The general lithography process has to go through the processes of silicon wafer surface cleaning and drying, bottom coating, spin coating photoresist, soft baking, alignment exposure, post baking, development, hard baking and etching.
Lithography is to make patterns (processes) with light;
The process of spreading glue on the surface of the silicon wafer, then transferring the pattern on the mask to the photoresist to temporarily "copy" the device or circuit structure onto the silicon wafer.
High-end projection mask aligner can be divided into step projection and scanning projection mask aligner. The resolution is usually between tens of nanometers and several microns. High-end mask aligner is known as the most sophisticated instrument in the world, and there are 70 million dollars of mask aligner in the world. High-end mask aligner is the flower of modern optical industry, and its manufacturing is very difficult, and only a few companies in the world can manufacture it. Foreign brands are mainly ASML in the Netherlands (the lens is from Germany), Nikon in Japan (high-end mask aligner, from which Intel once bought Nikon) and Canon in Japan.
SMEE, located in Shanghai, China, has developed a projection-type mid-range mask aligner with independent intellectual property rights, formed a product series, and achieved initial sales at home and abroad. At present, other series of products are under development and production.
The low-end mask aligner used in production line and R&D are proximity and contact mask aligner, and the resolution is usually above several microns. There are mainly German sus, American MYCRO NXQ4006 and China brands.
Mask aligner is generally divided into three types according to the simplicity of operation, manual, semi-automatic and fully automatic.
A manual: refers to the adjustment mode of alignment, that is, the alignment is completed by manually adjusting the knob to change its X-axis, Y-axis and thita angle, and the alignment accuracy is predictably low;
B semi-automatic: it means that the electric shaft can be positioned and aligned according to CCD;
C automatic: refers to uploading and downloading from the substrate, and the exposure time and period are controlled by the program. Mask aligner mainly meets the demand of processing capacity of the factory. NXQ8000 series of Enkeyou can process hundreds of wafers in one hour.
Ultraviolet light source editing
One of the core components of exposure system is ultraviolet light source.
Common light sources are divided into:
Ultraviolet (UV), G-ray: 436 nm; I-line: 365 nm
Deep ultraviolet (DUV), KrF excimer laser: 248 nm, ArF excimer laser: 193 nm.
Extreme ultraviolet (EUV), 10 ~ 15 nm
Requirements for light source system
A.it has a suitable wavelength. The shorter the wavelength, the smaller the feature size that can be exposed; The shorter the wavelength, the sharper the edge of lithography, and the higher the requirement of precision control during etching, because the diffraction phenomenon will be more serious. ]
B. there is enough energy. The greater the energy, the shorter the exposure time;
C. the exposure energy must be evenly distributed in the exposure area. [Generally, the concept of uniformity of light or parallelism of uneven light is used to measure whether light is evenly distributed]
The commonly used ultraviolet light source is high-pressure arc lamp (high-pressure mercury lamp), which has many sharp spectral lines. After filtering, G-ray (436 nm) or I-ray (365 nm) is used.
Excimer laser can be used as a deep ultraviolet light source with short wavelength. Such as KrF excimer laser (248 nm), ArF excimer laser (193 nm) and F2 excimer laser (157 nm) and so on.
The functions of exposure system mainly include: smoothing diffraction effect, realizing uniform illumination, filtering and cold light processing, realizing strong light illumination and light intensity adjustment, etc.
Alignment system editing
The manufacturing of high-precision alignment system needs nearly perfect precision mechanical technology, which is also one of the technical difficulties that domestic mask aligner can match. Many American and German brands of mask aligner have special patented mechanical process design. Such as mycon &；; Q mask aligner adopts the patented technology of all-pneumatic bearing design, which effectively avoids the process error caused by mechanical friction of bearings.
Another technical problem of the alignment system is the alignment microscope. In order to enhance the vision of microscope, many high-end mask aligner adopt LED lighting.
There are two sets of alignment systems with focusing function. Mainly 1 uses binocular double-field alignment for microscope body, eyepiece and objective lens (mask aligner usually provides eyepieces and objective lenses with different magnifications for users to combine).
The function of CCD alignment system is to enlarge the alignment marks between the mask and the sample and image them on the monitor.
As the name implies, the workpiece table is a platform for placing workpieces, and the most important workpieces in lithography process are mask and substrate.
Worktable is the key component of mask aligner, which consists of mask sample moving table (XY), mask sample relative moving table (XY), rotating table, sample leveling mechanism, sample focusing mechanism, wafer bearing table, mask fixture and drawing mask table.
Wherein, the sample leveling mechanism comprises a ball seat and a hemisphere. In the leveling process, pressurized air is first introduced into the ball seat and hemisphere, then the ball seat, hemisphere and sample are moved upward by the focusing handwheel to make the sample close to the mask to level the sample, and then the two-position three-way solenoid valve is switched to vacuum to lock the ball seat and hemisphere to keep the leveling state.
The sample focusing mechanism consists of focusing handwheel, lever mechanism and ascending linear guide rail. In the process of leveling, the initial focus is focused. After leveling, there will be a certain gap between the sample and the mask, so it is necessary to fine-tune the focus. On the other hand, after leveling, it is necessary to separate a certain alignment gap and fine-tune the focus.
The mask table is mainly used for quick loading and unloading, and consists of dovetail guide rail, positioning stop and locking handwheel.
The wafer stage and mask holder are designed according to different sample and mask sizes.
5 performance index editing
The main performance indexes of mask aligner are: supporting substrate size range, resolution, alignment accuracy, exposure mode, light source wavelength, light intensity uniformity, production efficiency, etc.
Resolution is a description of the thinnest line precision that lithography can achieve.
The resolution of lithography is limited by light source diffraction, so it is limited by light source, lithography system, photoresist and process.
Alignment accuracy is the positioning accuracy of interlayer patterns during multilayer exposure.
Exposure methods are divided into contact approach, projection and direct writing.
The wavelengths of exposure light sources are ultraviolet, deep ultraviolet and extreme ultraviolet, and the light sources include mercury lamps and excimer lasers.
6 exposure editing
A. contact printing: the mask plate is in direct contact with the photoresist layer. The resolution of the exposure pattern is equivalent to that of the pattern on the mask plate, and the equipment is simple. According to the different ways of applying force, the contact types can be divided into soft contact, hard contact and vacuum contact.
1. Soft contact is that the substrate is absorbed by the tray (similar to the substrate placement in the spin coating machine) and the mask covers the substrate;
2. Hard contact means that the substrate is pushed upward by air pressure (nitrogen) to make it contact with the mask;
3. Vacuum contact is to pump air between the mask and the substrate, so that they can cooperate better (think about the way of vacuuming the quilt)
soft