Johannes Faas Burger

The black silicon method II: The effect of mask material and loading on the reactive ion etching of deep silicon trenches

Very deep trenches in Si with smooth controllable profiles are etched using a fluorine-based Reactive Ion Etcher(RIE). The effect of various mask materials and loading on the profile is examined using the Black Silicon Method. It is found that most metal layers have an almost infinite selectivity. When the aspect ratio of the trenches is beyond five, RIE lag is found to be an important effect. Evidence is found that this effect is caused by the bowing of incoming ions by the electrical field.

Fabrication of a micro cryogenic cold stage using MEMS-technology

This paper describes the design and production process of a variety of reliable micro cryogenic coolers. The different cold stages are based on an optimized design found during a study which was done to maximize the cold-stage effectiveness. Typical cold-stage dimensions are 30 × 2 × 0.5 mm with an expected net cooling power varying from 10 mW to 20 mW at a tip temperature of 96 K. A cold stage consists of a stack of three fusion bonded D263T glass wafers. The production process has 7 lithography steps and roughly 100 process steps. In order to determine the maximum bend, shear and bond stresses inside a 175 µm thick D263T glass wafer, several pressure tests were performed.

Pressure drop of laminar gas flows in a microchannel containing various pillar matrices

The pressure drop of gas flows in a microchannel filled with a dense pillar matrix was investigated with specific attention to a pillar shape. Pillars of height 250 µm and aspect ratio of about 10 were etched in silicon using an optimized Bosch deep reactive ion etching process. The pressure drop head-loss coefficient due to compression and expansion of gas at the inlet and outlet of the pillar matrix was estimated to be about 1.4 for an opening ratio of 10. A comparison of friction factor correlations for circular pillar cross-sections agreed rather well with the correlations proposed for the macroscale. Experimentally determined friction factor correlations for several pillar cross-sections for Reynolds numbers in the range of 50–500 are presented. Among the various pillar cross-sections considered, sine-shaped pillars have the lowest friction factor. These pillar structures with low pressure drop but a rather large wetted area can be used quite effectively as regenerative materials enabling the development of microcryocoolers.

Vibration-free 5 K sorption cooler for ESA's Darwin mission

ESAs Darwin mission is an Infrared Space Interferometer that will search for terrestrial planets in orbit around other stars. It uses six free-flying telescopes that are stabilized with respect to each other to less than 10 nm by utilizing micro-Newton ion thrusters. As a consequence, hardly any vibration of the optical system with integrated cryocoolers can be tolerated. A sorption cooler is a favorite cooler option because it has no moving parts and it is, therefore, essentially vibration-free. An efficient two-stage helium/hydrogen sorption cooler is proposed with a cooling power of 10 mW at 5 K. It needs only 3 W of input power and applies two passive radiators at 50 and 70 K. Application of such low-temperature radiators is made possible by Darwins far-away orbit L2 where earth-radiation is limited. In this paper, first Darwins cooler requirements are discussed and different cryocooler options are compared. Next, sorption cooler operation is explained, after which six different sorption cooler configurations are described and compared.

Characterization of micromachined cryogenic coolers

Micro cryogenic coolers can be used to cool small circuitry and improve their performance. The authors present a variety of micro coolers which are fabricated using MEMS technology production processes only. The typical dimension of a micro cold stage is 30 × 2.2 × 0.5 mm. It cools down to 96 K, applying Joule–Thomson expansion in a 300 nm high flow restriction and has a cooling power ranging from 10 mW to 25 mW. This paper discusses the operation of the micro cold stage and the characterization measurements done.

Membranes fabricated with a deep single corrugation for package stress reduction and residual stress relief

Thin square membranes including a deep circular corrugation are realized and tested for application in a strain-based pressure sensor. Package-induced stresses are reduced and relief of the residual stress is obtained, resulting in a large pressure sensitivity and a reduced temperature sensitivity. Finite element method simulations were carried out, showing that the pressure-deflection behaviour of the structure is close to that of a circular membrane with clamped edge but free radial motion.

Long-life vibration-free 4.5 K sorption cooler for space applications.

A breadboard 4.5 K helium sorption cooler for use in vibration-sensitive space missions was developed and successfully tested. This type of cooler has no moving parts and is, therefore, essentially vibration-free. The absence of moving parts also simplifies scaling down of the cooler to small sizes, and it contributes to achieving a very long lifetime. In addition, the cooler operates with limited dcs so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESAs Darwin mission, a space interferometer in which the sensitive optics and detectors can hardly accept any vibration. The system design consists of a hydrogen stage cooling from 80 to 14.5 K and a helium stage establishing 5 mW at 4.5 K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5 W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80 K-radiators which are constructed at the cold side of the spacecraft. We developed, built, and tested a demonstrator of the helium cooler. This demonstrator has four sorption compressor cells in two compressor stages. Test experiments on this cooler showed that it performs within all specifications imposed by ESA. The cooler delivered 4.5 mW at 4.5 K with a long-term temperature stability of 1 mK and an input power of 1.96 W. So far, the cooler has operated continuously for a period of 2.5 months and has not shown any sign of performance degradation.

Insight into clogging of micromachined cryogenic coolers

Cryogenic microcoolers can be used to cool small electronic devices to improve their performance. The authors present a micro-cold-stage of only 0.05cm3 that cools to 96K, applying Joule-Thomson expansion in a 300nm high flow restriction. Critical in such a microcooler is the deposition of water molecules that migrate to the restriction and block the flow. Because the microcooler is made of glass the authors had the unique opportunity to monitor this phenomenon and combine this visualization with experimental data. This provides significant insight in the way this clogging develops and opens possibilities to realize stable operation.

Thermodynamic considerations on a microminiature sorption cooler

The sorption/Joule-Thomson cycle is a promising cycle for microscale cooling of low-temperature electronic devices because the cycle lacks moving parts. This facilitates scaling down to small sizes, eliminates interferences, and contributes to achieving a long life time. A thermodynamic analysis is presented in which the behaviour of compressor and cold stage are analysed separately, leading to a better understanding of sorption coolers. Some fundamental possibilities to improve the thermodynamic efficiency are discussed, and as a part of this a novel two stage compressor concept is proposed.

Miniature 10-150 mW Linde-Hampson cooler with glass-tube heat exchanger operating with nitrogen

Two demonstrator versions of a small Linde-Hampson cooler with glass tube heat exchanger are presented. The fabrication of these coolers are described and experimental results are presented and discussed. One cooler with a length of 270 mm and an outer diameter of 0.67 mm has a cooling power of 60 mW at 88 K with a mass flow of 7.3×10−6 kg/s (0.35 ln/min1) nitrogen gas. The lowest temperature measured is 82 K at a mass flow of 4.2×10−6 kg/s. The other shorter cooler has a length of 105 mm and the same outer diameter. It could also reach 82 K with a flow of about 2×10−6 kg/s. In both cases nitrogen gas was supplied at a pressure of 10 MPa.