The performance index of photoresist includes which main technical parameters of photoresist

Performance Index and Technical Parameters of Photoresist

The performance indicators of photoresist include resolution, contrast, sensitivity, viscosity/viscosity, adhesion, corrosion resistance, surface tension, pinhole, purity, thermal process, etc.

1-Resolution

Resolution (resolution,R) is the smallest size useful image that can be formed in a lithographic process. It is the ability to distinguish adjacent graphic features on the surface of a silicon wafer. Resolution is generally measured in terms of critical dimensions (CD,Critical Dimension). The smaller the critical dimension is formed, the better the resolution of the photoresist. This property is deeply affected by the physical and chemical properties of the photoresist material itself, and it is necessary to avoid the photoresist material from shrinking during the development process or flowing in the hard bake. Therefore, in order to make the lithography material have good resolution, it is necessary to carefully select the polymer substrate and the developer used.

2. Contrast

Contrast (Contrast) refers to the rate at which the chemistry (e. g., solubility) of a photoresist material changes before and after exposure. Contrast can be considered a measure of the ability of the photoresist to distinguish between bright and dark areas on the reticle, and the irradiation intensity varies smoothly near the edges of the photoresist lines and spaces. The greater the contrast of the photoresist, the steeper the line edge, the typical photoresist contrast is 2~4. For an ideal photoresist, if subjected to an exposure dose above this threshold, the photoresist is fully photosensitive; otherwise, it is not photosensitive at all. In fact, there is a distribution of the exposure threshold of the photoresist, and the narrower the distribution range, the better the performance of the photoresist.

3. Sensitivity

Sensitivity (Sensitivity) is the minimum energy value (or minimum exposure) of a certain wavelength of light required to produce a good pattern on the photoresist. Unit: mJ/cm2 or mJ/cm2. The sensitivity of photoresist is particularly important for deep ultraviolet (DUV) and extreme deep ultraviolet (EUV) with shorter wavelengths. Negative glue usually takes 5~15s exposure time, positive glue is slower, the exposure time is 3~4 times of negative glue.

Sensitivity reflects the extent to which a photoresist material reacts to a certain wavelength of light. Different photoresists are selective for different wavelengths of light. At the same time, high output requires short exposure time and higher sensitivity of photoresist. The exposure dose is usually used as an indicator to measure the sensitivity of the photoresist. The smaller the exposure dose value, the higher the sensitivity of the photoresist. The exposure dose of I-line photoresist material is about several hundred mJ/cm2, while the exposure dose of KrF and ArF photoresist materials is about 30 and 20mJ/cm2. The sensitivity can be reflected in the contrast curve of the photoresist.

4. Viscosity/Viscosity

Viscosity/viscosity (Viscosity) is a parameter that measures the flow characteristics of a photoresist. The viscosity increases as the solvent in the photoresist decreases; high viscosity produces a thick photoresist; the smaller the viscosity, the more uniform the photoresist thickness. The specific gravity (SG,Specific Gravity) of the photoresist is a measure of the density of the photoresist. It is related to the solids content in the photoresist. A larger proportion means that the photoresist contains more solids, higher viscosity and poorer fluidity. The unit of viscosity: Poise (P,1P = 10-1Pa-s), photoresist is generally measured in centipoise (cP,1cP = 10-2P). Percent poise, I .e. centipoise, is the absolute viscosity rate; kinematic viscosity rate is defined as: kinematic viscosity rate = absolute viscosity rate/specific gravity. Unit:% Stokes (cst)= 1 mm2/s. Most photoresist manufacturers measure viscosity by rotating a wind vane in the photoresist.

5. Adhesion

Adhesion (Adherence) is a characterization of the strength of the photoresist adhesion to the substrate. The main measure of photoresist resistance to wet corrosion. It is not only related to the nature of the photoresist itself, but also closely related to the nature of the substrate and its surface conditions. As an etch stop layer, the photoresist layer must adhere well to the surface of the wafer in order to faithfully transfer the pattern of the photoresist layer to the surface of the wafer. Insufficient adhesion of the photoresist will cause the pattern on the surface of the silicon wafer to deform. The adhesion of the photoresist must withstand subsequent processes (etching, ion implantation, etc.). Usually negative glue has a stronger bonding ability than positive glue.

6. Corrosion resistance

Anti-etching (Etching resistance) is the resistance of a photoresist material during etching. During the transfer of the pattern from the photoresist to the wafer, the photoresist material must be able to withstand high energy and high temperature (>150°C) without changing its original properties. The substrate surface is protected in the subsequent etching process. Thermal stability, anti-etching ability and anti-ion bombardment ability.

In wet etching, the photoresist printed with the circuit pattern needs to be placed in a chemical etching solution together with the silicon wafer for many times of wet etching. Only when the photoresist has strong corrosion resistance, can it be ensured that the etching solution etches the exposed pattern according to the desired selection ratio, and better reflects the performance of the device.

In dry etching, for example, when ion implantation is performed in the well region and the source/drain region in the integrated circuit process, it is necessary to have a better ability to protect the circuit pattern, otherwise the photoresist will volatilize in the implantation environment and affect the vacuum degree of the implantation chamber. At this time, the implanted ions will not play their role in the circuit manufacturing process, and the circuit performance of the device will be hindered.

7. Surface tension

Surface tension (surface tension) refers to the intermolecular attractive forces in a liquid that pull surface molecules into the bulk of the liquid. The photoresist should have a relatively low surface tension so that the photoresist has good flow and coverage.

8. Pinhole

A pinhole is a very small sized hole in the photoresist layer. Pinholes are detrimental because they can allow the etchant to seep through the photoresist layer and thus etch small holes in the wafer surface layer. Pinholes are caused by particulate contaminants in the environment during the coating process or by voids in the photoresist layer structure. The thicker the photoresist layer, the less pinholes, but it reduces the resolution, the photoresist thickness selection process needs to weigh the influence of these two factors. The aspect ratio of the positive glue is higher, so the positive glue can achieve the desired pattern size with a thicker photoresist film, and there are fewer pinholes.

9. Purity

Purity means that the photoresist must meet stringent standards in terms of particulate content, sodium and trace metal impurities and water content. The integrated circuit process has very strict requirements on the purity of photoresist, especially the content of metal ions. For example, when the G-line photoresist is developed into the I-line photoresist material, the content of metal Na, Fe and K ions is reduced from 10 to the -7 power to 10 to the -8 power.

10. Thermal process

There are two heating processes during the photolithography process: soft bake and hard bake. The craftsman bakes at high temperature to maximize the bonding ability of the photoresist as much as possible. However, as a material like plastic, photoresist will soften and flow when heated, which has an important influence on the final pattern size. The dimensional changes brought about by the thermal process must be considered in the process design. The more stable the thermal flow, the more favorable the process flow.

11, other

In the actual process, the selection of photoresist must also consider the type and nature of the film on the surface of the silicon wafer (reflectivity, hydrophilicity or hydrophobicity) and the required resolution of the product pattern.

A good photoresist must have high resolution, high sensitivity and high contrast to ensure that a precise image can be transferred from the reticle to the silicon wafer. Described in the industry as resolution, contrast, sensitivity. In addition, the technical requirements of photoresist are high, all technical indicators must meet the standards, so in addition to the above three hard indicators, a good photoresist must also have strong etching resistance, high purity, low solubility, high adhesion, small surface tension, low cost, long life cycle and high glass transition temperature.