Thin-Film Fabrication Experiment Discussion Examples | Effects of Film Thickness, Uniformity, and Drying Conditions

What Is a Thin-Film Fabrication Experiment?

A thin-film fabrication experiment is an experiment in which a material is formed into a thin film on a substrate and its film thickness, uniformity, surface condition, adhesion, transparency, functionality, and other properties are evaluated.
Thin films are used in a wide variety of fields, including optical materials, electronic materials, sensors, coatings, protective films, electrodes, and separation membranes.

The properties of a thin film are greatly affected not only by the material itself but also by the solution concentration, coating method, surface condition of the substrate, drying rate, and heat-treatment conditions.
Therefore, in a thin-film fabrication report, it is important to discuss the fabrication conditions in relation to the condition of the film.

Main Items to Include in the Results

In the results of a thin-film fabrication experiment, organize the materials used, solvent, solution concentration, substrate, coating method, drying conditions, film thickness, film uniformity, surface condition, presence or absence of defects, adhesion, and other information.
If the film thickness is measured at multiple locations, showing not only the average value but also the variation makes it easier to connect the results to a discussion of uniformity.

Main Items to Include in the Results

• Material used
• Solvent used
• Solution concentration
• Coating amount
• Type of substrate
• Whether the substrate was cleaned
• Fabrication method
• Rotational speed and time for spin coating
• Withdrawal speed for dip coating
• Drying temperature
• Drying time
• Heat-treatment conditions
• Film thickness
• Variation in film thickness
• Surface unevenness
• Presence or absence of bubbles, cracks, and peeling
• Transparency or cloudiness
• Adhesion
• Sources of error and points for improvement

Reference Experimental Values and Evaluation Examples for Thin-Film Fabrication

Here, the effects of film thickness, film-thickness variation, surface condition, drying conditions, and coating conditions obtained in thin-film fabrication experiments are organized as reference experimental values that are easy to discuss in reports.

In thin-film fabrication, film thickness and uniformity change depending on solution concentration, coating amount, rotational speed, drying temperature, drying time, cleanliness of the substrate, and other factors.
Fabricated films can be evaluated using film-thickness measurements, mass changes, optical-microscope observations, contact angle, transmittance, surface roughness, and similar methods.

Reference Experimental Conditions

Example of Film-Thickness Measurement Positions

Film thickness is measured not at only one point on the substrate but at multiple points to confirm the average value and variation.
Here, a reference example is shown in which a polymer thin film fabricated on a 25 mm square glass substrate was measured at five points.

In this example, the average film thickness is 1.19 μm, the maximum value is 1.24 μm, and the minimum value is 1.16 μm.
Because the differences among measurement positions are small, a relatively uniform film is considered to have been obtained.

Example Calculation of Film-Thickness Variation

Film-thickness variation can be evaluated by dividing the difference between the maximum and minimum film thicknesses by the average film thickness.

Film-thickness variation (%) = (Maximum film thickness − Minimum film thickness) ÷ Average film thickness × 100

If the maximum film thickness is 1.24 μm, the minimum film thickness is 1.16 μm, and the average film thickness is 1.19 μm,

Film-thickness variation = (1.24 − 1.16) ÷ 1.19 × 100 = 6.7%

The film-thickness variation within this thin film is therefore calculated to be approximately 6.7%.

Relationship Between Spin-Coating Speed and Film Thickness

In spin coating, the higher the rotational speed, the more thinly the solution generally spreads, and the smaller the film thickness after drying becomes.

Increasing the rotational speed decreases the film thickness, but at an extremely high rotational speed, the film may become too thin and pinholes or discontinuities may occur.

Relationship Between Solution Concentration and Film Thickness

Under the same coating conditions, the higher the solution concentration, the greater the amount of solids remaining after drying and the larger the film thickness becomes.

As concentration increases, film thickness increases, but the solution viscosity also increases, making it more difficult for the solution to spread sufficiently during spinning and potentially causing streaks and thickness variations.

Changes in Film Condition With Drying Temperature

The drying conditions of a thin film affect surface smoothness, residual solvent, cracking, and adhesion.

Increasing the drying temperature makes it easier to remove the solvent, but rapid drying may cause the film to shrink and result in cracking or warping.

Effect of Residual Solvent Depending on Drying Time

If the drying time is short, solvent may remain in the film and affect the film thickness, adhesion, conductivity, transmittance, and other properties.

Because changes in film mass and film thickness become small after 60 minutes, drying is considered to have been almost complete after approximately 60 minutes under these conditions.

Comparison of Film Thickness and Uniformity by Coating Method

Spin coating makes it relatively easy to prepare uniform thin films, whereas casting makes it easy to prepare thick films but tends to produce unevenness during drying.

Effect of Substrate Cleaning

If dirt or oil remains on the substrate surface, the solution may not spread uniformly and dewetting or pinholes may occur.

The smaller the contact angle, the higher the wettability of the substrate surface and the more easily the coating liquid spreads.
Therefore, a more uniform thin film with fewer defects is more likely to be obtained.

Relationship Between Film Thickness and Transmittance

In transparent thin films, increasing the film thickness may increase light absorption and scattering and reduce transmittance.

In samples with large film thickness, not only absorption but also scattering caused by surface roughness and internal nonuniformity may reduce transmittance.

Example Observation of Drying Unevenness

During thin-film drying, ring-shaped unevenness and thickening at the edges may occur because of solvent evaporation rates and solute movement.

Example of Adhesion Evaluation

Tape tests or rubbing tests may be performed to confirm whether the fabricated thin film is firmly attached to the substrate.

Adhesion changes depending on substrate-surface cleanliness, wettability, shrinkage stress during drying, and film thickness.

Example of Reproducibility From Repeated Fabrication

The following is a reference example in which thin films were fabricated multiple times under the same conditions and variations in film thickness and transmittance were examined.

In the fourth run, the film thickness and film-thickness variation were both larger.
The coating amount, solution viscosity, substrate levelness, or time before drying began may have changed.

Example of How to Write the Results

A polymer thin film was fabricated by spin coating, and the film thickness was measured at five points on the substrate.
Under conditions of 2000 rpm for 60 seconds, the average film thickness was 1.19 μm and the film-thickness variation was 6.7%.
Because the differences in film thickness among the center, upper, lower, left, and right positions were small, a relatively uniform thin film is considered to have formed under these conditions.

When the rotational speed was varied from 500 rpm to 5000 rpm, the average film thickness decreased from 3.80 μm to 0.45 μm.
This is considered to be because the higher the rotational speed, the more thinly the coating liquid was spread by centrifugal force and the smaller the amount of solution remaining on the substrate became.
On the other hand, pinholes were observed at 5000 rpm, suggesting that the film became too thin to cover the entire substrate uniformly.

Regarding the effect of drying temperature, the appearance was good and the film thickness was stable when dried at 80°C.
However, fine cracks and discoloration were observed at 120°C and above.
This is considered to have resulted from rapid solvent evaporation, which caused the film to shrink and increased internal stress.
Therefore, in thin-film fabrication, it is necessary to select drying conditions that remove the solvent sufficiently while avoiding rapid shrinkage.

Points for Connecting the Results to the Discussion

In a discussion of thin-film fabrication, it is important to explain not only the numerical value of the film thickness but also the relationships among coating conditions, drying conditions, substrate condition, and appearance observations.

• Was the film thickness measured at multiple points, and were the average value and film-thickness variation calculated?
• Can the reason the film thickness decreases as the rotational speed increases be explained?
• Can it be discussed that higher solution concentration increases film thickness but also increases viscosity, making unevenness more likely?
• Can it be explained that drying temperature and drying time affect residual solvent, cracking, and adhesion?
• Can it be explained that substrate cleaning and wettability are related to pinholes and dewetting?
• Can the relationship among film thickness, transmittance, scattering, and cloudiness be discussed?
• Can changes in film thickness and uniformity depending on the coating method be compared?
• For data with poor reproducibility, can factors such as coating amount, solution viscosity, drying timing, and substrate levelness be considered?

Example Discussion

In this experiment, a polymer thin film was fabricated by spin coating, and the effects of fabrication conditions on film thickness and uniformity were investigated.
The film fabricated at 2000 rpm for 60 seconds had an average film thickness of 1.19 μm and a film-thickness variation of 6.7%, and a relatively uniform film was obtained.
Because the differences in film thickness among multiple points were small, the coating liquid is considered to have spread uniformly over the substrate.

As the rotational speed increased, the film thickness decreased.
This is because higher rotational speed increases the centrifugal force, making excess solution more likely to be discharged from the substrate.
However, at 5000 rpm, the film thickness decreased to 0.45 μm and pinholes were observed.
This suggests that when the film thickness becomes too small, there may be insufficient material to completely cover the substrate, making defects more likely.

Increasing the solution concentration increased the film thickness, but streak-like unevenness was observed at 5.0 wt%.
High-concentration solutions contain more solids and therefore readily form thick films, but their viscosity also increases, making uniform flow during spinning difficult.
As a result, traces of the coating direction or flow remained on the film surface and the film-thickness variation is considered to have increased.

Regarding the drying conditions, drying at 80°C produced little residual solvent and good appearance.
On the other hand, cracks occurred at 120°C and above.
This is considered to have been caused by rapid evaporation of the solvent, which caused the film to shrink in a short time and increased internal stress.
In thin-film fabrication, the temperature and time must be set while considering not only rapid drying but also film shrinkage and adhesion.

Substrate cleaning also had a large effect.
On the uncleaned substrate, the contact angle was large and many pinholes and areas of dewetting were observed.
This is considered to have occurred because dirt and oil remained on the substrate surface, preventing the coating liquid from wetting and spreading uniformly.
On the plasma-treated substrate, the contact angle was small and the number of defects was low, indicating that improved surface wettability is effective for forming a uniform thin film.

Summary

In thin-film fabrication, film thickness, uniformity, defects, adhesion, and transmittance change depending on coating conditions, solution concentration, drying conditions, and the cleaning condition of the substrate.
Film uniformity can be evaluated by measuring the film thickness at multiple points and determining the average value and film-thickness variation.

This reference example dealt with spin coating, dip coating, casting, and bar coating, using rotational speed, solution concentration, drying temperature, drying time, substrate cleaning, transmittance, adhesion, and drying unevenness as examples.
In a report, it is useful to discuss not only the numerical value of the film thickness but also the relationship between the fabrication conditions and the appearance and physical properties of the film.

What Is Film Thickness?

Film thickness is the value representing the thickness from the substrate surface to the surface of the thin film.
Film thickness affects the optical properties, electrical properties, mechanical strength, permeability, reactivity, and drying state of the thin film.
For example, if the film is thick, light transmittance may decrease, drying may take longer, and internal stress may increase, making cracks more likely.

On the other hand, if the film is too thin, problems may occur such as incomplete coverage of the substrate, formation of pinholes, insufficient mechanical strength, and weak measurement signals.
In thin-film fabrication, it is important to reproducibly prepare a film thickness suited to the intended purpose.

Main Factors That Determine Film Thickness

Film thickness changes depending on solution concentration, coating amount, viscosity, substrate wettability, drying rate, rotational speed, withdrawal speed, and other factors.
In spin coating, the faster the rotational speed, the more excess solution is thrown outward and the thinner the film tends to become.
In drop casting, the amount dropped and the spreading of the droplet during drying greatly affect the film thickness.

The higher the solution concentration, the greater the amount of solids remaining after drying, so the film tends to become thicker.
In addition, a highly viscous solution is difficult to spread and may produce a thick or uneven film.

What Is Film Uniformity?

Film uniformity indicates whether the film thickness, composition, and surface condition are consistent at every position on the substrate.
In a uniform thin film, the difference in thickness between the center and the edges is small and there are few surface irregularities and defects.
In a nonuniform film, the thickness and drying state differ depending on the location, resulting in variations in performance.

Uniformity is affected by the coating method, cleanliness of the substrate surface, solution wettability, drying rate, air flow, temperature gradients, and other factors.
When a thin film is used as a functional material, uniformity is extremely important in addition to the average film thickness.

Discussion of the Spin-Coating Method

Spin coating is a method in which a solution is dropped onto a substrate and spread thinly by high-speed rotation.
Excess solution is thrown outward by centrifugal force, while the solvent evaporates at the same time, forming a thin film.
Film thickness is affected by rotational speed, rotation time, solution concentration, viscosity, and solvent volatility.

In general, the faster the rotational speed, the thinner the film tends to become, while a longer rotation time allows more solvent to evaporate and the film thickness to stabilize more easily.
However, if the solvent evaporates too quickly, the film may dry before it has spread sufficiently, resulting in unevenness or ring-shaped defects.

Discussion of the Drop-Casting Method

Drop casting is a method in which a solution is dropped onto a substrate and a film is formed by natural drying or heated drying.
The operation is simple, but the method is easily affected by movement of the solution and uneven evaporation during drying, making the film thickness more likely to become nonuniform.

In particular, if the solvent evaporates rapidly at the edge of a droplet, the solute may be transported toward the edge and remain concentrated in a ring shape, producing the coffee-ring phenomenon.
Therefore, drop-cast films may have different film thicknesses and concentrations between the center and the edges.

Discussion of the Dip-Coating Method

Dip coating is a method in which a substrate is immersed in a solution and then withdrawn at a constant speed to form a thin film.
Film thickness is affected by withdrawal speed, solution concentration, viscosity, surface tension, and drying rate.
A higher withdrawal speed may increase the amount of solution adhering to and being withdrawn with the substrate, resulting in a thicker film.

However, if the withdrawal speed is unstable, film-thickness variation may occur in the vertical direction.
In addition, if the substrate surface is dirty, the solution may not wet it uniformly, causing streak-like unevenness or film discontinuities.

Effect of Solution Concentration

Solution concentration is directly related to thin-film thickness.
In a high-concentration solution, more material remains after drying, so the film thickness tends to become larger.
In a low-concentration solution, the film becomes thinner, but the substrate may not be completely covered or pinholes may occur.

In addition, increasing the concentration may also increase the viscosity, making it more difficult for the solution to spread over the substrate.
As a result, thick and thin regions may form, reducing uniformity.

Effect of Solvent Volatility

Solvent volatility greatly affects the drying rate and uniformity of a thin film.
With a highly volatile solvent, drying proceeds rapidly and a film is formed in a short time.
However, if the solution solidifies before spreading sufficiently, unevenness, streaks, cracks, or cloudiness may occur.

With a low-volatility solvent, drying proceeds slowly, making it easier to allow sufficient time for the solution to spread uniformly, but drying takes longer and dust adhesion or solute segregation may occur.
Solvent selection is an important condition determining film thickness and uniformity.

Effect of Drying Conditions

Drying conditions greatly affect the uniformity, film thickness, surface condition, adhesion, and presence or absence of cracks in a thin film.
If the drying temperature is high, the solvent evaporates rapidly, but rapid drying may generate stress inside the film and cause cracking or peeling.
If the drying temperature is low, solvent tends to remain, which may cause problems such as a soft or sticky film and unstable thickness.

Drying time is also important.
If the drying time is short, a large amount of residual solvent remains and affects film-thickness measurements and physical-property evaluations.
On the other hand, excessive drying or high-temperature treatment may cause material alteration, oxidation, thermal decomposition, or shrinkage.

Effect of the Substrate Surface

The condition of the substrate surface greatly affects the wettability and adhesion of a thin film.
If dirt, oil, dust, or moisture remains on the substrate, the solution cannot spread uniformly and may cause discontinuities, dewetting, pinholes, or unevenness.
In addition, if the interaction between the substrate and film material is weak, the film may peel easily after drying.

Substrate cleaning and surface treatment may improve wettability and adhesion.
In thin-film fabrication, not only the material solution but also the cleanliness of the substrate surface is an important experimental condition.

Discussion of Wettability

Wettability is a property indicating how easily a liquid spreads over a substrate surface.
When wettability is good, the solution spreads uniformly over the substrate and a uniform film is easier to form.
When wettability is poor, droplets become rounded and film discontinuities or dewetting are more likely to occur.

Wettability is affected by the chemical state and surface roughness of the substrate, surface tension of the solvent, and viscosity of the solution.
The smaller the contact angle, the better the wettability, while the larger the contact angle, the more difficult it is for the liquid to spread.

Causes of Film-Thickness Variation

Causes of film-thickness variation include nonuniform solution concentration, variation in coating amount, substrate inclination, differences in drying rate, air flow, contamination of the substrate surface, and excessively high solution viscosity.
In spin coating, flow from the center toward the edges of the substrate and the rotation conditions have an effect.
In drop casting, movement of the solute during droplet drying is a major factor.

When film-thickness variation is present, functional properties such as optical and electrical properties vary depending on location.
In a report, discussing differences among measurement positions in addition to the average film thickness makes the discussion more persuasive.

Discussion of the Coffee-Ring Phenomenon

The coffee-ring phenomenon is a phenomenon in which, as a droplet dries, solute moves toward the edge of the droplet and remains there in a concentrated ring.
It is commonly observed in drop casting and droplet drying.
If evaporation is faster at the edge of the droplet, liquid flows from the interior toward the edge and transports the solute there.

When this phenomenon occurs, the center of the film becomes thin and the edges become thick, making it difficult to obtain a uniform thin film.
Drying rate, substrate wettability, solvent type, solution concentration, temperature, and humidity affect the coffee-ring phenomenon.

Discussion When Bubbles Are Present

When bubbles are present in a thin film, the film thickness and surface condition become nonuniform, and the optical properties and mechanical strength are also affected.
Bubbles may be produced by stirring during solution preparation, entrainment of air during coating, dust on the substrate, or rapid evaporation of the solvent.

Regions containing bubbles may become thinner or remain as holes or depressions after drying.
Effective measures for preventing bubbles include handling the solution gently, degassing, filtering, and cleaning the substrate before coating.

Discussion When Cracks Occur

Causes of cracking in a thin film include shrinkage during drying, excessive film thickness, excessively rapid drying, insufficient adhesion to the substrate, differences in thermal expansion, and generation of internal stress.
When the solvent leaves during drying, the film shrinks, but because it is fixed to the substrate, it cannot shrink freely and stress develops.
If this stress exceeds the strength of the film, cracks occur.

Particularly in thick films, the drying rate differs between the surface and the interior, making the stress larger.
Cracking may be reduced by lowering the drying temperature, drying in stages, or making the film thinner.

Discussion When the Film Peels

Causes of peeling from the substrate include insufficient adhesion, contamination of the substrate surface, shrinkage stress during drying, differences in thermal expansion during heat treatment, and excessive film thickness.
If the interaction between the film and substrate is weak, the film cannot withstand stress generated during drying or heat treatment and may lift or peel.

Possible ways to improve adhesion include substrate cleaning, surface treatment, introduction of an underlayer, gentler drying conditions, and adjustment of the film thickness.
In a report, peeling is discussed not simply as a failure but from the viewpoint of the interaction between the film and substrate.

Discussion of Pinholes

Pinholes are small holes or discontinuities formed in a thin film.
Possible causes include dust on the substrate, particles in the solution, bubbles, insufficient wettability, low solution concentration, and insufficient coating amount.
If pinholes are present, the substrate is exposed and the protective properties, insulation, barrier properties, and optical properties of the thin film decrease.

To reduce pinholes, it is important to filter the solution, clean the substrate, fabricate the film in a clean environment, and select an appropriate concentration and coating amount.

Discussion When the Film Becomes Cloudy

Causes of cloudiness in thin films include phase separation, crystallization, aggregation of fine particles, uneven drying, rapid solvent evaporation, and incorporation of moisture from the air.
If microscopic regions with different refractive indices form inside the film, light is scattered and the film appears white.

When the purpose is to fabricate a transparent film, cloudiness is an important observation indicating low film uniformity.
It may be improved by reviewing the solvent, drying rate, concentration, and mixing condition.

Discussion of Insufficient Drying

If drying is insufficient, solvent remains in the film.
Residual solvent may cause the film thickness to change over time, make the film soft or sticky, reduce adhesion, and make measured values unstable.
Residual solvent also affects optical, electrical, and mechanical properties.

If film thickness is measured before drying is complete, subsequent solvent evaporation may make the film thinner and prevent reproducible measurements.
Checking mass changes or film-thickness changes before and after drying makes it easier to evaluate the drying state.

Effect of Heat Treatment

Heat treatment after thin-film fabrication may cause removal of residual solvent, densification of the film, crystallization, crosslinking, curing, and improved adhesion.
On the other hand, if the heat-treatment temperature is too high, decomposition, oxidation, shrinkage, cracking, and peeling may occur.

If the film thickness decreases after heat treatment, removal of residual solvent or densification of the film may be responsible.
If transparency or surface condition changes, crystallization or phase separation may also have contributed.

Film-Thickness Measurement Methods and Discussion

Methods for measuring film thickness include profilometry, ellipsometry, interferometry, SEM cross-sectional observation, AFM, and gravimetric methods.
The meaning of the obtained film thickness and the sources of error differ depending on the method used.
For example, profilometry measures the step between the substrate and the film, while ellipsometry estimates film thickness based on an optical model.

If the film thickness is nonuniform, the measured value changes depending on the measurement position.
Therefore, it is important to measure at multiple locations and show the average value and variation.
A single film-thickness measurement cannot sufficiently evaluate the uniformity of the entire film.

Causes of Variation in Measured Values

Causes of variation in film-thickness measurements include actual film-thickness variation, differences in measurement position, surface roughness, imperfect step preparation, calibration errors of the measuring instrument, sample inclination, softness of the film, and insufficient drying.
Particularly for thin films, even small surface irregularities and measurement noise may greatly affect the film-thickness value.

If the variation in measured values is large, it is necessary to consider separately whether the film itself is nonuniform or whether the measurement method contains errors.
Measurements at multiple points and comparison with another measurement method are effective.

Discussion of Film Adhesion

Thin-film adhesion indicates how strongly the film is bonded to the substrate.
A film with high adhesion is resistant to peeling during drying, heat treatment, washing, and external force.
A film with low adhesion may lift from the edges, crack, or peel.

Adhesion is affected by substrate cleanliness, surface roughness, surface energy, interaction with the film material, and stress during drying.
If the film peels, insufficient substrate cleaning and drying shrinkage stress are considered together in the discussion.

Discussion of Surface Roughness

Surface roughness indicates the degree of irregularity on the surface of a thin film.
A film with a smooth surface tends to be optically transparent and exhibit uniform properties.
On the other hand, large surface roughness may cause light scattering, variation in electrical resistance, reduced adhesion, and adhesion of dirt.

Causes of increased surface roughness include particle aggregation, uneven drying, nonuniform solution, substrate roughness, unstable coating conditions, bubbles, and crystallization.
Surface-observation results are discussed in relation to film thickness and drying conditions.

Discussion of Transparency

Thin-film transparency is affected by film thickness, surface roughness, material absorption, crystallinity, phase separation, particle aggregation, bubbles, and other factors.
As the film becomes thicker, light absorption and scattering may increase and transmittance may decrease.
If irregularities, bubbles, or phase separation are present on the surface or inside the film, light is scattered and the film becomes cloudy.

For thin films that require transparency, it is important to control the film thickness appropriately, smooth the surface, and reduce internal nonuniformity.
In a report, the visible transparency can be discussed in relation to the film thickness and surface condition.

When a Thin Film Can Be Considered Good

A thin film can be considered good when it has a film thickness suitable for the intended purpose, spreads uniformly over the entire substrate, and has few bubbles, cracks, pinholes, peeling, or cloudiness.
It is also important that there be little variation in film thickness measured at multiple locations and that the film thickness and appearance remain stable after drying.

However, whether a thin film is good or poor depends on its purpose.
When transparency is important, surface smoothness and low cloudiness are important, while for a protective film, adhesion and a low number of pinholes are important.
In a report, judgment should be made by clearly identifying which properties are important for the purpose of the experiment.

Example Discussion When the Experiment Did Not Go Well

When thin-film fabrication does not go well, possible causes are considered from results such as film-thickness variation, bubbles, cracks, pinholes, peeling, cloudiness, insufficient drying, film discontinuities, and increased surface roughness.
Organizing the causes separately into solution conditions, substrate condition, coating conditions, drying conditions, and measurement conditions makes the discussion easier.

How to Write Points for Improvement

In a discussion of thin-film fabrication, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements are easier to organize when divided into solution preparation, substrate treatment, coating conditions, drying conditions, and film-thickness measurement.

Improvements to Solution Preparation

• Prepare the solution concentration accurately
• Mix the solution thoroughly
• Remove insoluble matter and aggregates by filtration
• Remove bubbles
• If the viscosity is too high, review the concentration or solvent

Improvements to Substrate Treatment

• Clean the substrate thoroughly
• Avoid adhesion of dust and oil
• Dry the substrate surface
• Perform surface treatment when necessary
• Keep the substrate level

Improvements to Coating and Drying Conditions

• Keep the coating amount constant
• Keep the rotational speed and time constant in spin coating
• Keep the withdrawal speed constant in dip coating
• Standardize the drying temperature and drying time
• Avoid rapid drying
• Reduce the effects of air flow and dust

Improvements to Measurement

• Measure film thickness at multiple locations
• Show not only the average value but also the variation
• Measure the film thickness after drying
• Record the measurement positions
• Check calibration of the measuring instrument

Difference Between a Superficial Discussion and a Good Discussion

In a thin-film fabrication experiment, writing only that “a film was formed,” “there was unevenness,” or “it dried” results in a superficial discussion.
Relating film thickness, uniformity, solution concentration, substrate wettability, drying conditions, and causes of defects produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of a thin-film fabrication experiment.
Adjust the necessary parts according to your own experimental results.

• Film thickness is affected by solution concentration, coating amount, viscosity, and drying conditions.
• The higher the solution concentration, the greater the amount of solids remaining after drying and the greater the film thickness tends to become.
• If the wettability of the substrate surface is insufficient, film discontinuities and film-thickness variation are more likely to occur.
• If the drying rate is too high, unevenness and cracking may occur because of solute movement and shrinkage stress.
• The coffee-ring phenomenon may explain why the edges became thicker.
• Bubbles and dust cause pinholes and surface defects.
• If the film becomes cloudy, light scattering caused by phase separation, aggregation, crystallization, or surface roughness may be involved.
• Possible causes of film peeling include insufficient adhesion to the substrate and internal stress during drying.
• To evaluate film thickness, it is necessary to measure at multiple locations and confirm the average value and variation.
• To obtain a uniform thin film, it is important to keep the solution preparation, substrate cleaning, coating conditions, and drying conditions constant.

Points to Check When Discussing a Thin-Film Fabrication Experiment

Checking the following points before writing the report makes the discussion easier to write.

• Is the fabrication method clearly stated?
• Is the relationship between film thickness and fabrication conditions explained?
• Is not only the average film thickness but also its variation considered?
• Is film uniformity related to the measurement positions?
• Are the effects of solution concentration and viscosity discussed?
• Are the wettability and cleaning condition of the substrate surface considered?
• Are the effects of drying conditions explained?
• Are the causes of bubbles, cracks, pinholes, and peeling considered?
• Is the coffee-ring phenomenon explained when necessary?
• Are the effects of insufficient drying and residual solvent considered?
• Are errors caused by the measurement method considered?
• Do the points for improvement correspond to the sources of error?

Summary

In thin-film fabrication experiments, a solution is applied to a substrate and a film is formed by drying or heat treatment.
Film thickness and uniformity are greatly affected by solution concentration, coating amount, viscosity, substrate wettability, drying rate, rotational speed, withdrawal speed, and other factors.
If the film is too thick, uneven drying and cracking are more likely to occur, while if it is too thin, pinholes and film discontinuities may occur.

To obtain a uniform thin film, it is important to keep the substrate clean, prepare the solution uniformly, and keep the coating and drying conditions constant.
If the drying rate is too high, cracking and unevenness occur because of shrinkage stress and solute movement, while insufficient drying may cause residual solvent to make the film thickness and physical properties unstable.

In a report, rather than simply writing that “a film was formed,” discuss film thickness, uniformity, drying conditions, the substrate surface, and the causes of defects in relation to one another.
In thin-film fabrication, it is important to evaluate not only the average film thickness but also film-thickness variation, surface condition, adhesion, and the presence or absence of defects comprehensively.