Magnetic-Abrasive Superfine Polishing: example of superfine polishing of parts of optics, lasers and electronics

Problem relevance

Polimag - MAM - А14 unit in the autonomous area
А14 unit

The problem of surfacing with nanoscale relief (Ra < 1 nm) and near-surface layers forming with a minimal amount of structural defects is of the most immediate interest to the heavy duty details of the optical, laser and electronic equipment. Surface-finishing improvement affects:

  • the optical systems resolution and microoptics performance capabilities
  • progress in the development of powerful and compact lasers, scope and effectiveness of their application in various fields
  • reduced dimentions of elements and devices for micro- and nanoelectronics

The industry circumstances

Polimag - MAM - Q22-XE
A14's analogue: Q22-XE

The traditional finishing technology for precision surfaces of optical lenses, single crystals – optically active elements of laser devices and wafers for integrated circuits is realized with grinding by wheels with fixed abrasive grains and with subsequent polishing by pastes or suspensions. The grinding wheels application provides hard contact with the treated surface. The high local temperature (up to 2000°C) and high normal pressure values are in the contact zone during processing. Under these conditions the surface layer is formed with a large number of structural defects. The layer depth is 25...75 μm. This layer is removed later by a number of polishing operations using free (unfixed) abrasive grains.

Currently the single crystal wafer and optical glasses are treated by the traditional method with the grinding powder – polishing pads and diamond pastes. This technology is time-consuming and very expensive. The polishing costs compose a considerate share (30% and over) of the wafer-plate cost. The best technology modes provide surface relief with roughness height of 2...5 nm. Finishing methods of chemical and mechanical surface treatment are also widely used in modern production (such as chemical etching, chemical-mechanical polishing, etc.). However, these methods are time-consuming and have limited technological capabilities, besides their use is associated with the environmental and economic problems of usage and recycling of large amounts of chemically aggressive acids and alkalis.

Polimag - MAM - A14 unit
A14 unit for superfine polishing

Today, there are only a few large companies with scientific and technical potential and sufficient production capacity worldwide, which are capable of mass-producing high-precision equipment for finishing of precision surfaces of parts for optics, electronics and laser optics: «Satisloh» (Germany), «Optotech» (Germany), «Speed Fam» (the USA), «Lapmaster» (England), «Fujikoshi» (Japan) and others.

In recent years, the magnetorheological finishing (MRF) technologies have widely spread at finish machining of optics and lasers elements. The MRF principles were developed in Belarus but the development has not been brought to the industrial application for lack of financial resources. The MRF technology was developed in the USA with more than 10 million US dollars investing. Nowadays the QED Company (USA) offers the MRF technology to the world market. The QED has a monopoly on the production of equipment and assistive technology environments. This technology provides the surface roughness in the range of Ra = 1...2 nm and shape accuracy of 6...10 nm.

However, the application of MRF technology is highly complicated because of unstable and nondurable magnetorheological suspension used as the polishing medium. In addition, the cost of QED system and interconnected monitoring equipment exceeds 400 thousand US dollars. The technological media-suspensions cost is about 500 US dollars per 1 liter. These costs are clearly overestimated and unpurchaseable for the majority of potential consumers. The MRF technology features of maintenance and economic factors significantly limit the scope of its industrial application. You can find out here more about comparison of MRF and MAM technologies.

The new solution: Superfine Magnetic-Abrasive Machining

Magnetic-abrasive machining method

The Magnetic-Abrasive Machining (MAM) method is performed with ferro-abrasive powder-tool that is compressed under the magnetic field influence, pressed against the treated surface and polishes it. More about Magnetic-Abrasive Machining (MAM). The MAM method is superior to the known analogues in technological opportunities, economic and environmental parameters.

Equipment for MAM

Polimag - MAM - M08 unit
M08 unit

The software-controlled system A09 was developed for the single-piece MAM of flat, spherical and aspherical surfaces of the parts with a diameter of 15...80 mm in order to improve the macrogeometry and reduce the surface roughness. MAM parameters are entered into the Computer-Numeric Control according to the initial (mechanically polished) surface interferogram. The MAM process is accomplished by automatic surface scanning with the flexible magnetic-abrasive tool; the material removal occurs selectively on the surface prominent parts. For example, the MAM of the flat optical glass plate with diameter of 28 mm provided the macrogeometry parameter PV reduction from 158 nm to 30 nm and the reduction of Ra from 20 nm to 1.4 nm in 6 minutes only.

Polimag - MAM - A09 unit
A09 unit

A09 technical specifications:

  • polished details diameter, mm 20...100
  • polished surface nanorelief parameter Ra <1.5A or better
  • polishing duration, min 2...15
  • power consumption, kW 1.5
  • unit overall dimensions L x B x H, mm 900x500x500
  • unit weight, kg 80
Polimag - MAM - M14 unit
M14 unit

This year (2015) we are planning to complete the implementation of:

  • the A14 unit, which exceeds in the technological capabilities the A09 unit and provides the surface polishing of parts with dimensions from 20x20 up to 200x200 mm with Ra <1 nm;
  • the M14 and M08 units for the single-piece and group parts MAP with a diameter up to 200 mm.

The above given performance specifications and quality of the MAP of the parts for optical, laser and electronic equipment are not limited and will be essentially improved.

Examples

Interferograms before and after Magnetic-Abrasive Machining
Interferograms of optical glass surface before (a) and after (b) Magnetic-Abrasive Machining
Before MAM Sa = 2,72 A
Before MAM image ISO 25178
Height Parameters
Sq0.000369 μm
SSk-0.256
Sku4.04
Sp0.00458 μm
Sv0.00366 μm
Sz0.00824 μm
Sa0.000272 μm
After MAM Sa = 1,42 A
After MAM image ISO 25178
Height Parameters
Sq0.189 mm
SSk0.137
Sku3.76
Sp1.13 nm
Sv0.735 nm
Sz1.87 nm
Sa0.142 nm
Polimag A14 unit
A14 unit in the autonomous clean area