Silicon Wafers for Spin Coating

university wafer substrates

What Silicon Wafer Should I use for Sping Coating?

The Spin coating process uniformly deposits thin layers on a substrate, such as silicon wafers, flat surface. Normally, a small amount of coating material is applied to the middle of the substrate, which rotates at low speed and does not rotate at all. The substrate is then rotated with centrifugal force distributed by the coating materials and rotation continues until the liquid turns around the edges of a substrate or reaches the desired thickness of the film.

So, what wafer spec should a researcher use for their spin coating?

A scientist asked us the following:

Could I have a quotation for the following please? - 10 boxes of 25 4-inch Si wafers, basic quality (for spin coating, but not electronics), 500µm thick and 1mm thick (both please) doping not important, orientation not important. 

Researchers have used the following inexpensive but high-quality substrate.

Si Item #452
100mm P(100) 0-100 ohm-cm SSP 500um Test Grade

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Spin Coating Explained!

Video: Spin Coating meaning, definition, explanation

Important Spin Coating Terms

  • film coatings
  • polymer solutions
  • force microscopy
  • casting solvents
  • nanoscale stretching
  • solvent viscosity
  • surface treatmen
  • spin coating
  • solvent molecules
  • surface properties 
  • cofacial polymer
  • polymer concentration
  • polymer films
  • surface effects
  • solution viscosity    

 

What is Silicon Wafer Spin Coating?

For decades, spin coating has been used to coat silicon wafers with a resistance film to produce high-yield, cost-effective, ultra-thin, high-performance and cost-effective silicon wafers. Spin coating usually involves coating or casting a solution of the desired material so that it can be easily coated or poured while it is being turned. [Sources: 7, 9]

In the traditional spin coating process, the coating material is deposited in the middle or back of the wafer and spun for a certain time until the remaining solvent evaporates and the thin film reaches from a few nanometers to a few micrometers thick. When the spinning layer is finished, a plate is placed on a hot plate, which is heated to about 100 oC, so that the solvent initially evaporates and solidifies in the plate. After the solvents have evaporated, it is put in a cold water bath for about 10 minutes until it solidifies. [Sources: 1, 8, 9]

Due to the influence of centrifugal forces on the spin coating process, the layer thickness and uniformity achieved by using the process solution in a spin coating process depends on both the size and shape of a wafer. Spin coating results in full coverage of wafers, but it suffers from the same problem of wasted coating as the traditional spinning process: it is too thin, too thick and not uniform enough. About 20% of the use is wasted because the excess solution is thrown away from the wafer substrate. [Sources: 1, 10]

This video shows poor uniformity of the coating caused by the capping of the spinning spreads on the dry surface of the substrate. In most cases, the two-stage spinning method involves drying the medium substrate before coating and removing the edge beads after coating to improve the uniformity of the spinning layer. Using a spin-coating spray jacket is much more efficient than coating the wafer with chemical vapor separation (CVD). [Sources: 9, 10]

As shown in examples, it is not necessary to coat the entire back of a semiconductor wafer with just one method. This video demonstrates the method of static dispensing coating using a 45 degree angle to avoid touching the edges of the substrate when moving the paint. It shows both static dispensing and spin coating methods, including touching substrate edges and inactive areas for applying paint. [Sources: 1, 9]

With spin coating at very low speeds, it is possible to produce higher nanorows than with drip casting. This provides a significant increase in uniformity compared to drip castings, although we cannot really describe the spin coatings because there are still centripetal forces. The repeatability is based on the fact that the parameters defined in the spinning process can lead to drastic fluctuations in coated films. [Sources: 2, 9]

In short, the properties of the film depend on its physical properties, such as thickness and uniformity. In most applications, the thickness of the films produced with spin coating is the most important consideration. [Sources: 9]

A resistance solution is dosed onto the surface of the wafer and spun quickly until it dries. After coating the resistance, a soft beacon is applied, in which the solvent is pushed away from the spinning resistance, improving the adhesion of the resistance to the wafer and exerting shear stress on it. Spin Coating normally performs the same task as Resist Coatings in the production of a thin layer of silicon wafers as they are produced for the production of high-performance electronics and other electronic components, but with a different purpose. [Sources: 3]

In most common spin coating techniques, the aim is to spin the substrate until the film is completely dry. The substrate is applied to a coating material that naturally covers the entire substrate well and then slowly rotates until it spreads well enough to cover it. However, if it rotates at all in silicon wafers, it does not rotate as quickly as in resist coating, but rotates so that the coating materials are well distributed over the entire substrate. [Sources: 6, 9]

A common defect in spin coating for beginners is dust, which can be seen both on the surface of silicon wafers and in the coating material itself. In spin coatings, the solution of the polymer solvent must be filtered through a 0.45 micron filter to remove dust (typically with a 1 / 4 micron filter). In addition, a solvent wash at the bottom of the wafer can eliminate the edge beads that form during the spinning process. [Sources: 8, 9, 10]

On the experimental side of the investigation, the textured silicon surface was first coated and then the reflection on the surface was measured. Figure 2 shows that the PS beads form a uniform monolayer and the etching direction is identical to the crystallographic orientation of silicon wafers with a chemical instability of 100%. When annealed experimentally at 125 degrees Celsius for 5 min, good surface uniformity is shown and surface reflection is significantly reduced. [Sources: 0, 4, 5]

Sources:

[0]: https://www.intechopen.com/books/new-research-on-silicon-structure-properties-technology/non-vacuum-process-for-production-of-crystalline-silicon-solar-cells

[1]: http://www.google.com/patents/US20110076858

[2]: https://www.scitek.com.au/applications/wafer-processing

[3]: https://www.eesemi.com/resist-processing.htm

[4]: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-19-S5-A1109

[5]: https://www.hindawi.com/journals/jnm/2014/439212/

[6]: https://openwetware.org/wiki/Spin_Coating_-_Yizhuo_Chen

[7]: https://www.s-cubed.com/spin-coater-equipment-and-processes/

[8]: https://www.spincoating.com/en/applications/spin-coating-theory-process/68/

[9]: https://www.ossila.com/pages/spin-coating

[10]: http://www.google.com.pg/patents/US5952045

What Silicon Wafers are used for Spin Coating on Polymers?

Scientists have used the following thin silicon wafers with tight Total Thickness Variation (TTV) for their research.

"I hope you are doing well. Just wondering if you would have single crystal, B-doped type silicon (100) wafers (native oxide layer) with thickness of 1 μm? Could you send me a quote for the #7 ultra thin wafers (5mm square, thickness 2 μm). I see there are 4 units in stock, right? By any chance, could you provide 5? If not, 4 wafers are fine."

5mm Square 2 micron TTV: 1 micron P/B 1-20 ohm-cm
Cz (100) Wafer Ring

Reference #2612548 for pricing.

What is Spin Coating on Polymers?

This article focuses on the technique and process of spin coating on polymers. We will examine the effects, problems, and other relevant information. To get started, we will first define what spin coating is. Then we will discuss the characteristics of the coating solvents used for spin coating on polymers. We will also review the different polymer classes used for spin coating. After that, we will discuss the effect of spin coating on polymers on the properties of the coatings.

Problems

Non-uniform film thickness is the most common problem associated with spin coating. To avoid this, lower the temperature of the spin coating solution. In addition, lowering the temperature will prevent the solution from evaporating too quickly. However, lowering the temperature of the spin coating solution will not completely eliminate the problems associated with non-uniform film thickness. Below are some common problems associated with spin coating on polymers.

The most significant disadvantage of spin coating on polymers is its low material efficiency. The final film thickness from this process is not proportional to o(-1/2), which is why larger substrates need a slower spin rate and thinner films. Another problem is that spin coating involves high-viscosity solutions, which tend to be less thinning than polymer films. Because of this, a larger film thickness requires a higher solution concentration to achieve the desired effect.

In addition to low throughput, spin coating on polymers is also problematic because it is a batch process. The solution used to coat the substrate is less than ten percent of the total material. Moreover, the solvent used during spin coating is not stable, which will change the surface properties and may damage other layers. If wetness problems are a concern, adjusting the volume will solve these issues. The video below shows a dynamic dispense spin coating procedure on PEDOT:PSS.

Technique

The spin coating technique is one of the most effective ways to deposit thin layers of materials. This technique can deposit high-quality layers of materials in a short amount of time. The high-speed spin coating process is particularly beneficial for industrial applications because it helps to achieve the most uniformity possible. In addition, spin coating speeds of over 1000 rpm help to achieve excellent uniformity, while those below 200 rpm may allow the polymer to dry slowly, allowing it to self-assemble.

The spin coating technique is an excellent choice for thin films, which are typically used for flat substrates. The coating solution is applied to the substrate, then spun off at high speed, ranging from 1000 to 8000 rpm. The spin speed and solvent concentration are crucial factors in determining the ultimate film thickness, as this can vary the thickness of the film. While this method works well for small substrates, it's not ideal for large scale applications because the film is thin and the process is expensive.

A significant challenge when using spin coating technique is using highly viscous solutions. The viscous solution is more resistant to deformation due to shear forces. As a result, the film formed may be very thick and not uniform. For this reason, it may be necessary to reduce the concentration of the solution before applying the film. However, this process can be a worthwhile investment if the desired film quality is sought. But be aware of the risks involved.

Process

The videomicroscopy images show the various stages of the spin coating process. The solution contains P3HT with a concentration of 1 wt% in chlorobenzene. The spin rate determines the film thickness. The initial film thickness h0 is proportional to the angular velocity o. If the spin speed is four times higher than o, the film thickness will be half the initial value. The spin curve can be derived by using this equation.

In this equation, C is the volume fraction of solute dissolved in the substrate. h0 is the thickness of the film at the transition between the film-thinning regimes. The coating solvent, k, is a constant that controls the film thickness. The k of typical spin coating solvents is approximately one centimeter per second. hf o (-1/2) is equivalent to Equation 1. The k of spin coating solvents is dependent on several assumptions. If any of them is wrong, the equation will fail.

The speed of the spin coater is critical. The higher the speed, the faster the film dries. The higher the airflow, the more consistent the film is. Spin coating reduces the need for post-desposition heat treatments. The process of spin coating is also highly cost-effective. Other batch-processing methods require expensive equipment and high energy. These processes are ideal for low-volume production. In addition, spin coating can be used on a variety of polymers, including silicone.

Effects

To understand the effects of spin coating, we must first understand the relationship between angular velocity and thickness. In general, angular velocity equals the spin speed, so the lower the spin speed, the thinner the film. In addition, spin coating processes at lower speeds allow more time for self-assembly. Pseudo-spinning can be a powerful tool for investigating order.

The final film thickness hf o is calculated by a similarity boundary-layer analysis. This result is consistent with limited experimental data. The constants k, initial polymer concentration, kinematic viscosity, solute diffusivity, and spin speed all affect the film thickness. The ratio of total spin time O-1 to O-1 is consistent with the data from experiments. The ratio between the film thickness and solvent evaporation is proportional to K, with a high correlation between kinematic viscosity and spin speed.

The physical mechanism governing spin coating is discussed. It consists of several components, including centrifugal and viscous forces, solute diffusion, and solvent evaporation. Solvent evaporation makes the solution viscous and reduces the flow of solution. Diffusion boundary layer thickness is directly related to the final solid film thickness. This relationship is essential for understanding spin coating thin-film formation.

Monitoring

Monitoring spin coating on polymers is a challenging problem due to the high speeds of the process and the non-equilibrium nature of the resulting film. Most studies of the process have relied on inferring the structure development from the final morphology, which is not always accurate. Therefore, in situ monitoring of spin coating on polymers is vital for the development of new materials. Here we discuss the challenges and the potential of in situ monitoring in spin coating.

The evaporation of the main solvent can be slowed by saturating the solution. However, this is not a practical method because of solvent waste and health and safety issues. Furthermore, the Marangoni defects are not always visible in high-volatility solutions, because viscous forces tend to dominate the Marangoni flow. Therefore, slowing the evaporation rate may result in films with improved crystallinity.

The process can be improved by using a non-standard pipettor. The normal pipettor technique involves pipetting the solution to the second stop, where the excess is sucked back into the tip. The second step causes additional drops on the surface, whereas the first method reduces bubbles. In addition to reducing bubbles, non-standard pipettor use can reduce the amount of solution on the surface.

Applications

Spin coating can be used to create thin, uniform films. During the spin coating process, fluid is spun off the edges of the polymer substrate and evaporates at the same time. The thinner the film, the higher the angular spinning speed. The solvent and viscosity of the solution influence the thickness of the film. Studies have shown that spin coating at low speeds can produce good results, but they are limited by poor uniformity and inconsistency.

High-speed spin coating is a batch process, which is very easy to scale. The spin process is relatively low-cost, making it an ideal coating method for large-scale industrial applications. In addition to being cheaper than batch dip coating, spin coating also has better thinness and uniformity. Unlike other, more exotic coating methods, spin coating has been widely used in high-volume production of high-end electronic devices. As far back as the 1950s, spin coating was used to deposit phosphor onto the glass surfaces of color television tubes.

The drying time of spin coating is directly related to the film's properties, and ambient conditions can have significant effects on this. Professional cleanrooms have well-controlled humidity and temperature. Glove boxes typically contain a nitrogen atmosphere to ensure sterility. Research labs, on the other hand, are not well-controlled environments. Extreme conditions can affect the consistency of the spin coating and can cause defects. This is why the solvents used to spin coat polymers are critical.

Video: What is Spin Coating?