Principles of IBF

Ion beam figuring (IBF) is a technique used for removing material from a surface by transferring kinetic energy from impinging neutral particles which is a nonabrasive technique for fine correcting the contour of precision optics. This figuring process utilizes a Kaufman-type ion source where plasma is generated in a discharge chamber by controlled electric potential (Kaufman et al. 1977). Its sketch graphic is shown in Fig. 1, where charged concave-type grids extract and accelerate ions from the chamber. The accelerated ions form a directional ion beam with Gaussian-type distribution. A neutralizer outside the grids supplies electrons to the directional ion beam to necessarily neutralize the beam to prevent charging optical component and to avoid bending the beam by extraneous electromagnetic fields from a charged workpiece or from other source. When the ion beam hits the optical component, the amount and distribution of material sputtered is a function of the material of optical component, the incidence energy, distance, and angle of ion beam (Sigmund 1973; Bradley and Harper 1988).

In IBF process, a non-varied ion beam energy maintains a constant sputtering rate (or material removal rate) and profile which is very important for optical deterministic figuring method to gain a constant beam removal function. Then, this temporally and spatially stable ion beam is held perpendicular to the optical surface at a fixed distance with ion source controlled by a 5-axis CNC system shown in Fig. 2 (Ion Beam Finishing Technology for High-Precision Optics Production). The optical deterministic figuring method described here is assumed a constant beam removal, so that the process can be represented by a convolution operation based on the CCOS (computer controlled optical surfacing) theory. If not a constant beam removal, its corrections would be required to model the process which will be discussed in the section “High-Gradient Optical Surface Figuring by IBF.”

The Typical Features and Its Purpose of IBF

Using ion beam to figure optical component, it has the following features and advantages:

1. High figuring precision. Based on the physical sputter effect, the material removal rate is in molecular or atomic level, so IBF figuring precision can be maintained in nanometer or sub-nanometer precision in the condition of ion beam stability.
2. Highly predicable and stable. Compared with traditional abrasive optical figuring methods, where chemical interaction (hydrated layer) and mechanical interaction (scratching) remove the material of optical component, IBF is figuring with ions, such as argon, which is like sandblasting. Its sputter rate can be accurately calculated by the physical law of elastic and inelastic scattering. Since no a chemical interaction, ion beam parameters that can be easily held in constant are the main reasons for the stability of IBF process. These features make the figuring process highly predictable and stable. That allows the figuring process to rapidly converge to the desired specifications and save significant time and cost.
3. Noncontact figuring. The noncontact nature eliminates the problems of tool wear, edge roll-off effect, loading force on the workpiece, and surface/subsurface mechanic damage generated by mechanical interaction in the conventional grinding and polishing methods. This feature is very useful for figuring very thin and lightweight optical component.
4. Good material removal function. The two main advantages of material removal function are Gaussian distribution and constant profile. The Gaussian distribution removal function is an ideal function for figuring process which can simplify the dwell-time calculation and improve figuring precision.
5. No or minimized support structure print effect. The so-called support structure print effect is the honeycombed support structure of the back side that is visible on the front optical surface for lightweight structure optical component, such as lightweight SiC optics. This is troublesome problem for traditional figuring process because of the loading force impact.

In addition, the ion beam figuring process is a clean figuring process which avoids the influences of polishing liquid and polishing abrasive in the conventional figuring process which usually lead to generate redeposition layer and insert polishing abrasives. IBF is an excellent complement to conventional figuring to gain very high optical surface and subsurface quality. However, it has some shortcomings, such as working in a vacuum chamber; component heating because the ions hit the optical surface with several hundreds to thousands eV, some of them being stopped by absorption which heats the workpiece; difficulty of improving surface roughness because of the ion “sandblasting” effect at the atomic level (recent work suggests that it could even be used to improve the roughness); slow material removal rate with normal values from tens to several hundreds nanometer per minute; etc.

Description of IBF Process

The basic flow of IBF process is shown in Fig. 3. Prior to any figuring, the material removal function (shown in Fig. 3) must be determined for the process as it is the base of deterministic figuring process. About how to gain this function, its detail will be discussed in the section “Removal Function Modeling and Analyzing of IBF.” The material removal function (or called beam function), analogous to a point spread function, provides a depth removal rate distribution as a function of radial distance from the ion beam center.

The ion beam figuring process begins with measuring the contour of the optical component by interferometer, such as ZYGO series of interferometer, which results in an x–y array map of relative surface height values. Then to gain a removal map, the difference between the measured surface contour and the goal surface contour, it describes the material to be removed. Based on the removal function and removal map, a dwell-time calculation is the third step in this flow. In CAMstep, its one aim is to make the ion beam moving routine and its velocities to realize dwell function according to the machine motion performance, such as axial maximal velocity and acceleration. The other aim is to automatically generate control codes or NC codes to the CNC system when the controller of an IBF machine is a standard CNC system. For figuring, the optical component is placed in the vacuum chamber, and the ion beam raster scans over the surface of optical component according to the dwell time and the moving routine. In the IBF process, the material removed and its distribution are represented by the convolution of the removal function and the dwell time:

where R(x, y) is the material to be removed, B(x, y) is the material removal function, and T(x, y) is the dwell time. If the material to be removed and a material removal function of ion beam were known, the dwell time could be solved by deconvoluting Eq. 1. This is the fundamental of IBF process.
There are some key problems that would be paid more attention to realize IBF and gain good figuring results. They are listed as (1) how to select suitable sizes and beam parameters of ion beam that is very important to control the low-, middle-, and highspatial-frequency errors of surface contour; (2) how to correctly evaluate the material removal function; (3) how to reasonably process the measuring contour data to make ion beam move smoothly as soon as possible to decrease the high-spatial-frequency errors generated; and (4) how to plan suitable ion beam moving routine including the suitable velocity, acceleration, pitch of raster scanning, and so on. Those four problems will be discussed in detail in the section “The Key Technology of IBF.”

Current Status and Future of IBF

The earliest work is done by Meinel et al. (1965), which is the first report to apply ion beam to polish optical glass. Since the over high beam energy generated by ion source, the polished glass surface was seriously damaged. The subsequent successful work on IBF should firstly thank Kaufman who invented a new low-energy ion source, so-called Kaufman ion source. Early work on IBF was re-performed using Kaufman ion source by Gale at the end of 1970s (Gale 1978). This work was deeply expanded at the University of New Mexico in the USA by S. R. Wilson et al. (1987, 1989). They did many initial figuring experiments on fused silica, Zerodur, and copper optical component with 2.54 cm Kaufman ion source. Their representative result is to figure a 30 cm fused silica optics from contour precision 0.41λ RMS to 0.042λ RMS (λ ¼ 632.8 nm) in one figuring cycle with 5.5 h. Lynn N. Allen et al. originally developed IBF system at the Eastman Kodak Company in 1988 and became operational in 1990 (Allen and Keim 1989; Allen and Roming 1990). This IBF system is designated for figuring large optics with up to 2.5 m  2.5 m  0.6 m of sizes. There are about 65 segments of 10 m Keck primary mirror was successfully realized their final figuring with 15 months. And 14 segments of them were measured after final figuring. Their average figuring accuracy was improved from 0.347 μm to 0.062 μm, and the maximal one-cycle error convergence ratio is 17.5 and the average value 5.6. The highly efficient figuring capability of IBF was successfully shown in Kodak Company which opened a new era for optical figuring technology (Allen and Roming 1991; Allen and John 1991).
Another representative work is the new Precision Ion Machining System (PIMS) research facility at NASA’s Marshall Space Flight Center at the beginning of 1990 (Drueding 1995), which is focused on the figuring of small optics using 3 cm ion source. Since the ratio of ion beam to the size of small optics is greater, figuring a smaller optics is more difficult. Currently, the small optics figuring by IBF is also an interesting and valuable research work in optical figuring (Fawcett 1994; Shanbhag et al. 2000).
Besides the above work, there many IBF systems were setup at the end of 1990s, such as CSL lab in Belgium (Tock et al. 1999), IOM & NTG in Germany (Fruit et al. 1999; Schindler et al. 2000), INAF–OAB in Italy (Ghigo et al. 2001), and so on. Cannon Co. Ltd in Japan set up its IBF system for EUVAL in 2004 (Ando et al. 2004). NUDT (National University of Defense Technology) in China set up a series of IBF system in 2006, 2010, and 2011 (Lin et al. 2007; Yuan et al. 2011).
One of these IBF systems is listed in Fig. 4. In addition, INAF–OAB in Italy and REOSC in France set up large IBF systems for 1.7 m- and 2 m-diameter space optics fabrication, respectively (Ghigo et al. 2009; Roland 2010). And also a large IBF for figuring 1.6 m diameter optics is building in the NUDT of China.
Currently, IBF method is universally used to fabricate ultrahigh-precision optical component, such as optics of DUV and EUV lithography, large space optics that many of them are the stitched primary optics whose segments are required no or very small error of edge roll-off effect. The interesting and focus research contents mainly include (1) figuring supersmooth optics, in which their key problems are how to control the middle- and high-spatial-frequency errors and how to hold or improve the surface roughness; (2) dwell-time calculation and figuring technique; (3) the heat effect control that is very serious for high thermo expand material and crystalline, such as BK7, BK9, CaF2, etc.; and (4) the optical material fabrication properties. The surface properties of the optical component influence the effectiveness of the process, and on the other hand, not all of the optical material can be figured by IBF.