Common measurement methods for wafers (4);

1. Atomic force microscopeAFM is a device that uses van der Waals forces to measure objects

1. Atomic force microscope

AFM is a device that uses van der Waals forces to measure objects. When the distance between two atoms gradually approaches, attraction is generated between them, but when they approach, repulsion is generated. The working principle is that when the tip encounters an object, it bends because the force between the atoms creates attraction or repulsion. Then, the reflection angle of the laser shining on the tip changes, and PSPD (Position Sensitive Photodiode) is a sensor that receives the reflected light and reads the position, which can recognize the position of the changing laser spot and achieve height detection.

Generally speaking, AFM has contact, tapping, and non-contact methods. In the case of contact or tapping, it has the advantage of more accurate measurement than non-contact methods, but the disadvantage is the wear of the probe and sample. The shape of the tip is as follows: no matter how small or light it is, if the nm tip passes through the wafer when scraping it, it will exert a huge force on the wafer (think of the destructive force of pressing with a needle or a blunt pencil tip, even if the same force is applied, the two destructive forces are different). The tip may break and contaminate the wafer, or the hard formed film may peel off. Therefore, taping and non-contact modes were designed. Both methods can minimize contact with the sample to the greatest extent possible. In the case of tapping, measure the tip by tapping the wafer. Tapping cannot avoid damage to the tip and wafer surface, but it is less than contact. The non-contact method is a completely non-contact method. When vibrating at a unique resonant frequency at the tip and approaching the chip, gravity dominates the van der Waals force (set resonance frequency oscillation variation), allowing the probe to be measured without contact with the sample. In this case, tip and wafer damage can be prevented from the source.

AFM varies depending on the axis or usage, but is typically a device with a resolution of 0.5A~1A (angstrom). Therefore, it is used for inspecting very small defects or measuring structures. For example:

(1) After CMP, error measurements such as erosion and dishing;

(2) The roughness of the deposited film;

(3) Line width, such as LER (line edge roughness) and LWR (line width roughness)

In addition, the advantage of AFM is its simplicity as it does not require additional vacuum equipment. However, due to the limitations of the probe, there is a drawback, which is that it is difficult to measure in high aspect ratio grooves. In addition, the tip is also a consumable of AFM, which is expensive and has a recommended service life, so it must be replaced regularly. This is a very important factor, as the measurement results may vary depending on the state of the tip.

2. Overlay

In silicon wafer manufacturing, the control of etching accuracy is a necessary control of pattern and pattern alignment in silicon wafer manufacturing.

Currently, silicon wafers are manufactured in a series of steps, with each stage placing a material pattern on the wafer; In this way, transistors, contacts, etc. made of different materials were placed. In order for the final device to function properly, these individual patterns must be correctly aligned - for example, contacts, circuits, and transistors must all be aligned.

3. Ellipsometer

When the measured light with known polarization degree (which can be adjusted by using a polarizer or other means to emit light from the light source) is irradiated on the sample to be measured, the polarization state reflected on the surface of the sample changes, and the thickness and refractive index of the thin film can be determined by measuring the polarization state of the light.

However, it is difficult to obtain information about the thickness and refractive index of the thin film solely using the obtained data, so it is necessary to refer to appropriate models.

*In addition, there is a non-destructive method that can be used to confirm the thickness of SiO2.

Use the monitoring color card, as shown in the following figure:

If there is a target value, similar colors can be found nearby and the thickness of SiO2 can be checked. For example, if 500nm SiO2 is formed, there will beBlue green.

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