have capability of controlling the temperatures of the substrate and master to +/- 1 degree Celsius accuracy. The controller powers 2 x 200 watt heaters that can heat the platens to 150 Celsius in 8 minutes. The Instron has a test speed range of 1 micron to 500mm/min (0.00004in/min to 20in/min) with a 50KN load cell attached to the platen with an accuracy of +/- 1N. The platen cooling system uses water where flow rate will vary the resulting cooling rate. To ensure even heating of the substrate, shape factor analysis was used in the design of the copper platens. The shape factor analysis reveals the heat flow patterns and regions of isotherm within the platens originating from the cartridge heaters and thus using this information, the platens can be designed to have the cartridge heaters located so as to minimize temperature variation of the surface of the platen. The apparatus has been used so far to study hot embossing of sub-millimeter sized features using a copper master on a 1mm thick PMMA material. This presentation will provide greater details on the design of the machine, its initial performance tests and some preliminary process variation experiments.Singapore-MIT Alliance (SMA

Revitalizing us manufacturing to capitalize on innovation

We find that a conventional engineering degree approach to education is not sufficient to meet the new challenges in the ecosystem of manufacturing, design and business innovation, and product realization. Instead a new form of engineering education, the “Professional Masters” is required that takes the grounding provided by typical Bachelor of Science in engineering degree and provides condensed, formalized, experiencewith systems,applications, projects, and non-technical topics to create a true professional ready to maximize their value to the company and ready to use their experience to lead. The Master of Engineering in Manufacturing (MEngM) at MIT was developed over a period of 10 years, and has more than 200 alumni. It is based on the notion of a need for graduate level education in the profession of engineering that is not fulfilled by the conventional research- oriented Master of Science degree. We have learned that there is a large pool of outstanding students who will seek out this degree once it is offered, and who have as alumni drawn strongly positive reviews from their employers. Students in the program are drawn to the notion that manufacturing is how technological advances and innovations become rooted in a nation's economy. They want to understand the essential components and growth opportunities of the foundation - manufacturing and innovation - of an economy. There are many indicators of the decline of manufacturing in the US, most of them economic. One troubling indicator is the persistent lack of interest in careers in this field, particularly at the collegiate and post-graduate level. While there are continual calls for better labor force training and government programs to support the same, there are actually disincentives for promising young professionals to enter this field. Societal perception and industry needs seem to run counter to one another. We propose that the MEngM can serve as one example of a new national model for professional manufacturing engineering education. It can profoundly impact the US’s innovation ecosystem which is the foundation of our manufacturing based economy today and in the future

Multiple Input-Multiple Output Cycle-to-Cycle Control of Manufacturing Processes

Cycle-to-cycle control is a method for using feedback to improve product quality for processes that are inaccessible within a single processing cycle. This limitation stems from the impossibility or the prohibitively high cost of placing sensors and actuators that could facilitate control during, or within, the process cycle. Our previous work introduced cycle to cycle control for single input-single output systems, and here it is extended to multiple input-multiple output systems. Gain selection, stability, and process noise amplification results are developed and compared with those obtained by previous researchers, showing good agreement. The limitation of imperfect knowledge of the plant model is then imposed. This is consistent with manufacturing environments where the cost and number of tests to determine a valid process model is desired to be minimal. The implications of this limitation are modes of response that are hidden from the controller. Their effects on system performance and stability are discussed.Singapore-MIT Alliance (SMA

Measurement and Process Control in Precision Hot Embossing

Microfluidic technologies hold a great deal of promise in advancing the medical field, but transitioning them from research to commercial production has proven problematic. We propose precision hot embossing as a process to produce high volumes of devices with low capital cost and a high degree of flexibility. Hot embossing has not been widely applied to precision forming of hard polymers at viable production rates. To this end we have developed experimental equipment capable of maintaining the necessary precision in forming parameters while minimizing cycle time. In addition, since equipment precision alone does not guarantee consistent product quality, our work also focuses on real-time sensing and diagnosis of the process. This paper covers both the basic details for a novel embossing machine, and the utilization of the force and displacement data acquired during the embossing cycle to diagnose the state of the material and process. The precision necessary in both the forming machine and the instrumentation will be covered in detail. It will be shown that variation in the material properties (e.g. thickness, glass transition temperature) as well as the degree of bulk deformation of the substrate can be detected from these measurements. If these data are correlated with subsequent downstream functional tests, a total measure of quality may be determined and used to apply closed-loop cycle-to-cycle control to the entire process. By incorporating automation and specialized precision equipment into a tabletop “microfactory” setting, we aim to demonstrate a high degree of process control and disturbance rejection for the process of hot embossing as applied at the micron scale.Singapore-MIT Alliance. Manufacturing Systems and Technology Programm

Cycle to Cycle Manufacturing Process Control

Most manufacturing processes produce parts that can only be correctly measured after the process cycle has been completed. Even if in-process measurement and control is possible, it is often too expensive or complex to practically implement. In this paper, a simple control scheme based on output measurement and input change after each processing cycle is proposed. It is shown to reduce the process dynamics to a simple gain with a delay, and reduce the control problem to a SISO discrete time problem. The goal of the controller is to both reduce mean output errors and reduce their variance. In so doing the process capability (e.g. Cpk) can be increased without additional investment in control hardware or in-process sensors. This control system is analyzed for two types of disturbance processes: independent (uncorrelated) and dependent (correlated). For the former the closed-loop control increased the output variance, whereas for the latter it can decrease it significantly. In both cases, proper controller design can reduce the mean error to zero without introducing poor transient performance. These finding were demonstrated by implementing Cycle to Cycle (CtC) control on a simple bending process (uncorrelated disturbance) and on an injection molding process (correlated disturbance). The results followed closely those predicted by the analysis.Singapore-MIT Alliance (SMA

Direct-Write Photolithography for Cylindrical Tooling Fabrication in Roll-to-Roll Microcontact Printing

The scale-up of microcontact printing (μCP) to a roll-to-roll technique for large-scale surface patterning requires scalable tooling for continuous pattern printing with μm-scale features (e.g., 1–50 μm). Here, we examine the process of creating such a tool using an optical direct-write or “maskless” method working on a rotating cylindrical substrate. A predictive model of pattern formation is presented along with experimental results to examine the key control factors for this process. It is shown that factors can be modulated to vary the cross-sectional shape in addition to feature height and width. This feature can then be exploited to improve the robustness of the final printing process.Center for Clean Water and Clean Energy at MIT and KFUP

Process Control in Micro-Embossing: A Review

Abstract— A promising technique for the large-scale manufacture of micro-fluidic devices and photonic devices is hot embossing of polymers such as PMMA. Micro-embossing is a deformation process where the workpiece material is heated to permit easier material flow and then forced over a planar patterned tool. In this work we review the basic process and the state of research with respect to manufacturing process control, where the latter is defined as methods for minimizing variation in the product while maximizing production rate. From this review we conclude the following: Several investigators have reported success at creating micron scale features using this process, but none have performed a formal characterization or optimization of the process.Singapore-MIT Alliance (SMA

Effect of sputtering power on friction coefficient and surface energy of co-sputtered titanium and molybdenum disulfide coatings and its performance in micro hot-embossing

Si micromolds are common for fabrication of polymer-based microfluidic devices by hot-embossing because of the well established fabrication methods for Si, e.g., deep reactive ion etching, for favorable surface finish and accuracy. The problems with low yield, poor reproducibility, premature failure and limited lifetime of a Si micromold are induced by high friction and surface adhesion generated during demolding. Therefore, Titanium (Ti) and molybdenum disulfide (MoS[subscript 2]) coatings were deposited on Si micromolds via magnetron co-sputtering at various combinations of target powers to improve its surface properties. Coating composition, crystallographic orientation, roughness, critical load, hardness, friction coefficient and surface energy were measured by X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy, scratch testing, nanoindentation, ball-on-disc tribometry and the contact angle method respectively. A statistical design of experiment matrix was used to investigate the effect of the Ti and MoS[subscript 2] target powers on the friction coefficient and surface energy of the coatings. From this designed experiment, it was observed that increasing MoS[subscript 2] target power was associated with increasing surface energy and decreasing friction coefficient and target powers had statistically significant effects on these parameters. Crystallinity, roughness and hardness of the coatings increased with increasing Ti concentration. A mathematical model of the effects of Ti and MoS[subscript 2] target powers on the friction coefficient and surface energy of the coatings has been fit to the experimental results using the response surface method. Uncoated and MoS[subscript 2]–Ti coated Si micromolds were used in hot-embossing for a comparative study on replication performance of uncoated and various coated micromolds. Hotembossed PMMA microstructures showed that coating improve replication performance of Si micromolds. Si micromold coated with co-sputter of Ti and MoS[subscript 2] at power of 300 and 75 W respectively, showed better replication quality among the selected target powers

Process Variability in Micro-Embossing

A promising technique for the large-scale manufacture of micro-fluidic devices and photonic devices is hot embossing of polymers such as PMMA. Micro-embossing is a deformation process where the workpiece material is heated to permit easier material flow and then forced over a planar patterned tool. While there has been considerable, attention paid to process feasibility very little effort has been put into production issues such as process capability and eventual process control. In this paper, we present initial studies aimed at identifying the origins and magnitude of variability for embossing features at the micron scale in PMMA. Test parts with features ranging from 3.5- 630 µm wide and 0.9 µm deep were formed. Measurements at this scale proved very difficult, and only atomic force microscopy was able to provide resolution sufficient to identify process variations. It was found that standard deviations of widths at the 3-4 µm scale were on the order of 0.5 µm leading to a coefficient of variation as high as 13%. Clearly, the transition from test to manufacturing for this process will require understanding the causes of this variation and devising control methods to minimize its magnitude over all types of parts.Singapore-MIT Alliance (SMA