Technology

Technology

Technology
全部分类
×

Low-Temperature Bonding Process

Process Overview

 

To meet the special requirements of high assembly precision, microscale structural stability, high power resistance, and harsh environmental applications, low-temperature bonding (deepening photoresist) technology is increasingly widely used in the production of optical components. This process can be summarized as: dehydration bonding of clean, flat, and active surfaces. It is briefly described in three points:

1. Polishing Surface Roughness

Surface roughness determines the contact area of ​​the bonding surface, and the bonding strength of pre-bonding is proportional to the area. Results from some literature (G-W model) are directly presented to define the conceptual direction for quantifying roughness. The G-W model is based on the following boundary conditions:

The height of the micro-protrusions on the rough surface follows a normal distribution.

The tops of the micro-protrusions are spherical, and all spherical radii are equal.

There is no interaction between the micro-protrusions (rigid contact), and the surface material is isotropic with small deformation (optical glass materials are considered isotropic).

                       

Here are the results directly:

Where: N is the total number of micro-protrusions on the effective area

R is the radius of curvature at the tip of the micro-protrusion

d is the distance between the reference plane of the rough surface and the ideal rigid plane (this is considered to be the RMS value of the optical surface roughness)

Z is the profile height; contact occurs only when the profile height > d

Contact probability:

                  

Integrating the above quantities, we obtain the surface contact area result:

                   

In summary, the optical bonding area is directly proportional to the surface roughness; an increase in contact area corresponds to an increase in bonding strength. (This only presents the conclusion: the better the optical surface roughness, the better the surface bonding strength. The theoretical explanation of how surface roughness is achieved during polishing is not discussed in detail.)

 

2. Optical Low-Temperature Bonding Surface Characteristics:

Optical low-temperature bonding is mainly based on SiO2 as a medium. This primarily depends on the dehydration bonding of Si-OH molecules, hence the "bonding" part of the technology's name.

The SiO2 surface contains two types of water molecules: one is (triSi-OH), where hydroxyl groups are linked to (Si-) via hydrogen bonds (bound water), which has higher surface energy; the other is Si linked to H2O molecules (free water), where water molecules easily detach and do not contribute to bonding.

In the initial pre-bonding stage, the substrate, at room temperature, allows the two surfaces to adhere tightly through intermolecular forces. After a period of bonding, hydroxyl groups on the surfaces of the two substrates form hydrogen bonds in the contact area. Relying on the mutual attraction of these hydrogen bonds, a polymerization reaction occurs, forming cross-links. At this point, a large number of dangling bonds form on the surface. As the pre-bonding temperature increases, the diffusion of ions and holes on the surface gradually intensifies, the structure is restructured, and eventually stabilizes. The dangling bonds gradually disappear, and silanol bonds are generated. Due to the presence of alkali ions in the glass, the bonding temperature of the glass is lowered, the hydroxyl mobility increases, causing elastic deformation of the substrate, increasing the bonding area, and consequently increasing the bonding strength. The main reaction formulas are as follows:

          

The above reaction equations show a significant dehydration effect during the bonding process. Water molecule diffusion increases exponentially with increasing temperature. These water molecules diffuse into the surface's porous structure, creating a local vacuum during their expulsion, inducing local elastic deformation, increasing the bonding area. Simultaneously, SiO2 undergoes viscous flow, eliminating micro-gaps. With increasing annealing time, the viscous reflow on the surface reduces the total volume of the bonded oxide layer, leading to stress relaxation and completion of bonding.

From the quantitative and qualitative analysis of this part of the bonded surface, some key process points can be identified:

How to increase the amount of surface-bound water (increase surface energy)?

How to promote free water diffusion (free water aggregation induces bubble formation)?

How to promote the surface diffusion and penetration process—selection of SiO2 thin-layer medium characteristics?

 

3. Baking (Bond Acceleration and Annealing)

As the core part of the optical low-temperature bonding process, the selection of baking parameters requires careful consideration. Here are some guidelines for reference:

3.1 Changes in Bonded Surface Energy with Temperature and Time

At a constant bonding temperature, the bonded surface energy increases over time to a certain value and then tends to stabilize:

                 

3.2 Relationship between bond strength and annealing time:

                  

3.3 Utilizing the spectral absorption peaks of numerous interfacial hydroxyl groups and free water to promote the bonding process (see attached water peak absorption table):

     

The selection of baking temperature, based on theoretical support, also needs to be adjusted according to factors such as the temperature characteristics of the substrate material and the setting of coating parameters. If it is a crystalline material, the lattice characteristics also need to be considered. Therefore, the corresponding empirical summaries become particularly important.

 

 

 

Optical components based on low-temperature bonding technology:

 

Low-temperature bonding (see attached introduction), also known as deep optical bonding, is a crucial processing method in optical assembly based on the low-temperature (below 300℃) dehydration bonding effect of Si-OH chemical bonds. It has been widely applied in numerous high-precision optical components. The use of low-temperature bonding technology has deepened with the development of the optical industry, while the requirements have also increased. In applications requiring high resistance to laser damage and high extinction ratios, low-temperature bonding technology has become the most important process implementation method.

Through specialized surface-active treatment technologies and dedicated baking equipment tailored to the characteristics of various optical materials, Zhongke Jingchuang provides high-performance low-temperature bonded optical components, including: etalons, high-power (N) PBS, deep waveplates, PBS+waveplate assemblies, AR glasses modules, etc.