In the field of optical processing and coating, quartz glass is utilized as a fundamental component in the manufacture of various high-end optical lenses. It is widely used in a variety of applications, ranging from deep ultraviolet lenses in semiconductor lithography machines to protective windows for medical endoscopes and output windows for high-power lasers. Quartz glass is widely used in optical design due to its wide transmitting range, low thermal expansion and excellent chemical stability.

The optical processing of quartz glass presents significant challenges. It is characterized by high hardness and brittleness, rendering the efficiency of traditional processes is relatively low. Some processing methods (such as single-point diamond turning) are not applicable in this case. In optical manufacturing, the core issues revolve around leveraging strengths and avoiding weaknesses, processing quartz glass into optical components that meet requirements, and coating their surfaces with functional films.

1. Why is quartz glass suitable as a substrate for optical coating?

The process of coating optical films (including anti-reflection films, reflective films and filter films) onto quartz glass surfaces is a valuable strategy for maximizing its benefits as substrates. The aforementioned advantages can be reflected in the conventional coating process.

(1) Strong film adhesion

The silicon hydroxyl group on the surface of quartz glass is capable of forming chemical bonds with various oxide film materials, including SiO2, TiO2, and Ta2O5. Following standard cleaning procedures, the coating should pass the tape test without delamination.

(2) Excellent thermal stress

Quartz glass has a coefficient of thermal expansion that is nearly equivalent to that of silica film and comparable to other frequently utilized film materials. It is guaranteed that the film will not crack due to thermal expansion and contraction within the working temperature range of -40°C to +200°C.

(3) Smooth substrate surface after polishing

Following conventional optical polishing, quartz glass surfaces typically exhibit surface roughness in the range of 0.5-1nm. This paves the way for coatings with minimal scattering loss.

(4) Resistance to high temperatures during coating

The substrate is typically heated to between 200 and 300°C during electron beam evaporation coating, and quartz glass can withstand this temperature without deforming or generating internal stress.

2. Which processing methods are not suitable for quartz glass?

In the field of optical processing, certain methods are effective for standard optical glass, but they can present challenges when applied to quartz glass. The following are the key issues that regular factories should be aware of.

(1) Single-point diamond turning

It is evident that this method is not completely applicable to quartz glass processing. Quartz glass is a brittle material. Local stresses occurring during turning can result in chipping and micro-cracks, making it impossible to achieve an optical-grade surface.

(2) Conventional CO2 laser polishing

Its use in precision optical components is limited. Local heating by laser is susceptible to generating thermal stress, which can result in micro-cracks or even explosions. Additionally, surface shape accuracy is challenging to control. This technology is primarily used for the rough smoothing of non-optical surfaces.

(3) Long-term etching with pure hydrofluoric acid

While it is available, it must be used with caution. Hydrofluoric acid etching is isotropic, which will compromise dimensional accuracy and increase surface roughness. In standard production operations, the use of hydrofluoric acid is restricted to brief periods for deburring or chemical thinning, and is meticulously controlled.