Pattern Generator

On the video the photolithography process at the CIDESI Queretaro clean room is observed being used in the temperature sensors manufacturing. Depending on the device, the number of times that a photolithography can be required could increase, for example, a thin film transistor requires less than 10 photolithography processes, and an integrated circuit a few more.

The photolithography implementation in the sensors and integrated circuits manufacturing

The photolithography techinque is used in the electronic devces manufacturing such as sensors and integrated circuits. With this technique the patterns are transferred of a photomask to a substrate, this substrate may be crystalline silicon or semiconductors, metals and dielectrics thin films. This pattern transference has the object to limit areas or very thin geometries or with a high resolution in the substrate or the material that is part of in the device or integrated circuit that is to be fabricated.

In the CIDESI Queretaro clean room we have an exclusive area to develop this process. It is formed by two class 100 bays with yellow lighting, due this process has to be developed in suspended particles free rooms. The room is yellow in order to prevent the photoresing to light of short wavelenghts.

1.- Photoresin depostition and thermal treatment

This first step consists of the application of a positive or negative photoresin to the substrate where the patterns wil be transferred of the sensor or integrated circuit, the coating of the photoresing is done through "spin coating" technique. This step commonly uses an adherence promoter between the resin and the substrate where the pattern will be transferred, usually HDMS (Hexamethyldisilazane), see Figura 1.

The next step to the photoresin coating, is to take the wafer or substrate to a thermal treatment in a hot plate for a short time and temperature around 115°C. This step promotes the adherence of the photoresin to the substrate.

Habitually the thermal treatment temperature for the photoresin is suggested by the supplier.

Fig. 1. Sequence of the deposition process in a silicon wafer, afterwards the wafer with the photoresin passes to a thermal treatment. This process is carried on in two class 100 bays with yellow lighting to prevent the exposure of the photoresin to short wavelenght light.

2.- Exposure of the substrate with coated photoresin at ultraviolet (UV) light

In this step, an UV light source is used (see Figure 2) and a photomask, the photomask has dark and transparent areas that represent the geometry of the paterns that are to be tranferred, this photomask is placed between the UV light source and the photoresin covered substrate.

As mentioned before, the photomask desgin will determine what geometry will be recorded in the photoresin and eventually in the substrate, this is due the dark parts of the mask will prevent determined areas of the film with photoresin to be exposed to UV light. In this step, UV light modifies the coated photoresin, which changes its physical chemical properties making it sensitive to the revealer.

Depending of the manufacture complexity of the sensor or integrated circuit, several stages or photolithography processes could be necessary where it is required to couple more than a level on the device structure, this coupling is performed with an alignment system which allow to locate the patterns on the desired positions. In this apparatus, the UV light exposure is done.

Fig. 2. Semi-automatic aligner, the CIDESI Queretaro clean room has an aligner of the brand "OAI" with several contact modalities like soft, hard and of proximity. the alignment gap is programmable from 0 to 28 micrometers and the lighting it uses is UV (Hg-Xe lamps) of 350W, this system allows working with 4-inches wafers and reaching a 5 micrometers resolution between geometries, furthermore of an optical resolution up to 5x. Among these capabilities of this system there is the possibility of back alignment.

3.- Revealing and thermal treatment

The consequence of sensitize the photoresin with UV light, is that when the substrate with the photoresin is immersed in an alkali substance known as revealer, the photoresin is removed from the exposed areas or from those that were not exposed, depending on whether the photoresin is positive or negative, respectively. This process is known as revealing, where the photomask design is recorded on the substrate with the photoresin, see Figure 3.

Fig. 3. En la figura a) se observa una oblea de silicio con fotoresina S1813 que fue expuesta a luz UV y que esta sumergida en revelador MF-319. En la figura b) y c) se puede observar que el diseño de la fotomáscara quedo grabado en el sustrato con fotoresina.

After the revealing process, the substrate still with photoresin is submitted to a second thermal treatment on a hot plate, which has the objective to harden the resin on the substrate and improve the adherence of both.

4.- Humid and dry etching

After the revealing stage and the second thermal treatment, the wafer passes through a stage where it is humid (acids) or dry (plasm) etched. These two etching modalities, can be observed on Figure 4 a) and b). The photolithography process ends after the substrate is etched, it gets cleaned with acetone, which will take over to remove the leftovers of the whole resin that are still deposited over the substrate, Figure 4c).

Fig. 4. Wafer with devices submitted to a humid etching, dry and photoresin removal.

5.- Checking

Fig. 5. The manufactured devices are inspected with the help of optical or electronic microscopes.

Once the etching is done and the wafer or substrate is cleaned correctly, the process is checked through observation, whether electronic or optical microscopical microscopy, depending on the requirements of the device or integrated circuit in the manufacture.

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