How is it possible to arrange several optical filters on one substrate in order to enable miniaturized sensors? Or to deposit a high reflective mirror on a small and well-defined area in the middle of a substrate? This can be done by the combination of structuring and coating.
In the simplest case a metal mask is used. This is attached to the substrate before deposition and covers the areas which have to remain uncoated. But this can be used only for limited applications: Various shapes like covering a closed area in the middle of a substrate cannot be realized with a metal mask. Additionally, due to shadow- ing effects at the edges of the mask a transition zone between coated and uncoated areas occurs, so the achievable structure size is limited. Therefore, LASEROPTIK combines high quality custom coatings with photolithography in order to meet the requirements especially of the sensor and lighting technology.
The basic principle of photolithography is the creation of a structured surface by means of light shining through a mask onto a light-sensitive photoresist. Depending on the resist’s type the exposed areas become soluble (positive resist) or insoluble (negative resist) in a liquid chemical solvent called developer. After removing the soluble areas you receive a structured resist which can be used as a mask for further processes, e.g. etching or lift-off.
Principle of the two different types of resists
To pattern our coatings we use a process called lift-off. A negative resist is applied onto an uncoated or regular coated substrate. Then it is structured as described above. Depending on the exposure time the edges of the resist do not have a rectangular but an undercut shape (1).
In the next step the desired coating is deposited on top. Because of the structured resist layer the coated areas are directly on the substrate or on the resist (2).
Lift-off process using a negative resist
Finally a liquid chemical solvent dissolves the resist whereby the coating above is removed (lifted off), too and the structure of the mask is transferred positively to the coating (3).
For a successful lift-off you have to ensure that the coating areas on the resist disconnect with the ones on the substrate, so the solvent can penetrate into the resist layer. To achieve this its thickness has to be larger than the coating layer. Additionally, it is helpful to create the greatest possible undercut profile in the resist structure by using an optimized exposure and developing time.
With our equipment we are currently able to process round substrates with a maximum diameter of 150 mm and rectangular substrates with a maximum length/width of 100 mm. Only sputtered coatings can be used.
Due to the precise process of photolithography structures with sharp edges and very small widths (down to 25 µm) can be achieved. To give you an impression three examples are shown below. The measurements / pictures are done by Leica DCM 3D (courtesy of LNQE, Leibniz University Hanover) or our Zeiss Axio Imager.