The ophthalmic lenses

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1. Introduction

Ophthalmic lenses are coated for variety of reasons. For example, ophthalmic lenses have to withstand scratching and friction. There are also some applications requiring special optical properties such as UV- resistant and better transmittance. Other requirements include the ability to be easily cleaned and to adapt variety of surrounding. None of these requirements is able to be attained using single lenses without coating .

Surface analysis techniques such as Auger electron spectrum (AES), X-ray photoelectron spectroscopy (XPS), and Secondary Ion Mass Spectrum (SIMS) are quite suitable for characterising thin solid films such as lenses coating. Firstly, they are sensitive within a thickness of several monolayers to tens of nanometres, which is measurable to the scale of lenses coating. Secondly, variety of information about solid films such as the thickness, chemical state and composition are provided by these techniques. Moreover, they are also quite accurate and well-established techniques.

The result of surface analysis is of great significance to the development of technical processing. This case study give a series of research about the lenses coating, aiming at showing how variety of surface analysis techniques are used to develop and improve the processing of the coating. These examples also help to understand the configuration of coating system, and currently methods used for processing.

2. The lenses coatings

2.1. Glass and polymeric lenses

Conventionally, the ophthalmic lenses are made of silicate glass because of their special optional property. However, there are some evident disadvantages. They are too brittle. Many people like me once had the memory to break an eyewear glass unintentionally. The time and money consumption for lenses manufacturing is also remarkable, which is not favoured for the entrepreneurs who purchase high productivity.

These problems encountered by glass ophthalmic lenses are overwhelmed by polymeric lenses. In China, more than sixty percent of market is occupied by polymeric lenses, and the market fraction is increasing [1]. Nevertheless, there are several disadvantages for the application of plastics. For example, the composition of plastic lenses varies from one batch to another. The thermal expansion coefficient for plastic is also much large, meaning plastic lenses might wrap or distorted during processing involving heat . The lenses are too soft, and is quite vulnerable to mechanical damages.

The most commonly used thermoplastic for eye wear applications is thermosets such CR39 or allyl diglycol carbonate. There is small fraction of lenses made using thermoplastics, for example, PMMA and polycarbonate. In comparison with thermosets, those thermoplastic materials are much less resistant to heat .

2.2. multilayer coating

The coating for ophthalmic lenses has a multi-layer configuration. For glass substrate which performs high scratch resistance, the coating system is an antireflective layer protected by top layer. The antireflective property of the lenses is improved by the coating layer. For polymeric lenses substrates, the coating is composed of an a hard layer, an antireflective layer, and a top layer.

A typical multilayer configuration for the coating system on plastic glass wear is shown in the Figure 2.1. It can be found that antireflective layer also involves more than two layers, which is favoured for reducing the reflective loss of light in lenses. A single layer decrease the reflective loss from 4.2% to 1.5%, while a double layer decrease the loss slightly more than that. Although there is significant decrease of reflective loss in double layer system at mediate wavelength. However, the reducing effect of a double layer system is largely limited within narrow wavelength band range, meaning a colour shift for the scope of eye glass wearer. In contrast to single and double layers systems, a multilayer system decrease the reflective loss from 4.2% to less than 0.5% in much broader wavelength range. This means a much better improvement for the performance of the lenses.

A significant problem encountered by the ophthalmic lenses coating is the internal stress between the layers due to multilayer configuration. The internal stress tends to initiate crack at the hard layer which is of highest brittleness. This problem is largely solved by using a two layers of different mechanical property instead of a single hard one . In the past an adhesive layer of Cr2O3 was used between the hard layer and substrate. Yet it has been integrated into the processing of hard layers, which is going to be introduced in later session.

3. The processing of multilayer coating

3.1. The preconditioning of adhesion

In order to obtain good adhesion, the surfaces of substrates are normally pre-treated using plasma or ions. A plasma is formed by charging gases by AC resources in a low pressure chamber filled with argon and oxygen atmosphere. The voltage and pressure should be controlled. More detail about plasma would be given in later session about plasma polymerisation. Normally a combination of argon and oxygen are used to generate plasma. It has been proved that a plasma using blend gasses is more effective than that of single composition [5]. For CVD and PVD process, the application of plasma and ions are integrated into the coating stage of the process, which is called plasma enhanced CVD(PECVD) or PEPVD.

3.2. Hard layers

One significant disadvantage of thermoplastic ophthalmic lenses, lack of scratch resistance, is largely overcome by the coating of hard layers. Silicon oxide is an idea material as hard coating because of its high strength covalence bond and hardness. Therefore, variety of efforts has been made in order to make silicate particles embedded into the substrate matrix to form the coating film, including the chemical vapour deposition (CVD), sol-gel process, and plasma polymerisation.

3.2.1. Processing method

Chemical Vaporous Disposition (CVD)

The most mentionable process used to making hard layer in ophthalmic lenses coating is chemical vapour deposition, in which the coating layer is formed through vapours reaction of precursors containing silicon. The most evident advantage of coating obtained through this method is the accurate control to the composition of the layer.

In order to stop crack propagation and to decrease internal stress, two or three layers of different mechanical properties should be processed by controlling the composition of precusor. For example, in the work by Martin-Palma and co-workers, the SiOxCy: H coating was formed using vapours reaction between oxygen and hexamethyldisiloxane (HMDSO) as precursor. Through controlling the ratio between HMDSO and oxygen, adhesion layer, flexible hard layer and hardening layer are formed outside the substrate by controlling the oxygen content in the plasma, and hence relative content to the precursor at different level . By the same token, using plasma assisted chemical vaporous disposition, Kreissig et al also produced hard coating of multilayer structure on the surface of polymeric substrate.

Liquid phase processing

Another technique worth mentioning is the liquid phase deposition processing, which is based on the reaction of liquid solution. One apt illustrating example involve the research by Hozumi, Kato and Takai who produced two layer hard coatings on PMMA and polycarbonate substrates through curing liquid film obtained through spin dipping. The beneath layer is cured by ultraviolet radiation, while the upper layer is cured by heat, and the additive might be added into the solution to making beneath layer . Coating film formed during the reaction of the liquid film after dipping. The role of the solution allows to be concluded as the precursor for the silicon oxide film.

One disadvantage which limits the application of liquid phase disposition is its requirement for the type of polymeric substrate. For polymer types of high polarity such as PMMA and polycarbonate, which are largely used in ophthalmic lenses, the substrate has to be coated in advance, or certain types of pre-conditioning is needed . This means extra process to be carried out in the manufacturing of the ophthalmic lenses, which is not favoured by the entrepreneurs.

Plasma polymerisation

Plasma polymerisation is a potential technique which can be used for the producing of hard layer on ophthalmic lenses. The principle of the plasma polymerisation and its application in ophthalmic lenses coating has been largely introduced by Wohlrab and Hofer [9]. In this process, the gas induced in a low pressure chamber was ignited to form plasma in which gas molecules decompose into positive ions and electrons. Since the electrons have much smaller probability to be scattered, the energy of electrons can be so high that they are able to initiate the polymerisation of monomers at room temperature is applied. Such reaction of appropriated monomer would form a coating layer which is a combination of SiOx and polymer. In comparison with liquid processing, plasma polymerisation has several advantages, such as less toxic chemicals involved and more easily multilayer coating. Using plasma polymerisation, it is possible to eliminate some undesired optical properties .

3.3. he antireflective layer(AR)

3.3.1. Function of anti-reflective layer

Anti-reflective layer benefit customers in many aspects. For example, after the interference of light transmitting through multilayer coating, the refraction index of the lenses is more similar to the medium. Drivers would get better vision in the night with the help of antireflective layer, and antireflective layer help to enhance contrast sensitivity and visual performance . The antireflective layer has contributions far more than optical performance. The deposited titanium oxide particles lead to self-cleaning ability of the lenses [10]. Many lenses coated by single antireflective layer have certain degree of ultraviolet resistance [11]. The composition of antireflective coating involves a variety of metallic oxides such as TiO2, ZrO2, Hf2O3, Al2O3 of different refractive index in laminate structure. Mg2F is also used in the antireflective coating for glass substrate [1]. The design principle for antireflective layer was reviewed by Schallenberg .

3.3.2. The processing of antireflective layer

Antireflective coating is deposited on either polymeric or glass lenses substrate using physical vapour deposition (PVD) method, in which a stoichiometric composition of the coating is obtained. In this processing, the molten metallic oxide evaporates rapidly in high vacuum, and then is sputtered to the surface of the polymeric or glass substrate at high rate. However, one big problem for such process is the decomposition of the oxide during sputtering. The oxygen in some oxide tends to leave the molecule under high vacuum, resulting in non-stoichiometric composition of the coating. Thus, a low pressure of oxygen is usually used to compensate oxygen loss during the process. Similar to the processing of hard coating, plasmas or UV irradiation are employed for preconditioning of the substrate, aiming at good adhesion of the coating layer. For polymeric substrate, the application of ion has more significance. The assistance of ion helps to avoid high temperature of the substrate. More detail about instruments and preconditioning has been largely reviewed by Pulker .

There are some other methods to deposit titanium oxide antireflective coating upon on polycarbonate substrates, such as sol-gel. In the process of sol-gel, a sol of titanium or silicon contained precursor forming a film on pre-treated surface of substrate, and loss water during the curing of the film, producing solid film which is composed of metallic oxide. Relevant examples involve the work reported by Yaghoubi, Taghavinia and Alamdari [10]. There is also research using chemical vapour deposition to produce antireflective layer.

3.4. Top coating

A layer of hydrophobic top coating is also deposited upon the substrate. The main reason to do so is to enhance the ability to be easily cleaned, and to protect the substrate from scratch to some extent. Top coating is processed through evaporation or wet chemical method, and chemical vapour deposition is also used. Nitrogen are sometimes added in order to release residual stress in the coating [15].

4. Application of surface analysis techniques

4.1. Compositional characterisation

There is a lot research in which elastic recoil detection analysis (ERDA) is used for compositional analysis. For example, in the research cited in prior session by Martin-Palma and co-workers, they used RF-PECVD method to produce hard coating upon polymeric substrate, and ERDA was used for compositional analysis of the coating. The composition result shows that there are at there are disparage chemical state of carbon when high and low RF power is used. When high power is used, the carbon is completely dissociated from the -CH3 group. Similarly, the compositional analysis result as a function of gas pressure shows that higher pressure lead to decrease amount of carbon and increase amount of oxygen in film. The content of silicon is independent with the content of carbon in plasma [4]. The compositional information help to control the processing parameter of coatings.

Another apt example showing the application of ERDA involves the depth profile of compositional analysis. In this study, the depth profile of heavy ion ERDA was carried out on a three layers coating with gradient of silica content. It was indicated by the depth profile that a thin layer forming adjacent to the substrate, in which the content of carbon and hydrogen is fairly high, is highly polymeric characteristic. The polymeric composition of the adhesive layer means that silica particles adhered, and entangled the molecular chains of substrates. The result obtained using heavy ion ERDA provided convince explanation for the adhesive mechanism .

In sum, it can be found that surface techniques such as elastic recoil detection analysis are quite useful technique for the development of coating method, because the information help to explain the evolution during processing, and to optimize the processing variables.

4.2. Determination of refined structure

Although there are some microscopy techniques like atomic force microscope (AFM) able to show the roughness and morphology on the surface, meaning that the change roughness become visiible, surface analysis techniques show their advantages in some aspects such as refined structure of the coating. These techniques include Rutherford Backscattered Spectrometry (RBS), neutron reflectivity and sum-frequency generation (SFG).

For example, in the work by Batagllin et al, single TiO2 layers on glass substrate were formed through reactive DC magnetron sputtering in argon and oxygen, and several samples of multilayer coating of TiO2//SiO2 were obtained through industrial disposition. Rutherford Back Scattering was used to indicate the thickness of each coating and the result is expressed in terms of the amount of monolayer within the coating. Another surface analysis technique, neutron reflectivity, was used to refine the coating structure for complex coating system. Using this technique the thickness and roughness of each coating were got, and the result is correlated to the optical property of the coating [16]. The refined structure of coating layers lead to new way to assess different processing method.

Another technique in which the coating configuration and refractive index are characterised simultaneously is spectroscopic ellipsometry. The research by Latella et al justify apt example. Spectroscopic ellipsometry were used in combination with contact angle measurement and XPS. The fine structure of the coating is sometimes important for the development of coating technology. In this case, the surface of as-received sample was treated using water plasma. After preconditioning , the contact angle of the sample surface have changed a lot, which is reflected in XPS spectrum pattern showing there was more C-OH bond. The roughness of the surface, nevertheless are still comparable, leading to the conclusion that enhanced adhesion derived from chemical evolution rather than mechanical mechanism .

In the research by Miyamae and Nozoye, a coating of silicon oxide is deposited upon PMMA substrate using electron beam source. The surface morphology experience very negligible change after coating of silicon oxide, as is indicated by atomic force microscope. Using surface analysis technology of sum-frequency generation (SFG), the change of molecular stereotype configuration during silicon oxide coating is the investigated. The comparison between the sps and ppp signal in C=O and C-H region indicates the molecular reconstruction on the surface. The ester methyl pendant groups tend to lay parallel to the surface after coating .

The cases given above might show the variation of structural evolution might be discernible only when techniques as delicate as SFG and RBS are employed. Thus, in order to understand the refined structure of coating layers, variety of surface analysis techniques are almost the only choices.

4.3. Solution for possible problem

In addition to film characterisation, which is of significance for the development of the processing route, surface analysis techniques are also used solve potential problems for lenses coating.

In the research by Kuhr et al, it was reported that both antireflection and hard layer are formed on PMMA substrate. Since the silicon-contained organic compounds are always added during the injection moulding process, the PMMA substrate was cleaned warm acidic bath with the help of supersonics. Secondary ion mass spectrum (SIMS) was used to identify residual silicon oil on the surface, showing that the silicon oil has been largely eliminated [19]. The result of SIMS proves that the rinsing to PMMA is a useful method to get rid of the contamination. This is of significance for the application of PICVD practice, since injection moulding is the most promising method to produce articles whose dimension is accurately required, and many additives used during injection moulding affect the coating adhesion negatively. The research imply possible solution for the problem due to injection moulding of polymeric lenses, which help to improve the performance of coating on plastic lenses.

By the same token, the effect of oxygen plasma on injection moulded Bisphenol A polycarbonate (BPA-PC) plate was also investigated utilizing a combination of techniques including SIMS, XPS and ellipsometry. Using a combination of XPS and ellipsometry, the relative content of carbon and oxygen in samples tend to remain constant after a threshold treating time, which is the optimal treating time in practices. Using principal component analysis (PCA) mode of SIMS, it become clear that aromatic structure within polymer molecules is highly vulnerable, which is the main reason for its degradation when too long time for curing was used. The formation of some small molecular weight material also occurred at the surface layer of the material during the curing [20]. The result suggests that excessive pre-treatment might be counterproductive in terms of adhesive enhancement.

The risk for polymer degradation was also proved by result of Electron spectroscopy for chemical analysis (ESCA), a technique quite sensitive to depth less than 20. Using ESCA, it was found that during ion bombardment to CR39, the oxygen contained groups was destroyed, while newly formed carbon groups contain no oxygen. Such effect is responsible to the worsened adhesive ability when the cure time is long. The depth profile of AES reveals the formation of intermixing layer between the coating and the substrate, which is the adhesive mechanism .

4.4. Surface analysis for adhesive mechanism

Although XPS result provides information about composition, in more circumstances the XPS is used to detect the change of chemical state, which indicate underlying mechanism during processing. This is also important to determine the optimal processing variables.

4.4.1. Hard layers

In the processing of hard layer through CVD, it is possible to use ammonia and other nitrogen-contained compounds as precursor to relieve the internal stress brought by multilayer configuration. Both XPS and XAES are carried out on samples made using different fraction of ammonia to determine the chemical state of each element. It was found that the peak position and the FWHM did not change, while the position and FWHM of Si 2p and Si KLL peaks changed. These phenomena are attributed to the formation of many silicon tetrahedrical bonds, which is verified by many the change of O 1s feature shape [22]. The surface analysis result help to understand underlying philosophy during process.

Liquid phase deposition for Hard layer coating is also improved by XPS result. In the research by Hazumi, Kato and Takai, double layer of the coating are obtained through liquid phase deposition. The XPS result shows that oxygen contained in the additives was present on interfaces between two layers, which is the reason for enhanced inter-coating adhesion. The understanding for the evolution during process make it clear that it is possible to use similar method to improve performance of solid film fabricated through liquid phase deposition.

There is some other research using XPS to explain adhesion of films obtained from liquid phase deposition. For example, in order to get good adhesion of the coating upon polymeric substrate of high polarity in liquid solution deposition, one method of pre-conditioning is surface oxidization of the substrate. Using X-ray photoelectron spectrum upon ZEONNOR substrate, it was found the surface oxidation arise increasing amount of OH- groups, which is the reason for fabulous adhesive .

4.4.2. AR layer

The research carried out by Battiston and co-workers further illustrate the application of XPS. Antireflective layer of titanium oxide was coated upon silicate glass through PE-CVD process. Titanium tetraisopropoxide was used as precursor, and blend of oxygen and nitrogen was used to generate plasma. The substrate was cleaned in advance, and etched using plasma for preconditioning. A variety of processing parameters are used, such as the substrate temperature, power and atmosphere for plasma. The XPS result shows that with increasing processing temperature, there is more nitrogen contained in the coating . In this case, the XPS results indicate not only the change of chemical state, but also the change of composition. The result shows the significance to control the atmosphere for pre-treatment.

Other method such as ion beam might also be used to pre-treat substrate surface. In the research by Lee, Chob, Kohb, and Kimathe, polycarbonate sample was subjected to Ar ion irradiation using a cold-cathode ion source in vacuum for preconditioning, followed by deposition of SiO2/TiO2 multilayer coating. It was shown that during ion conditioning the contacting angle of the sample become smaller, and higher ion doze result in smaller contacting angle, which is the evidence for more effective pre-conditioning. Such phenomenon is explained by XPS result on treated and untreated polycarbonate substrate surface, .showing increase amount of C=O bond after treatment .

4.4.3. Summary: the significance of mechanism

In session 4.4, a lot of research relevant to mechanism during process in introduced. The research focuses on improvement to adhesion of coating upon substrate during process, ending up with description to possible mechanism of evolution. The conclusion of the research can be concluded to be that an inducing of oxygen contained group on interface would strengthen the adhesion, and any possible method would induce oxygen to the surface would be favoured. These conclusions help to develop new method of adhesion enhancement.

5. Conclusion

a. How is coating system deposited?

In order to obtain good surface adhesion, the substrate are rinsed for cleaning first, and plasma or ion beams are employed for pre-treatment.

The main processing method of each coating layer is tabulated in Table 5.1.

Layer

Processing Method

Ref

Composition

Top layer

Evaporation,

Nitrogen induced polymer

CVD

[19]

Anti-reflective layer

PVD

[14][16]

Al2O3, HfO2, ZrO2, TiO2, MgF2 (for glass),SiO2(for polymer)

Sol-gel

[11]

Hard layer

CVD

[5][6][10]

SiO2 embedded in polymer

Liquid phase deposition

[8]

Plasma polymerisation

[9]

Table 1. The main processing method and composition for each coating layer

b. How does development of the technique benefit from surface analysis technique

Through characterizing solid films or treated substrates prepared in laboratory, engineers are able to evaluate the influence of processing parameters toward the sample, and to understand the change occurred during a process, in the promise that the experimental method is of certain similarity with the real condition of industrial practice. The mechanisms during each process, which is suggested by surface analysis techniques, are also of significance for the technique evolution,. It is possible to develop new method to enhance performance based on mechanism revealed by surface analysis result. Moreover, the surface analysis result suggest possible problem during process.

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