Spin Coating Has Been Widely Used Biology Essay

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From early 20th century spin coating has been widely used for producing thin films of polymer on different surfaces. Spin coating is used in metal and glass industries to produce homogenous coating of high thickness and uniformity for a long time. Using spin coating method homogenous film of 2 to 5000nm thickness can be achieved[1]. The application of these films are very widespread, some of their application includes microelectronics, sensors, membranes, optical coating and protective coating [2-5]. The huge applications of polymer film is mainly due to advances in the properties of these films; such as barrier (mainly water and oxygen), protective (resistance to environmental conditions & robustness), optical and flexibility of modifying and/or imprinting film surface with 3 dimensional structures[6, 7].The list of properties and applications of these films are so wide spread and that is the reason that polymer films are of great pharmaceutical interest in last few years.

Spin Coating

Process of Spin coating

Dip coating[8], flow coating[9], spray coating, thermal spray coating[10], plasma polymerisation[11] and pulsed laser deposition[12] are some of the other techniques to obtain polymer films. But spin coating is by far the best technique to obtain uniformly spread homogenous film of polymer on a planner substrate. Spin coating is a technique in which excess solution of dissolved polymer is placed on to substrate, which than spun at very high speed to spread the solution under centrifugal forces. The centrifugal forces and the adhesive forces acting at liquid substrate interface cause radial flow, by which the solution is ejected of the surface. The remaining liquid slowly starts to evaporate causes reduction in thickness of the remaining liquid film. This causes the polymer concentration to increase and hence reduction in viscosity at liquid vapour surface. This drives the evaporation of solvent from the liquid film. After evaporation of remaining solvent, uniformly distributed solid polymer film can obtained. Schematic representation of spin coating technique can also be seen in .

Radial flow

Radial flow

Evaporation

Substrate

Solution

Figure Process of Spin Coating.

Factors effecting film thickness

The process of spin coating and various parameters affecting the thickness of the polymer films are extensively researched, understood and well documented[2]. It has been found from the previous studies that thickness of the film mainly depends on concentration of polymer [2], molecular weight of polymer [1, 13], spinning velocity [2, 14] and evaporation rate of solvent [1, 15] . there is also a evidence there from these studies that the amount of solution deposited, the rate at which it's deposited ,rotational acceleration prior to the final acceleration and the spin time have no or very little effect [2] . There are various models available for the process of spin coating which allows mathematical prediction of film thickness considering all the factors. But due to complex nature of the spin coating process and the various mechanisms involved, it is very difficult to come up with a model that covers every single parameter involved. There are many literatures available where mathematical models have been developed to correlate different parameters and film thickness [2, 16, 17].

Application of spin coated polymers

Microelectronics

The most popular applications of spin coating is in microelectronics, where light sensitive and/or electron sensitive polymers and polyimides are spin coated on to different surfaces[2]. This gives a polymer film with low dielectric constant, good adhesion and good thermal and mechanical properties[2], which are very desirable in microelectronics.

Optical coating

One of the other applications of spin coating of polymer film is in optical coating mainly due to their anti-reflective properties to improve light transmission[2, 18]. Low refractive index of material is very essential to obtain films that improve light transmission. The other properties that optical coating provides include selective wave transmission, reduction in viewing angle, hydrophobic repelling and protection for LCD and plasma display. Optical coatings are mainly used for optical sensors, emissive display and integrated optical circuits.

Protective coating

Polymer films are widely used to give protective coating to provide scratch resistance and protection against different environmental conditions. One of the most popular applications of this is in CD, DVD and blue-ray DVD industries to provide protection against scratch, static electricity and UV light[2, 19].

Membranes

Polymer films are also used get desired quality membranes especially porous membrane. In recent time film with cylindrical prose of uniform size are developed using block copolymers [2]. The size of the pores and properties of film can be controlled by different ratio of block copolymers.

Sensors

The other application that is getting particular attention in recent years is sensors such as especially biosensors. To name a few there are many different types of sensors including chemical, physical and biological. Example of these sensors include glucose sensor (biosensor) based on cellulose film derived from bacterial origin [20], pH sensor(chemical sensor) made by electrode position of polyaniline film [21] and humidity sensor based on poly(ethylene oxide) film[22]. In the last few years characteristic of polymer films for their application in biosensor has been extensively researched. Biosensor is a device that measure concentration or presence of different biological molecule at sensor surfaces. Once the sensor receives a signal it can causes a biological reaction to give quantifiable response; it is normally detected in form of electrochemical or optical signal. Biosensors are normally made of three different parts receptor, transducer and some measuring method. Normal biological receptor include tissue, microorganism, enzymes, anti-body and DNA, in addition, artificial receptors can also be used. These receptors can specifically bind to an analyte to give response to transducer. Transducer is a chemical or detecting element that can send signal in physiochemical or electrical mode which can be easily measured. For ease of use, the measured signal can be given on a friendly display: these can be anything from a colour changing strip to much more sophisticated LCD display that gives quantifiable results. This is usually the most expensive part of the device.

The instant measurement of analyte is of utter most important in treatment of various diseases. The problem is biological species are extremely hard to immobilise and will lose some activity depending on chemicals in solution and type of substrate used. Also, biological molecules like blood and saliva have lots of various reacting species in them; this can bind to sensor and cause fouling or loss of sensor activity. Sensors with electrochemical measurement system can have problems with other electro-active substance as it will affect the measurements. So it is very important to come up with membranes which are selective to one particular analyte and also stop interference of other substance with receptor. Polymer film provides the solution and it also gives good biocompatibility compared to conventional materials used. Amphoteric polymer films are being used increasingly since maines et.al successfully produced cellulose acetate films to enhance biocompatibility by selective permeation of small molecules[23]. There is a review article by Kingshhot and Griesser[24], which details polymer used to prevent biofouling.

Polymers for spin coating.

There are many research articles available out there, which utilise polymer films in many applications including biosensors. Most commonly used polymer materials are polystyrene (PS)[1, 25-27], poly vinyl alcohol (PVA), hrdroxy propyl methyl cellulose (HPMC)[28], hydroxyl ethyl cellulose (HEC)[28, 29], poly vinyl chloride (PVC)[1] and poly (methyl methacrylate) (PMMA)[1, 25, 27]. In most case combinations of polymers are used to get desired quality and characteristic of film. The properties considered are strictly looking at their application in biosensor field. Water soluble polymer PMMA, PVA, HEC, Polyvinylpyrrolidone (PVP) and hydroxyl methyl cellulose (HMC) were considered for the study. But due to higher molecular weight and low water solubility PMMA, PVA, and HMC were not dissolved after long time. During initial experiment PVA and HMC were not dissolved after left stirring for 7 days. Therefore, PMMA, PVA and HMC were not considered for further experiment.

Methods of analysis

Polymer films can be characterised by many ways including Atomic force microscopy (AFM), optical microscopy, scanning electron microscopy (SEM), Raman mapping and screening, X-ray diffraction and infrared spectroscopy (IR). Science the development of AFM in 1986, it has been extensively used to study surface characteristic of various materials. It has been one of the most important tools in understanding surface morphology of polymer films. AFM consist of cantilever with sharp tip at its end and it is used to scan sample surfaces. The tip of the cantilever is brought in contact with surface of sample and the sample, which cause deflection of cantilever according to hook's law. Laser id reflected on the cantilever and the response is detected on array of photodiode. represents schematic diagram of AFM.

Figure Schematic presentation of Atomic Force Microscopy.

AFM has many advantages over other techniques as it don't require any sample preparation, provides 3 dimensional images rather than 2 dimensional with some of the other techniques. AFM provides film resolution of up to nanometre scale and also the thickness of the film can be measured. Also samples are jet under normal condition there is no pressure or temperature requirement as with other techniques. Some of the other techniques were also used to evaluate film in initial stages was IR, SEM and optical microscope. The results suggested that it was not possible to evaluate film using IR and optical microscope. SEM gave some useful information and confirmed deposition of polymer film.

Gap in the knowledge

All the articles looked at so far mainly produces a film using two different block copolymer. There is very limited research available on characteristic of polymer film produced from spin coating of single polymer. Most of the study done using either organic or inorganic solvents but there is very limited articles which involve water as solvent. Hence, in this study characteristic of polymer film produced by water soluble polymer was investigated. It has been proven that there is relationship between speed of spin coating and thickness of the film. For most of the cases the process of spin coating is performed at speed ranging from 2000 rpm to 6000 rpm. So in this article effect of spin coating at higher speed (> 10000 rpm) is also investigated.

Aim

Produce a thin film of polymer by a process of spin coating using water as solvent. Investigate spin coating at higher speed (>10000) related to other factors and what effect it has on characteristic of film.

Objective

One of the main objectives is to produce a polymer film using water as solvent.

The other one is to produce a spin coated film using high speed of around 11000 rpm.

Investigate if thickness of film is correlated to concentration of polymer used.

Find a ways of analyse the characteristic of film and their thickness.

Materials and Equipments

Materials used

Table List of ingredients used for the

Equipment used

Methods

All the spin coated films obtained in this study was prepared using same method of preparation. The only difference was polymers used and concentration of polymers used. Therefore the following method demonstrates general method used to produce polymer film of spin coated polymer.

Preparation of polymer solution

Required amount of polymer was weighed in a weighing boat using top pan balance. The polymer was than dissolved in required amount of purified water, measured using measuring cylinder in a 50 ml glass beaker. The solution was left stirring on a magnetic stirrer with flea until all the solid polymer particles are dissolved completely. Table details the different concentration of solution made.

Table List of polymer solution and concentration made

Polymer

Concentration

Stirring

HEC

1.25 % w/w

1 % w/w

0.1 % w/w

0.01 % w/w

Preparation of polymer film by method of spin coating

Once the solution was prepared the next step was to cut a glass slide using a glass cutter into approximately 2cm x 2cm. Care must taken handling glass slide and glass cutter. Label the glass slide appropriately and stick a small piece of stick tape at the centre of the back side. Wash the glass slide with propane -2-ol and clean it with tissue to remove any oil deposited during handling. From this point onwards hold the glass slide from the edges to avoid finger print.

Once glass slide is labelled and washed, turn the vacuum pump on and stick the glass slide firmly using the stick tape side onto the spin coater. Then use a big plastic funnel and hold it upside down in a way that it covers the top of the spin coater. The reason behind doing this is to protect the user in case of glass slide flew off the spin coater. Turn the spin coater on and put approximately 1 ml of polymer solution on to the glass slide using dropper through funnel. Let it spin for at least 1 minute and after 1 minute turn the spin coater and vacuum pump off. Carefully remove the spin coated glass slide off the spin coater.

Method of analysis used for polymer films prepared.

List of techniques used for the analysis of polymer films produced

Optical microscopy

Infrared Spectroscopy

Scanning electron microscope

Atomic force microscopy

Figure AFM images for 1% HMC film

(a) Topography of 1% HMC film

(b) Topography of scratched 1% HMC film

Results

Figure Film thickness of 1 % HMC film.

Figure AFM image showing topography of 1% HEC film

Figure Dimension of black spot found on 1% HEC film.

Figure Topography of scratched 1% HEC film.

Figure Film thickness of 1% HEC film

Figure AFM image showing topography of 0.1% HEC film.

Discussion

The process of spin coating is normally done at speed ranging from 2000 rpm to 6000rpm. But in this study we looked at characteristic of film produced at speed of around 11000 rpm. During the early stages of the study 1% HMC in water was spin coated at 11000 rpm to produce a film. But after spin coating of HMC on glass slide, it wasn't visible on the glass slide. The glass slide was checked under optical microscope but due to both colourless substrate and film, optical microscope was unable to give any evidence on presence of film. AMF is the best tool for studying surface morphology and was considered as the best option to analyse the slide. But due to its unavailability at early stages, SEM and FT-IR was used to detect presence of film.

The SEM results showed that there is some deposition of HMC film on glass slide, but the SEM images () didn't give any information on characteristic or thickness of the film. The deposition of HMC film was further validated by Edx mapping method, in which it gives information on the elements presence on the surface. This method showed some carbon on the surface of the film , which can only be due to deposition of HMC film on glass slide. This was compared against map of empty glass slide used as control. The graph of edx mapping for both control and sample is given in figure. The empty class slide did not show any carbon and the edx map for slide with HMC film show reduced amount of silicon compare to control. These can be clearly seen on edx graphs as the silicon peak for control is bigger than that of slide with HMC film on. Peak for carbon can be observed on only slide with HMC film on. The figure shows FT-IR results for glass slide with HMC film and control. The transmission method of FT-IR was used to detect HMC film but it was not able to give any information on film. The reason for this was that it's not possible to get any transmission through colourless sample. The film and glass slide both being colourless didn't give any information. The FT-IR ATR method of reflectance would have been more use for to detect sample of this nature but was unavailable at that time.

Now that we know that the film is deposited, next step was to identify thickness of the film deposited. Therefore, 1 % HMC film was observed under AFM. shows the AFM images for 1% HMC films. (a) show very uniformly spread HMC film. The light spots on image are due to dust particle contaminating the film surface. (b) shows the topography of scratched 1% HMC film, this film was scratched to get information on thickness of the film deposited. The dark side in the images is the scratched part and the light part is where the film is still intact, this can be clearly distinguished in the topography showed in image b. In the middle part of the (b), there is 2 to three 3 different layers are visible and it is mainly due to folding of polymer film during scratching. 3 different points on this image was used to obtain the thickness of the 1% HMC film. Results in shows graphical representation of thickness of the film obtained. The thickness of the film is calculated as the difference between the scratched and unscratched region. All 3 areas suggesting film thickness above 50nm, with the mean thickness of 51.83 nm (50.5+52.7+52.3/3). This is a thin film and all 3 graph shows film thickness within 2 nm of each other, further proving uniformity of the film.

It has been proven from previous studies that polymer concentration has effect on film thickness. To look at this effect at higher speed of spin coating and also to study surface characteristic of film, 3 HEC film of different concentration was produced. These films were produced by 1%, 0.1% and 0.01% of HEC in water. From the surface of the film looks somewhat rough, as there are quite a few big dark parts and lots of small light spots. The dark spots are due to polymer not being stuck to substrate in that region, so there is no HEC deposited in that area. This is due to glass slide was not clead with propan-2-ol before spin coating and hence, oil not allowing polymer to stick into that area. The light spots are due to lots of polymer deposition, giving a huge peak like structure. Apart from that, films looks evenly spread out. The depth of the black spots was obtained; this is also a thickness of the film. details depth and width of, one these black spot. The results shows mean depth of 67 nm, which is essentially the thickness of the film. The film was scratched to obtain thickness; the shows topography of scratched 1% HEC film. The 2 different areas 1 and 2 on scratched film were selected to get much accurate thickness of the film, which shows the thickness of the film to be 50.7 nm. The showing graphs of difference between scratched and unscratched part for 3 different regions of 1%vHEC film. The graphs showing mean thickness of 55.2 nm (58+52.6+55/3). All the 3 results showing film thickness of 1% HEC vary around 6nm to each other, suggesting that it's a rough film compare to 1% HMC film. Film thickness for 1% HEC films also varies by 12nm compare to thickness obtained using the black spot (67nm) on the films.

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