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Electronic Imaging & Signal Processing

Microshutter arrays will paint the picture of the early universe

The James Webb Space Telescope will incorporate new technology capable of blocking light from unwanted sources in order to observe extremely faint objects.
15 February 2008, SPIE Newsroom. DOI: 10.1117/2.1200802.1023

The James Webb Space Telescope (JWST) Near Infrared Spectrometer (NIRSpec) instrument is the next generation of space-borne astronomy platforms. It is scheduled to be launched in 2013. The JWST mission has four overriding goals: to identify and study the very first bright objects that formed after the Big Bang, to determine how galaxies were formed and how they have evolved from their birth to the present, to study the birth and subsequent formation of stars and planets and, to study the properties of solar systems and locate where conditions for life may exist1.

The astronomical observations needed to achieve these goals require the most sensitive measurements possible on objects and phenomena that are extremely faint and very far away (and very far back in time). The light from these objects must be integrated over time and then separated into its spectral components where the desired information resides. Hence, the JWST uses the NIRSpec spectrograph to achieve the necessary resolution while maintaining sufficiently low read-out noise and dark currents. However, while the accumulation of light from these faint objects is taking place, light from other unwanted sources must be completely blocked, otherwise their signals will corrupt and essentially destroy the signal from the sources of interest.

The microshutter array (MSA) is a key component of the new system. Its purpose is to allow transmission of light from specific selected objects while completely blocking the transmission of light from all other sources. The MSA is a micro-electro-mechanical systems (MEMS) device consisting of 62,415 individually addressable shutters that operate like trap doors. Each shutter cell is 105μm x 204μm, and the entire array of 62,000+ shutters is contained in a 4.1cm x 4.5cm silicon structure: see Figure 1(a). The basic system components necessary to accurately operate and control the microshutter array include high-voltage CMOS driver chips, analog and digital control electronics, and a magnet and magnet transport system. The subassembly, then, consists of the MSA, CMOS address integrated circuits (ICs), a substrate to which both the CMOS ICs and the MSA are mounted, and a mounting plate with flexure arms to attach the substrate. The entire assembly operates at a temperature of 35 degrees Kelvin (-238C or -396F).

Figure 1. (A) A microshutter array containing 62,415 individual shutters. (B) SEM image of the front side of a small section of the microshutter array with a SEM image of a single microshutter (inset). (C) SEM image of the backside of the microshutter array. (D) the microshutter array mounted on the silicon substrate with the CMOS ICs.

Figures 1(b) and 1(c) show scanning electron microscope (SEM) images of the MSA. Coated with a magnetic film, each microshutter has an electrode on the shutter surface as well as on the back wall. A magnet is swept across the array, pulling each shutter toward the magnet. When the shutter is opened, a voltage potential is applied between the surface of the shutter and the back wall. This potential difference holds the shutter in the open position once the magnet has passed (Figure 2). Once all the shutters are placed in the open (or latched) position, the voltage potentials can be selectively removed from the shutters that need to be closed. For the JWST mission, a typical configuration would have up to 100 shutters open for any given observation (permitting the simultaneous observation of up to 100 objects). This multi-object selection capability is essential to the science requirements and the mission's success.

Figure 2. Schematic illustrating the microshutter actuation and latching. In this image, the magnet would be approximately 3mm below the back wall frame.

Since the MSA is a MEMS device, it must be mounted on an interface substrate to facilitate connections. For the NIRSpec instrument, this substrate was a custom developed 10cm square metal patterned silicon wafer (referred to as a “quad”): see Figure 1(d).

Tests were performed in a cryogenic chamber with a magnet mounted on a transport stage. The quad is mounted with the array approximately 3mm from the bar magnet. A light source is located behind the magnet and illuminates the array. Light passes through the open shutters and is imaged by a high resolution CCD camera. Figure 3 shows a test pattern and an image formed by the open shutters transmitting the light.

Figure 3. Two images of the microshutter array in operation: (A) a test pattern (B) an image of the Czech Republic coat of arms.

Murzy Jhabvala, Harvey Moseley 
NASA Goddard Space Flight Center
Greenbelt, MD

Murzy Jhabvala is chief engineer of the Instrument Systems and Technology Division at the Goddard Space Flight Center. With Goddard since 1974, he has designed, fabricated, flight-qualified and delivered components and subsystems to more than a dozen NASA space missions including: ISEE, IUE, Cosmic Background Explorer, Spitzer Space Telescope, and Gravity Probe B. He is currently leading the development and delivery of the microshutter subsystem for the James Webb Space Telescope.

Harvey Moseley is the principal investigator for the microshutter array subsystem for the NIRSpec instrument on the James Webb Space Telescope.