Precision electroforming mirrors open new vistas, both tiny and cosmic

The European Space Agency (ESA) decided in the early 1990s to create the XMM-Newton telescope. According to ESA mission objectives, it would be an “x-ray telescope four times more powerful than any other telescope in the world and with the capability of observing ten times more x-ray sources in the universe." Optical specialists were handed a challenge: how to produce 200 square meters of highly reflective thin optical mirrors just 0.4–1mm thick possessing an ultrasmooth surface with roughness less than 0.4nm.

Figure 1. One mirror module in the European Space Agency's XMM-Newton telescope. (Image courtesy of Media Lario Technologies, MLT.)

We developed high-precision electroforming (e-forming) replication technology to make mirrors of the necessary quality, size, and performance requested by the scientific community (see Figure 1).¹ In the e-forming process, metal ions from an anode are deposited through electrolyte solution onto an electroconductive surface called a master. A metal skin is built up to desired thickness and then removed from the master, forming a self-supporting structure. With the support of ESA and the Italian Space Agency, we progressively improved and optimized the technique. Stringent control over the internal stress and thickness of the metal ion deposit allows an easy release of the mirror from the master, enhancing the optical performance and increasing the lifetime of the master copy.

The core advantage of our approach is the ability to precisely reproduce the master surface many times over without performing any further polishing steps on the final mirror. The result is significant cost and cycle time savings of over 50% when compared with traditional technologies. The savings remain true even for complex surface profiles such as toroidal and off-axis shapes. We are currently applying precision e-forming replication to challenges in advanced EUV lithography and single-aperture multispectral visible long-wave IR (VIS-LWIR) sensors for space and defense applications.

Figure 2. Grazing incidence collector for advanced extreme-UV lithography equipment during integration at the optical bench. (Image courtesy MLT.)

Figure 3. Interferometric measurement of mirror accuracy. λ/4 PTV (peak to valley) and λ/16 RMS were obtained through the replica process. λ: Wavelength.

Microchips are produced by photolithography using short-wavelength UV light. As more and more circuits are packed onto ever smaller chips, the semiconductor industry has shifted to new lithographic-manufacturing technologies that use shorter wavelengths in the EUV spectrum. The shorter the wavelength, the finer the circuits it can draw on a chip. Conventional lenses cannot focus, or collect, EUV rays. Consequently, developers of advanced EUV lithography equipment need an efficient mechanism with reflective optics for collecting and transporting EUV light. Their requirements are stringent both in terms of operative environmental conditions and performance of the optical system.

The solution to this problem turned out to be the ‘reverse’ configuration of the space telescope (see Figure 2).² Accordingly, we designed and produced a new EUV collector. This design provides integrated thermal management capable of dissipating a thermal load of 15kW, maintains the optical performance of the collector under full source power operation, has a collection efficiency up to 32%, and is capable of 500W EUV power at the intermediate focus. The optics have a reflective layer with a one-year lifetime and flexible configurations tailored to the specific source and scanner requirements, e.g., the number of mirror shells, reflective layer, and alignment and mounting interfaces.

Figure 4. Single-aperture multispectral sensor prototype for defense surveillance. (Image courtesy MLT.)

Precision e-forming replication technology also has promise for multi- and hyperspectral sensors for defense surveillance and space-based Earth observation. A key challenge presented by the developers of such sensors is how to produce compact, lightweight imaging systems at a reasonable price, working in a wide range of wavelengths (typically VIS-LWIR corresponding to the range 450nm–12μm). These multispectral sensors consist of several optical systems based on lenses using expensive materials and with significant mass and overall dimensions. Alternative solutions based on a reflective approach, which requires totally aspherical off-axis optical design, have been studied. But they were too expensive, heavy, and complex to be produced at the time.

Now, our precision e-forming technique has made the use of multispectral off-axis mirrors possible. We can produce such lightweight and low-roughness optics with complex design at reasonable cost and cycle time (see Figure 3 and Table 1). We are currently working on the design and production of a multispectral off-axis demonstration project for defense surveillance and space-based Earth observation applications (see Figure 4 and Table 2). We believe the technology can be pushed even further. We have several ongoing studies in cooperation with various scientific institutes to improve the thermo-mechanical performance of the e-formed mirrors, and hope to expand their application in the future.