Materials
Why Glass Encoder Discs Use Chrome for the Lines
The black lines on a glass encoder disc are a chrome film about 0.1 μm thick. It is thin yet dark enough, adheres firmly, resists wear and corrosion, and can be made into micron-level lines by photolithography and etching. This article explains why glass discs generally use chrome, looking at light blocking, adhesion, durability and processability.
Hold a glass encoder disc up to a lamp: the lines are black and the gaps are clear. The black part is a chrome film only about 0.1 μm thick — taking a hair as 50–100 μm, that is 500 to 1,000 times thinner than a hair. Chrome is not the only metal that could form the lines, so why do glass discs generally use it?
The tests a line material must pass
A film that can serve as the lines must satisfy at least all of the following:
- Dark enough: blocks light even when very thin;
- Adheres firmly: does not peel or flake off the glass;
- Durable: survives cleaning, wiping and long-term use;
- Can be made fine: can be patterned by photolithography and etching into micron-level lines with clean edges.
Chrome is well balanced on all of these, which is the fundamental reason it is so widely used.
A thin layer is dark enough
How well a line blocks light is expressed by optical density, OD = −lg T, where T is the transmittance:
| Optical density | Transmittance |
|---|---|
| OD 1 | 10% |
| OD 2 | 1% |
| OD 3 | 0.1% |
| OD 4 | 0.01% |
Chrome absorbs and reflects light strongly, so a thickness on the order of a hundred nanometres reaches a high optical density. A quick comparison: take the light through a gap as 1; with lines at OD 3 only 0.001 remains, and the modulation M = (Imax − Imin) ÷ (Imax + Imin) = 0.999 ÷ 1.001 ≈ 0.998 — almost perfect black and white. Conversely, if the opaque layer has pinholes or is locally too thin, light leaks there and signal contrast drops.
A thin film has another advantage: etching time is short, so there is less lateral undercut and the line edges are cleaner.
Firm adhesion, long life
Chrome adheres well to glass — which is why a chrome underlayer is often deposited first when coating gold or silver. The chrome film is hard and scratch-resistant, and forms a dense oxide on its surface that is fairly stable against moisture and common cleaning agents. Once a disc is installed in an encoder it must last many years, so these points matter.
Some discs also have a chrome oxide layer on top of the chrome to reduce surface reflection and cut stray light inside the readhead.
It can be lithographed and etched
The chrome line process is shown in Figure 1: clean the glass, deposit a chrome layer by vacuum sputtering or evaporation, spin-coat photoresist, pre-bake, expose with UV light through a mask, develop to uncover the chrome to be removed, etch that chrome away, and finally strip the resist.
The chrome etchant (commonly a ceric ammonium nitrate system) attacks only chrome, not the glass or the photoresist, so the pattern is “copied” accurately. The photomask industry has used chrome as its opaque layer for decades, and the materials, equipment and processes are mature enough for disc manufacturing to borrow directly.
Wet etching causes a little lateral undercut, so lines end up slightly narrower than the resist pattern; this has to be compensated in the mask design and the process parameters.
Compared with other materials
| Material | Light blocking | Adhesion and wear | Stability | Typical use |
|---|---|---|---|---|
| Chrome | Very dark even when thin | Good adhesion, hard | Good | Opaque layer of glass discs and photomasks |
| Aluminium | Blocks light, strongly reflective | Softer, scratches easily | Fair | Mostly used as a reflective film |
| Gold, silver | Blocks light | Soft; poor adhesion directly on glass | Silver tarnishes | Expensive; generally not used for lines |
Summary
Chrome became the line material for glass encoder discs on its overall performance: about 0.1 μm is dark enough, it adheres firmly, resists wear and corrosion, and mature lithography and etching processes turn it into clean fine lines. When judging a disc’s light-blocking quality, focus on optical density, pinholes and line edges.
Translated from the ENC7 Chinese technical library (article 0014). Read the Chinese original. Need a disc built to your drawing? Request a quote or see our custom encoder discs.
© Langfang Jingzheng Optoelectronic Technology Co., Ltd. All rights reserved. This article was created with AI assistance.