Dawn M. Skala

Mechanical properties of CRETE, a polyurethane foam

The room-temperature mechanical properties of a closed-cell, polyurethane encapsulant foam were measured as a function of foam density. Over the range of densities examined, the modulus could be described by a power-law relationship with respect to density. This power-law relationship was the same for both tension and compression testing. The basis for this power-law relationship is explained in terms of the elastic compliance of the cellular structure of the foam using a simple geometric model put forth by Gibson and Ashby. The elastic collapse stress, a property relevant to compression testing, also is found to exhibit a power-law relationship with respect to density. The density dependence of this property is also found in the work of Gibson and Ashby and is explained in terms of the Euler buckling of the struts that comprise the cellular structure. Energy absorption during deformation is also reported for both tension and compression testing.

Resist substrate studies for LIGA microfabrication with application to a new anodized aluminum substrate

Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate the successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 µm, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 µm. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 µm depending on the resist thickness.

LIGA: metals, plastics, and ceramics

LIGA, an acronym from the German words for Lithography, Electroforming, and Molding, is being evaluated worldwide as a method to produce microparts from engineering materials. Much of the work to date in LIGA has focused on producing metal microparts, with nickel as the most common material of choice. There is a growing interest in producing plastic parts replicated from LIGA metal masters due largely to microanalytical instrumentation and medical applications. These plastic replicates are generally made by either hot embossing or injection molding. Ceramic replication, of particular interest for high temperature applications or to produce piezoelectric or magnetic microparts, is also emerging as an area of interest. In this paper, a model of the LIGA exposure and development processes is presented along with the result of numerical optimization of mask design and process cost. The baseline processes for a cost- effective method to produce metal microparts are discussed, along with replication methods and result for plastics and ceramics.

Investigation of sidewall cracking in PMMA LIGA structures

During x-ray exposure in the LIGA process, the polymethylmethacrylate (PMMA) photoresist undergoes chain scission, which reduces the molecular weight of the exposed materials. Under some exposure and development conditions, sidewall cracking is observed on the PMMA sidewall, creating surface texture that is undesirable. In this research, exposed and developed PMMA sidewalls were examined for evidence of crack formation using optical profilometry. PMMA thickness, exposure dose and delay time between the end of exposure and beginning of development were varied. Our analysis of samples, with three different radiation doses and four different delay times from the end of exposure to the beginning of development, indicate that the first occurrence of cracking and the extent of cracking are affected by both the dose and the development delay time. This work includes the examination of the depth of cracks into the PMMA, distance between cracks, the width of cracks and the relationship between crack occurrence and dose profile. An empirical predictive model to correlate the delay time to the observance of sidewall cracking based on the deposited dose is presented. This information has direct implication for predicting processing conditions and logistics for LIGA fabricated parts.

Final-part metrology for LIGA springs, Build Group 1.

The LIGA spring is a recently designed part for defense program applications. The Sandia California LIGA team has produced an initial group build of these nickel alloy parts. These are distinctive in having a macroscopic lateral size of about 1 cm, while requiring microscopic dimensional precision on the order of a few micrometers. LIGA technology capabilities at Sandia are able to manufacture such precise structures. While certain aspects of the LIGA process and its production capabilities have been dimensionally characterized in the past, [1-6] the present work is exclusive in defining a set of methods and techniques to inspect and measure final LIGA nickel alloy parts in large prototype quantities. One hundred percent inspection, meaning that every single LIGA part produced needs to be measured, ensures quality control and customer satisfaction in this prototype production run. After a general visual inspection of the parts and an x-ray check for voids, high precision dimensional metrology tools are employed. The acquired data is analyzed using both in house and commercially available software. Examples of measurements illustrating these new metrology capabilities are presented throughout the report. These examples furthermore emphasize that thorough inspection of every final part is not only essential to characterize but also improve the LIGA manufacturing process.

An aluminum resist substrate for microfabrication by LIGA.

Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development, and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development, and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 {micro}m, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 {micro}m. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 {micro}m depending on the resist thickness.