cee, Author at Cost Effective Equipment

March 22, 2023March 22, 2023 cee Spin

The Cotton Candy Effect

TL;DR Summary

The cotton candy effect is a common issue in the semiconductor manufacturing process where dried material comes off the substrate in string-like filaments instead of droplets. To address this issue, process parameters and environmental factors should be adjusted. Optimization testing is required to fine-tune the process until the cotton candy effect is eliminated.

The Cotton Candy Effect

The cotton candy effect occurs when the material at the edge of the substrate dries too quickly and, instead of spinning off in droplets, the material comes off in filaments, which can be redeposited back onto the substrate. There are a variety of factors that influence the drying time of the material including evaporation of the solvent, substrate size, spin speed and acceleration, and exhaust parameters.

Addressing the Cotton Candy Effect

To reduce the cotton candy effect, we recommend the following actions:

Reduce spin speed & acceleration

Consider a reduction in spin speed and acceleration during the casting step. Higher spin speed and acceleration introduce air turbulence, more quickly evaporating solvent and prematurely drying the material. If you are unable to reduce your spin speed and acceleration or have already done so and are still experiencing the cotton candy effect, you may need to adjust your exhaust parameters.

Reduce exhaust flow rate

Excessive exhaust parameters pull solvents too quickly, leading to premature drying of the material. Mitigation can be managed in two ways: by reducing facility exhaust flow rate or with the use of Cee®’s proprietary programmable exhaust solution. As opposed to the static adjustment of facility exhaust which may detrimentally affect other processes and violate organizational safety practices, programmable exhaust allows for recipe-driven dynamic adjustment. Variances in material viscosity, solvents, substrate size, etc. can all influence your success in eliminating the cotton candy effect. Experiment with various parameter adjustments to fine-tune your process. For more information on how to effectively use programmable exhaust, see our [blog post] on programmable exhaust.

In the event that adjusting the exhaust parameters doesn’t eliminate the cotton candy effect, you will need to manufacture a solvent-rich environment using backside rinse (BSR) and/or edge-bead removal (EBR).

Create a solvent-rich environment with BSR & EBR

In this process you will artificially enrich the spin bowl to encourage the formation of droplets instead of strings. BSR and/or EBR are turned on at the beginning of the cast-off process when the cotton candy effect is encountered. When adequate coverage is achieved, turn both off and spin according to your normal process parameters for desired thickness.

*Be sure to consult your material manufacturer for a list of appropriate solvents for BSR & EBR.

Conclusion

The cotton candy effect occurs when the material at the edge of the wafer dries too quickly and comes off in cotton candy-like strings instead of droplets. To prevent this, it’s necessary to fine tune the parameters during the casting step. These parameters include adjusting spin speed & acceleration, experimenting with different exhaust parameters, and creating a solvent-rich environment. By following one or more of these suggestions, the cotton candy effect will be eliminated.

March 21, 2023March 21, 2023 cee Bake

Bake Plate Process Theory

TL;DR Summary

Hotplate baking is a popular technique for film drying and curing, offering advantages over traditional convection ovens such as decreased bake time, increased uniformity, reproducibility, and decreased particle contamination. Hotplate baking heats the substrate from the bottom up, preventing the formation of a skin on the film surface and offering advantages for thick films.

Bake Plate Process Theory

Hotplate bake processing has increased in popularity since the early 1980s. Previously the most common technique for film drying and curing was the convection oven. Bake plates (also known as hotplates) offer several advantages in the form of increased throughput, increased uniformity and reproducibility, and decreased particle contamination. In a typical bake process the substrate is placed into contact with a heated surface of known temperature. The substrate quickly rises to temperature. Drying and curing steps generally take about 1 minute. This is in contrast to traditional oven processes taking 30 minutes or more.

Bake Plates vs Conventional Ovens

Hotplates have several advantages over convection type ovens:

Decreased bake time
Increased reproducibility
Better film quality.

reproducibility, and decreased particle contamination. In a typical bake process the substrate is placed into contact with a heated surface of known temperature. The substrate quickly rises to temperature. Drying and curing steps generally take about 1 minute. This is in contrast to traditional oven processes taking 30 minutes or more.

Particle generation also occurs within a standard oven. In a forced-air, convection oven, substrates are commonly exposed to a flow of particle laden air for at least 30 minutes.

During resin film cures, the substrates will be exposed to considerable particulate contamination. The substrates

are vulnerable since the film may still contain solvents and during this ‘soft’ state, the film is very susceptible to having particles adhere to it.

The Skin Effect

Another disadvantage in normal oven baking results from baking substrates from the “outside in”. Since heat is applied to the outer surface of the film first, a skin forms on the surface of the film thus trapping solvents. Upon vaporizing, these solvents form blisters or bubbles which results in adhesion loss or even bulk film failure. This problem prevails in processes involving thick film resins, e.g. polyimides.

No skin effect occurs on a hotplate since hotplate baking heats the substrate from the bottom up. This “inside out” approach offers advantages for thick films since solvents in the film nearest the substrate are baked off before the fim surface seals over.

Hotplate Bake Variables and Methods

A typical bake process consists of preheating the surface to a known temperature, loading the substrate onto the surface for a specific length of time and removing it promptly at the end of the cycle. The selection of the temperature and time values used as well as the bake method employed all affect the overall performance of the process.

Bake Temperature

The bake temperature used is dependent on several factors. The material and substrate being baked as well as the results desired are key factors to be considered in developing a bake process.

In general, hotplate baking will be performed at temperatures slightly higher than those used in oven bake processes. The film being baked will reach a temperature somewhere between the temperature of the hotplate and the ambient air above the film. As an example, with a hotplate surface temperature of 115°C, a layer of photoresist on a silicon wafer will reach a final temperature of about 105°C after a few seconds. Thicker substrates and/or substrates with lower coefficients of thermal conductivity will require even higher temperatures to compensate for this phenomenon.

Bake Methods

Another important factor is the method of bake. Cee® hotplates allow for three distinct bake methods; proximity, soft contact, and hard contact.

In a hard contact bake the substrate is held onto the hotplate surface by the application of vacuum to the underside of the substrate. Small holes are machined into the hotplate surface in a pattern which optimizes vacuum distribution without the formation of cold spots or warping of the substrate. This method is usually preferred for silicon and other flat substrates where back side contact is not a problem.

Soft contact baking uses gravity alone to hold the substrate onto the hotplate. This method generally offers less uniformity since the substrate-hotplate thermal interface is not as efficient.

Proximity baking is accomplished by forcing nitrogen through ports in the hotplate surface. This forces the substrate to float at a distance of one to four mils (25-100µm) above the hotplate surface. Proximity baking allows a slower warm-up than contact bake methods and can be advantageous when baking thick films where blistering would otherwise be a problem.

Another advantage of proximity baking in this manner is that in many cases cambered or warped substrates can be baked with a high degree of uniformity. This is usually not possible with the contact methods since it is not possible to achieve a vacuum under a substrate that is not flat to start with. Processing cambered substrates with the soft contact method creates hot spots where the substrate touches the hotplate and cold spots where it does not. It should be noted as well that this type of proximity process is “self-leveling” in that the substrate will tend to form a uniform gap to the hotplate surface.

Proximity baking also offers the unique advantage of allowing hotplate processing without touching the bottom side of the substrate. An example of this application is photomask processing. In processing these relatively thick glass plates it is important that the back side of the glass not directly touch the hotplate since this causes micro-fractures in the glass itself from rapid heating. By performing the entire bake process in the proximity mode the integrity of the substrate is not endangered and the uniformity is excellent.

Exhaust Cover

The design of the Cee^®exhaust cover promotes the dissipation of vapors removed from a substrate placed on the chuck, without actually drawing air across the chuck surface.

Oven vs Bake Plate Examples

The information below presents process examples for commonly used resins. These figures should not be used as a rigid guideline, since the best method with a particular baking application can only be achieved through experimentation.

APPLICATION: Positive Photo Resist

OVEN BAKE

- 90°C – 30 minutes, Polyimide beta (partial imidization)
- 135°C – 30 minutes, Polyimide alpha (solvent removal)

HOTPLATE BAKE

- 115°C – 30 secs w/Hard Contact bake, Polyimide beta (partial imidization)
- 150°C – 15 secs w/Proximity bake, 150Â°C – 90 secs w/Hard Contact bake, Polyimide alpha (solvent removal)

Hotplate Process Troubleshooting

As with the spin coating process, there are no absolute rules for hotplate baking, only general guidelines. Following is a list of issues to consider for specific hotplate process problems.

Film Overbaked

bake temperature too high

select lower temperature
bake time too long

decrease bake time

Film Underbaked

bake temperature too low

select higher temperature
bake time too short

increase bake time

Film Blistering or Cracking

unstable balance in temp/time parameters

Decrease temp/increase time
warm-up time too fast

use proximity bake to preheat substrate

Non-Uniform Bake

operating with exhaust lid raised

lower lid exhaust
unstable ambient conditions

protect against major fluctuations
bake time too short

increase bake time
hotplate surface contamination

clean surface of hot plate
unstable balance in time/temp parameters

decrease temp/increase time

March 20, 2023March 21, 2023 cee Develop

Developer Configurations

TL;DR Summary

The use of immersion tank processes in photolithography has decreased due to excessive material consumption, non-uniform resolution, and poor clearing from high-aspect-ratio features. Spin developing has become the more popular method for development of features in photolithography, with three main methods emerging: puddle/stream dispense, side spray dispense, and direct spray dispense. The side spray dispense has become the most popular option, as it allows for a variety of wafer sizes, materials, and feature sizes, while direct spray dispense is useful for continuous-spray applications and accelerated development of thick films with high aspect ratios.

Developer Configurations

Development of features is a critical process step in photolithography. There are several processes to perform this step such as tank (bath) immersion and spin developing.

The use of immersion tank processes has steadily declined in MEMS fabrication and advanced lithography over the past decade due to excessive material consumption, non-uniform resolution, and poor clearing from high-aspect-ratio features. Additionally, increased throughput requirements and smaller features have further shifted mainstream applications to spin developing. As the popularity of the spin developing has grown, three main methods have emerged.

Puddle/Stream Dispense

The most common and oldest process is the stream/puddle dispense. Developer is dispensed into a puddle at the center of the wafer. The wafer is then spun at low speed (0-50 rpm) long enough for the developer to cover the entirety of the wafer. The rotation is then stopped allowing the chemical reaction to dissolve the soluble areas of the patterned film. This process is very popular for thin films ≤ 1 µm thick and/or for small wafer applications. For thick films and/or high-aspect-ratio features, the stream puddle step may be repeated over several iterations to reapply fresh developer. However, this method is problematic for many of today’s process requirements. Often features along the wafer perimeter are underdeveloped, and center features are over-developed due to the inherent over exposure of a puddle dispense.

Side Spray Dispense

The use of side-angle spray nozzles (45º) significantly enhances fluid deposition uniformity. The standard configuration utilizes two side-spray nozzles to evenly apply developer solution across the substrate. These nozzles are positioned to spray from the center of the wafer out. The side spray nozzles are factory positioned outside the wafer plane and ensure uniform deposition for many substrate sizes. Cee® recommends the side spray dispense option for aspect ratios up to 5:1.

Over the years, the side spray dispense has become the most popular option as it allows for a variety of wafers sizes, materials, and feature sizes. Cee® Developers use side spray dispense as the standard setup configuration.

Direct Spray Dispense

Direct-angle nozzles (90º) are often used for continuous-spray applications and allow accelerated development of thick films (5-100 µm) with features having high aspect ratios greater than 5:1. The direct angle provides sufficient agitation to penetrate the film and remove soluble material.

Cost Effective Equipment’s developers can be configured to accommodate process techniques including stream, spray, or direct spray dispense. As the predominant lab-scale equipment supplier for advanced R&D and prototyping, the Cost Effective Equipment product team is eager to meet your specific application needs.

March 20, 2023March 21, 2023 cee Bake

Multi-Stage Baking with Programmable Lift Pins

TL;DR Summary

Smart-Pin technology enables flexible temperature ramping and uniform baking for thermal shock sensitive materials. The lift pin height recipe utilizes a temperature matrix for soft bake, PEB, and post-develop hard bake. The programmable system eliminates the need for multiple bake plates and is suitable for thick-film materials and mitigating the “skin” effect.

Multi-Stage Baking with Programmable Lift Pins

Cee® Apogee™ Bake Plates now incorporate Smart-Pins. The lift-pins utilize a motor that drives the lift-pins to specific proximity height above the baking surface in any sequence or combination. The heights can be programmed in 0.1 mm increments, with an overall operating window from 0.1 mm to 19 mm. This feature allows flexible temperature ramping and can emulate several bake plate temperatures, while maintaining a high degree of bake uniformity. Programmable lift-pins are extremely valuable for the safe handling of thermal shock sensitive materials such as gallium arsenide, lithium niobate, indium phosphide, gallium nitride, silicon carbide, sapphire, and etc.

Cost Effective Equipment product engineers have utilized the KLA-Tencor® SensArray 17-point measurement probe to measure, track, and record temperature conditions produced by Cee® bake plates. The engineers have performed these process trials and developed user-friendly temperature matrixes for a variety of common temperatures used for a soft bake (100°C), post-exposure bake (PEB), and post-develop hard bake (205°C) for final curing.

The charts below represent several temperatures and incremental descent from 15.2 mm to hard vacuum contact in downward steps in increments of 2.5-0.5 mm. Each positional height is allowed to stabilize for a period of 300 seconds (5 minutes), and uniformity is recorded at these heights.

By eliminating the need for multiple bake plates, this programmable system is a cost-effective option in a space-saving design. This configuration is ideal for performing multiple stage baking, baking “from the inside out” for thick-film materials such as MicroChem SU-8 materials, MicroChem KMPR® materials, Shipley BPR™-100 photoresist, and Brewer Science® WaferBOND® HT-10.10 materials, and mitigating concerns associated with the “skin” effect.

The Cost Effective Equipment Apogee™ Bake Plate product family brings together all these capabilities in a compact footprint designed for a lab-scale environment.

March 17, 2023March 17, 2023 cee Spin, Spin Chucks

Spin Chucks for Square Substrates

TL;DR Summary

Spin-coating is a process used to evenly distribute material on a substrate by spinning it at a high speed. Round substrates are commonly used because their smooth edges create minimal turbulence, resulting in uniform evaporation rates. However, square and rectangular substrates create unique challenges due to increased air turbulence and can result in uneven coating thickness and poor uniformity. A recessed spin chuck, which emulates a round substrate, can be used to solve these problems by reducing turbulence and providing auto-centering and vacuum grip capabilities.

Overcoming Spin-Coating Challenges for Square Substrates

Typical spin-coating processes involve dispensing a small puddle onto the center of a round substrate. This material spreads due to the centrifugal force and evenly flows across the entire surface of the substrate. Final coating thickness and coating uniformity are affected by final rotational speed, acceleration, airflow turbulence, and fume exhaust. Process engineers have utilized spin-coating techniques for decades producing predictable results with standard round substrates.

Round substrates feature an advantage in airflow dynamics compared to other shapes because the smooth, contoured edges create minimal turbulence. This shape produces relatively uniform evaporation rates, as the fluid moves toward the edge during the spreading and subsequent drying steps. The resulting edge profile effect is very consistent and can be optimized through many standard process techniques.

However, square and rectangular shapes are commonly processed for a wide variety of applications. These square and rectangular substrates create unique and complicated challenges for spin-coating due to increased air turbulence. The leading edge causes significant turbulence which leads to uneven evaporation of the film resin and poor uniformity. Common imperfections seen on square or rectangular substrates are often referred to as “edge buildup,” “fringing,” or corner “interference bands.”

Looking for the perfect solution for your process needs? Our sales team is here to help you find the right products and services. Contact us now to get started!

March 17, 2023March 17, 2023 cee Spin, Spin Chucks

Spin Chucks for Thinned Substrates

TL;DR Summary

Thinner and delicate substrates are commonly used in the semiconductor industry, and their safe handling requires special techniques and spin chucks. These substrates can be made of various materials, including silicon, metal foils, and flexible polymer films, and are highly sensitive to mechanical and thermal shock.

Thin Wafer Processing Trends

Thin and fragile substrates are now common throughout the semiconductor industry. Thicknesses of the substrates often range from 50 to 100 µm (0.001 to 0.004 inch) and are cut into various shapes (round, square, and rectangular). Safely handling these fragile materials requires specially designed spin chucks and thin-wafer handling techniques.

Thinned substrates can be made of a wide variety of materials such as flexible polymer films (fluorinated ethylene propylene [FEP] and polyester [PET]) and metal foils (titanium, aluminum, and steel). Other substrate materials include silicon, gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), and silicon carbide (SiC). These CS and III-V materials are extremely brittle and far more sensitive to both mechanical and thermal shock.

Alternative Spin Chuck Designs

For thinned substrates (< 250 µm thick), we have developed a porous ceramic insert design that has a distinct advantage of completely supporting the backside of any given substrate dimension. The chuck distributes the vacuum equally through a porous surface

and mitigates any potential deflection, eliminating detrimental effects to your substrate or coat quality. These chucks are design specific and available for a wide array of shapes and sizes.

Film frames are commonly used to support silicon (Si) and CS materials. A device wafer can be mounted onto a film frame after backside processing but while it is still supported by its carrier, as recommended in the mechanical debonding process, or following separation (debonding) from the carrier, as in the thermal slide debonding process, for subsequent transport, cleaning, and packaging.

The porous ceramic design can also be adapted for thinned substrates mounted to film frames. Mechanical clamps and a porous ceramic insert combine for spin processing thinned substrates (< 250 µm thick) that have been taped to frames. The ceramic insert ensures complete and uniform backside support, while it distributes the vacuum source across the taped surface.

This design also utilizes vacuum O-rings and mechanical clamps for securing the outer film frame to the chuck assembly and maintaining positive lock.

Ready to take your thin films to the next level? Our sales team is standing by to show you how our products and services can help you get there. Contact us today to learn more!

March 17, 2023March 17, 2023 cee Spin

Selecting Maximum Spin Speed and Acceleration

TL;DR Summary

In spin coating, the speed and acceleration of a spinning substrate affect the thickness and quality of a film. The substrate’s rotational speed affects the centrifugal force and air turbulence, while its acceleration affects the film’s properties. The initial spin cycle is critical because the resin dries quickly, and up to 50% of base solvents can evaporate in the first few seconds. A fast “snap” process is preferred to cast the material evenly and overcome surface tension. The subsequent drying step is slower or stopped immediately.

Selecting Maximum Spin Speed and Acceleration

Velocity and acceleration are critical factors in determining the film thickness. The rotational speed of the substrate controls the amount of centrifugal force applied to the resin and the turbulence of the air above it. Acceleration of the substrate towards the final spin speed can also dramatically affect the properties of the film. Since the resin begins to dry during the initial part of the spin cycle, it is important to control acceleration to the desired set point. In many cases, up to 50% of the base solvents in the resin will be lost to evaporation in the first few seconds of the dispense and cast steps. Therefore, utilizing a “snap” process technique to aggressively cast the material from the center to the radius edge in less than 1 second is preferred. This aggressive ramp rate drives material towards the substrate edge, minimizes uneven evaporation, and overcomes surface tension to improve uniformity. The high velocity, high acceleration cast step is followed by a much slower drying step and/or immediately halted to 0 rpm.

However, too high velocity and acceleration can have detrimental effects on coating uniformity due to the creation of excessive turbulence. This turbulence is amplified with larger substrates because the velocity at the outer edge increases with diameter. This phenomenon is often characterized by the Reynolds number Re, and uses the following formula for a rotating wafer:

Re=ωr²/v, where:

Re is the Reynolds number
ω is the angular velocity of the substrate in revolutions per second
r is the radius of the substrate in meters
v is the kinematic viscosity of air, or 1.56×10^-5meters²/sec

The turbulence threshold limit is the square root of the Reynolds number. In the case of a spinning wafer, the turbulence threshold limit is 549.58. Therefore, any value greater than 550 is considered to be too turbulent to achieve uniform film thicknesses. The following table reflects the maximum speed for standard diameter substrates based upon this theory.

Substrate Diameter

Maximum Velocity

< 1″ (< 25mm)

288K RPM
2″ (< 50mm)

72K RPM
3″ (< 75mm)

32K RPM
100mm (4″)

18K RPM
125mm (5″)

9.8K RPM
150mm (6″)

8K RPM
200mm (8″)

4.5K RPM
300mm (12″)

2K RPM
450mm (18″)

802 RPM

Questions about this topic? For more information, contact our customer support team today!

March 17, 2023March 17, 2023 cee Spin

Programmable Exhaust for Optimal Thick-Film Spin Coating

TL;DR Summary

Overview

To achieve ultra-low uniformity on thick films in spin-coating processes, precise control of solvent vapors is crucial. A closed bowl environment and programmable exhaust module enable this control.

One of the most critical results in spin-coating processes is coating uniformity. In many cases, engineers are challenged with achieving ultra-low uniformity on thick films. To achieve such a high uniform coating, automated control of the solvent vapors is essential. A closed bowl environment combined with a programmable exhaust module allows the solvent vapor concentration to be precisely controlled at various stages in the spin-coating process.

Creating a Solvent-Rich Environment: Prewet Dispense Step

Typically, creating a solvent-rich environment is optimal for the initial dynamic dispense. This step is often accomplished through a prewet dispense step that is performed immediately before thick-film deposition. The step consists of dispensing a small volume of solvent onto the substrate surface, which quickly casts the solvent onto the interior bowl surfaces. This preparation has the dual benefits of increasing vapor enrichment in the spin chamber and prepping the surface of the substrate for optimal spreading characteristics. The spin chamber exhaust is typically programmed to 0% flow during this phase, which mitigates the risk of solvent evaporation prior to the dispense step.

Applying the Material: Dynamic Dispense Step

The dynamic dispense step uniformly spreads the coating material across the wafer surface. The material is applied while the substrate is spinning at relatively low speeds of 200 to 500 rpm. The dynamic method enables optimal coverage of the surface area while minimizing the overall volume of wasted material. Following the deposition step, the spin speed is slowly ramped to establish the final thickness. Depending on the material composition, molecular weight, and viscosity, the exhaust is generally throttled at 0% to 50% closed. The combined effects of spin speed, vapor concentration, and time will determine the final coating thickness.

Stabilizing the Film: Drying Step

Finally, a drying step removes residual solvents and increases the physical stability of the film. Thin-film applications often utilize a higher spin speed for this effect; however, many thick films must remain at a lower speed to prevent additional thinning. We recommend the programmable exhaust be set at 100% during this phase to assist in faster solvent evaporation. This setting provides the additional benefit of removing the residual vapors from the chamber before removal of the substate.

Spin speed, acceleration, and drying rate are the most important variables in determining the final film thickness and uniformity. Cost Effective Equipment products provide these precision controls to enable optimal coating results for lab-scale and pilot-line applications. Contact our sales team for more information today!

March 17, 2023March 17, 2023 cee Spin

Spin Coat Theory

Spin Coating Overview

Spin coating has been used for several decades as a method for applying thin films. A typical process involves depositing a small puddle of a fluid material onto the center of a substrate and then spinning the substrate at high speed. Centripetal acceleration will cause the resin to spread across the substrate, leaving a thin film of material. Final film thickness will depend on the fluid material properties (viscosity, drying rate, percent solids, surface tension, etc.) and the spin process parameters (rotation speed, acceleration, and fume exhaust). One of the most important factors in spin coating is repeatability, as subtle variations in the parameters that define a spin-coating process can result in drastic variations in the coated film.

Spin Coating Process Description

A typical spin process consists of a dispense step in which the resin fluid is deposited onto the substrate surface, a high speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. Two common methods of dispense are Static dispense, and Dynamic dispense.

Static dispense is simply depositing a small puddle of fluid on or near the center of the substrate. This can range from 1 to 10 cc depending on the viscosity of the fluid and the size of the substrate to be coated. Higher viscosity and or larger substrates typically require a larger puddle to ensure full coverage of the substrate during the high speed spin step. Dynamic dispense is the process of dispensing while the substrate is turning at low speed. A speed of about 500 rpm is commonly used during this step of the process. This serves

to spread the fluid over the substrate and can result in less waste of resin material since it is usually not necessary to deposit as much to wet the entire surface of the substrate. This is a particularly advantageous method when the fluid or substrate itself has poor wetting abilities and can eliminate voids that may otherwise form.

After the dispense step, it is common to accelerate to a relatively high speed to thin the fluid to near its final desired thickness. Typical spin speeds for this step range from 1500-6000 rpm, depending on the properties of the fluid as well as the substrate. This step can take from 10 seconds to several minutes. The combination of spin speed and time selected for this step will generally define the final film thickness. In general, higher spin speeds and longer spin times create thinner films.

A separate drying step is sometimes added after the high-speed spin step to further dry the film without substantially thinning it. This can be advantageous for thick films since long drying times may be necessary to increase the physical stability of the film before handling. Without the drying step problems can occur during handling, such as pouring off the side of the substrate when removing it from the spin bowl. A moderate spin speed will aid in drying the film without significantly changing the film thickness.

Spin Speed

Spin speed is one of the most important factors in spin coating. The speed (rpm) affects the degree of centrifugal force applied to the resin and the turbulence of the air immediately above it. Relatively minor speed variations at this stage can result in large thickness changes. Film thickness is largely a balance between the force applied to shear the fluid resin towards the edge of the substrate and the drying rate of the resin. As the

resin dries, the viscosity increases until the radial force of the spin process can no longer move the resin over the surface. At this point, the film thickness will not decrease significantly with increased spin time. All Cee^® spin coating systems are specified to be repeatable to within ±0.2 rpm at all speeds.

Acceleration

In addition to spin speed, acceleration can also affect the coated film properties. Since the resin begins to dry during the first part of the spin cycle, it is important to accurately control acceleration. In some processes, 50% of the solvents in the resin will be lost to evaporation in the first few seconds of the process.

Acceleration also plays a large role in the coat properties of patterned substrates. In many cases the substrate will retain topographical features from previous processes; it is therefore important

to uniformly coat the resin over and through these features. While the spin process in general provides a radial (outward) force to the resin, the acceleration aids in the dispersal of the resin around topography that might otherwise shadow portions of the substrate from the fluid. Cee® spinners is programmable with a maximum acceleration of 30,000 rpm/second (unloaded).

Fume Exhaust

The drying rate of the resin is defined by the properties of the fluid, as well as by the air surrounding the substrate during the spin process. It is well known that such factors as air temperature and humidity play a large role in determining coated film properties. It is also very important that the airflow and associated turbulence above the substrate itself be minimized, or at least held constant, during the spin process.

All Cee® spin coaters employ a “closed bowl” design. While not actually an airtight environment, the exhaust lid allows only minimal exhaust during the spin process. Combined with the bottom exhaust port located beneath the spin chuck, the exhaust lid becomes part of a system to minimize unwanted random turbulence.

The distinct advantage to this system is slow drying of the fluid resin. The slower rate of drying offers the advantage of increased film thickness uniformity across the substrates. The fluid dries out as it moves toward the edge of the substrate during the spin process. This can lead to radial thickness non-uniformities since the fluid viscosity changes with distance from the center of the substrate. By slowing the rate of drying, it is possible for the viscosity to remain more constant across the substrate.

Drying rate and hence final film thickness is also affected by ambient humidity. Variations of only a few percent relative humidity can result in large changes in film thickness. By spinning in a closed bowl the vapors of the solvents in the resin itself are retained in the bowl environment and tend to overshadow the affects of

minor humidity variations. At the end of the spin process, when the lid is lifted to remove the substrate, full exhaust is maintained to contain and remove solvent vapors.

Another advantage to this “closed bowl” design is the reduced susceptibility to variations in air flow around the spinning substrate. In a typical clean room, for instance, there is a constant downward flow of air at about 100 feet per minute (30m/min). Various factors affect the local properties of this air flow. Turbulence and eddy currents are common results of this high degree of air flow. Minor changes in the nature of the environment can create drastic alteration in the downward flow of air. By closing the bowl with a smooth lid surface, variations and turbulence caused by the presence of operators and other equipment are eliminated from the spin process.

Process Trend Charts

These charts represent general trends for the various process parameters. For most resin materials the final film thickness will be inversely proportional to the spin speed and spin time. Final thickness will also be somewhat proportional to the exhaust volume although uniformity will suffer if the exhaust flow is too high since turbulence will cause non uniform drying of the film during the spin process.

Spin Coating Process Troubleshooting

As explained previously, there are several major factors affecting the coating process. Among these are spin speed, acceleration, spin time and exhaust. Process parameters vary greatly for different resin materials and substrates so there are no fixed rules for spin coat processing, only general guidelines. Following is a list of issues to consider for specific process problems.

Film too Thin

Film too Thick

Poor Reproducibility

Poor Film Quality

Root Cause for Common Defects

Air Bubbles on Wafer Surface

Comets, Streaks, or Flares

Swirl Pattern

Center Circle (chuck mark)

Uncoated Areas

Pinholes

Air Bubbles on Wafer Surface

Comets, Streaks, or Flares

Swirl Pattern

Center Circle (chuck mark)

Uncoated Areas

Pinholes

March 16, 2023March 17, 2023 cee Spin

Edge Bead Removal and Backside Rinse Demystified

TL;DR Summary

Edge bead removal is used to eliminate build up while backside rinse prevents contamination. Using an automated system for both processes results in precise, efficient, and consistent results with minimal operator intervention.

Introduction

Edge bead removal (EBR) and back-side rinse (BSR) are critical processes in the fabrication of high-quality micro and nanoscale devices such as sensors, solar cells, and microelectronics. In this post, we’ll delve deeper into edge bead removal and back side rinse, outlining their use cases and traditional methods for achieving them.

Edge Bead Phenomenon

In spin coating, a liquid solution (such as a photoresist) is dispensed onto the center of a substrate. The substrate is then rotated rapidly, typically at speeds of several thousand rotations per minute, causing the liquid to spread outwards towards the edge of the substrate to form a thin, uniform film over the surface. During this process, the material tends to accumulate along the edges of the substrate, forming a ridge or bead.

Edge Bead Removal (EBR)

EBR is a process that removes excess material from the edge of a coated substrate preventing any defects that may arise due to non-uniformity. Traditionally, there are three methods for edge bead removal: mechanical scraping, solvent wiping, and via the use of a specialized automated edge bead removal system. Mechanical scraping (e.g., razor blade) is a simple and inexpensive method but causes damage to substrates and/or compromise the photoresist pattern. Solvent wiping is far more gentle than mechanical scraping but requires time and effort to achieve full removal of edge bead, especially in the case of materials that do not dissolve easily. Automated edge bead removal systems are designed to selectively remove the edge bead without affecting the rest of the photoresist pattern as the spin process completes.

Back Side Contamination

During the spin coating process, material is dispensed and as the substrate spins, material may migrate or splash onto the back of the substrate. If the back side of the substrate is not rinsed properly the contaminants and residues on the back side may contaminate the substrate and equipment in subsequent processing.

Back Side Rinse (BSR)

BSR is achieved by applying a rinse solution to the back of the substrate while it is spinning to remove residual photoresist or developing solution. The rinse is applied to the exposed underside of the substrate and is evenly distributed to the edge as the substrate spins. The result is a clean underside which is important for avoiding unwanted contamination and achieving precise and reliable results.

Conclusion

In summary, edge bead removal (EBR) and back side rinse (BSR) are critical processes in the fabrication of high-quality semiconductor devices. While methods vary, integrated automated systems for both EBR and BSR provide precise, efficient, and consistent results with minimal operator intervention. If you’re interested in learning more about our offerings, please contact our sales team today.