Photolithography: An Overview

The History and Development of Lithography

Discuss about the UV-Visible Spectrophotometric Method Development.

Photolithography is the combination of photography & lithography which generates an image and which is useful for various applications. The word lithography is derived from Greek words lithos and graphia which means writing on the stones. This process is used in transferring geometric shapes to a surface or a film during microfabrication. The geometric shapes and patterns which makes the structures on a semiconductor allows the electrical properties, dopants and wires to make a circuit which in modern days is used in most of the devices like mobiles, tablets, computers, sensors etc.

In early age, a German actor by the name Alois Senefelder and playwright, invented lithography as an inexpensive way to publish their theatrical works. It was the first of it kind of new printmaking technique that emerged after 300 years. This is a method of printmaking by using a stone or metal or a polyester plate on a completely smooth surface. It works on the principle of mutual repulsion of water and oil. The image obtained on the smooth surface through the print plate with an oil-based medium like wax crayon, while the negative image remains hydrophilic. Once the plate is brought in contact to a compatible printing ink and water mixture, the ink adhere to the positive image and water cleans the negative image. Today lithography is widely used in producing posters, maps, books, newspapers and packaging.

Technically, lithography is transferring of any shapes and patters to another surface while photolithography is directly related to lithography in semiconductors (Sungyong Choi, 2010). In the fabrication of integrated circuit, the semiconductor undergoes various physical and chemical processes. The process of making an IC can be categorised into semiconductor doping, film deposition and patterning. To connect and isolate transistors and its components films of both conductors and insulators are used.

Photolithography is one of the most commonly used technique for microfabrication. It is commonly used because of its extremely precise work of incisions. It is one of the most complex process as it needs ideal conditions to perform. It needs very clean substrate and its temperature condition should be idea. This technique is only used to make flat surfaces. Though it has some limitations, this process is widely used to make integrated circuits.

In modern days, the significance of lithography is briefed as:

The first being the various steps involved in manufacturing of integrated circuit which constitute 30% of manufacturing cost.

The second being, lithography acts a technical limiter when it comes to advancement in feature in reduction of size and further affecting speed of transistor and the silicon surface area. So, it is important to have the idea of operation cost and competence while manufacturing a circuit through lithography process for cost cutting and increasing its efficiency.

The photoresist which is sensitive to light when it is exposed it develops in the formation of 3D relief images on the surface of the substrate. Generally, the shape of the intended or designed pattern is same with vertical walls through the thickness of substrate in ideal photoresist image. Hence, the final pattern in binary meaning some parts are completely uncovered while the other parts are covered with resist. Binary pattern plays a significant role in transfer of pattern as the covered parts is protected from implantation of ion (Acikgoz, 2011).

How Lithography Works for Microfabrication

The steps of photolithography are as follows:

Photoresist is a material that is light sensitive material and when it is applied in the core of oxidized silicon wafer, it results in acceleration of wafer to a velocity ranging from 3000 to 7000 rpm for 30 to 60 seconds. This process results in spreading of the solution into thin and uniformity coating. Thereby spinning off the excess material (Young, 2008). The thickness generally ranges from 500 to 10000 A.

In some cases, before the application of photoresist the wafers are baked at 100 degrees Celsius to dry out the moisture from the silicon wafer surface and which result in better adhesion.

After the application of photoresist, now the wafers are put into an oven at a temperature of 80 degree Celsius for about an hour to toughen so that it transforms into a semisolid and it drive off solvents present in the photoresist (Moreau, 2017).

Afterwards, the silicon coated wafer is kept in a vessel known as mask aligner. It is placed very near to photo mask i.e. 25-125μm. Now the mask and silicon wafer are placed in a way that they are aligned properly with the reference mask (Ping, 2010).

The appearance of photomask is like a glass plate of about 125mm square in area and thickness of 2mm. The photomask consists of a photographic emulsion on one side. This thin metal pattern has both opaque and clear areas. The photomask to the wafer alignment is very important and has to be specific to within less than 1mm or 0.5mm in some cases. Once, the accurate alignment is attained, the wafer is placed with photomask so that they in contact for further process.

Afterwards, the ultraviolet is turned on, it is applied on wafer that is not covered with photomask so that it is exposed UV rays. The duration of UV exposure is around 10 seconds and is monitored carefully so that the UV radiation is in required amount.

They are basically of two types: Positive and Negative photoresist.

The negative photoresist is applied on the surface of photoresist that is not exposed to UV rays and polymerization occurs. Because of this process the length of molecules increases which constitute the photoresist. This makes the photoresist harder and indissoluble in the solution. Afterwards, the resisting photoresist will be a replica of photomask design with the areas on photomask corresponding to the photoresist on the wafers.

A reverse process occurs with the positive photoresist. The exposure of the areas of the photoresist to the ultra violet radiation results in depolymerization. This process makes those areas of the photoresist soluble in developer solution, while the unexposed areas remain insoluble. Hence, the exposed or depolymerized regions of the photoresist is removed from the developer solution while the unexposed areas remain on the wafer. The replication process is again repeated but, in this case, the clear areas of the photomask is produced on the wafer from where photoresist has been removed.

Once the development and rinsing are done the wafers are again baked at a temperature of 150 degree Celsius for an hour to harden the resist present on the silicon wafer. This is done for making the adhesion strong and resilient to the HF acid which is used later for engraving of oxide.

Applications of Lithography Today

The photoresist which are still there is hardened and acts like a mask from where oxide layer could be engraved for exposing semiconductor areas. Now this surface is further set for impurity diffusion.

This process of engraving oxide involves either immersing wafer into hydrofluoric acid solution or sprayed the same over the wafer. The ratio of the solution is normally 10:1 (water: Hydrofluoric acid) or it can be 10:1 (NH₄F: hydrofluoric acid). This etches the silicon dioxide but leaves the silicon and photoresist layer unharmed. Furthermore, silicon wafers are exposed to this solution so that it removes the silicon dioxide completely in the areas which are not covered by photoresist.

The duration of this process is carefully monitored so that only existing oxide is removed from the photoresist. If the process is carried out for the longer duration, it will lead to widening of the oxide opening more than required limit. This procedure is described as wet etching process as in this the reagents used are in liquid state. There are the other processes which are dry process known as plasma etching and ion milling.

This is the final process in which the remaining resist is removed by the application of H₂SO₄ and H₂O₂ and followed by abrasion process. This process of cleaning and drying helps to obtain the required opening in oxide layer. The negative photoresist is difficult to remove in comparison to positive photoresist which are easily removable in organic solvents like acetone. Commercially, the organic strippers are phenol based which are better and avoids slum formation during the process. However, the commonly used wet strippers in case of positive photoresist are inorganic acid based normally used at high temperature.

The wet stripping has various problems which incurs during the etching process. It is most often important to use plasma descum after the wet process to completely remove the resist residues on wafers. It is often observed that photoresist which has undergone harsh processing and excessive hardening can be almost impossible to strip off the chemicals. Hence, for these reasons plasms stripping has been widely used in semiconductors processing.

• It is a type of lithography that helps to produce geometric patterns on the tiny surfaces. With the help of process of photolithography integrated circuits and many internal parts of computers are produced(Minseok, 2015). It is also used to produce microscopic computer systems and nanomachines.
• It is used in typical patterning at laboratory level and manufacturing of several MEMS devices.
• Through projection printing it is used in production of both commercial products and advanced electronics such as IC_s and CPU chips.
• By electronic beam lithography, it can be used in patterning in research and development including channels for nanofluidics, photonic, bull’s eye structure, plasmonic lens.
• Nanoimprint lithography technique is used in bio sensors, bio electronics, nano wires. etc
• Dip-pen lithography technique is used in bio electronic sensors, gas sensors, etc

 One of the main advantage of this techniques is that it can engrave any geometric patterns in Integrated circuit for which it needs ultraviolet light only. It does not need any other materials for the process. This is the reason it is considered as the most inexpensive and effective technique as it also produces tiny incisions in a film or surface. It also regulates the shape and size of the whole substrate.

The UV-Visible spectrometer is an analytical instrument used for UV-Vis spectroscopic analysis.  UV-VIS spectroscopy is an analytical method in which we can measure the absorbance of ultraviolet or visible radiation via an analayte (Werner, 2017). An analayte molecular absorption corresponds to both excitation of valance electrons and excitation of electrons in various atomic orbitals. UV-Vis is very effective & convenient technique for both qualitative and quantitative analysis for both organic and inorganic compound. It’s a mature technique used in most of the industries like food and beverage, pharmaceutical, environmental analysis, etc. Its based-on Lambert-Beer principle, according to which absorbance of solution is directly proportional to its concentration (c) & its pathlength (l) when the wavelength of the incidence light remains constant.

Photolithography vs Lithography

This can be summarised in the following equation where ? is molarity absorptivity:

                                                A= ?/C

Spectrometers are of various configurations but the most common can be categorised in single beam, split beam or double beam which depends upon the design of their optical system. The common components used in the construction of these instruments are monochromator, light source, detector, cell compartment, signal processing system.

The single beam spectrometer has single cuvette; hence the control group and treatment group samples can be measured one after another (R S Shah, 2015). But with the double beam UV Vis spectrometer both the groups can be measured simultaneously. So, the double beam spectrometer is more accurate as we don’t need to recalibrate the instrument to measure the second group sample.

As shown in the diagram, when a ray of incident light from a source falls on the prism it gets separated into various wavelengths. After this, each monochromatic beam divides into 2 equal beams of same intensity through a half-mirror. As a result, the sample beam enters through a tiny container which is transparent and contains compound solution  (Kourosh kalantar-zadeh, 2008). Secondary beam, also known as reference beam enters via the similar cuvette which contains only the blank solvent. Then the intensity of both beams are determined and compared with a detector. The reference beam is denoted as I_o and the sample beam is denoted as I. In the meanwhile, the spectrometer scans all the wavelength automatically.

I= I_o, when the sample compound is not able to absorb light and I is less than Io when the sample compound is able to absorb the light. This absorption can be presented as transmittance where T is equal to I/I_o or absorbance can be presented as A= log I_o/I.  In case if there is no absorption T= 1 and A =0. Commonly, the absorbance in spectrometer is displayed on vertical axis ranges from 0 to 2. The maximum wavelength is denoted by λ_max. The absorbance maxima and absorbances varies with different compounds. The compounds are observed in that are in dilute state as it helps detector to receive sufficient light and it needs transparent solvents which are non-absorbing in nature. The common solvents that are widely used are hexane, ethanol, water and cyclohexane (Behera, 2012). The solvents which have single bonds are preferred.

The application of UV Vis spectrometer can be classified into two groups:

1. Chemical application
2. Structural application

(i) Identification of unknown substance: The UV-Vis spectrometry can be used in the identification of various classes of compound in both biological preparations and pure state. This can be done by plotting the absorption spectrum curves which represent specific class of compounds and by studying these curves helps in identification of any substance. As for example, the substance which do not get absorbed in 220-280nm are normally alicyclic hydrocarbon or aliphatic or their derivatives (Ambadas R. Rote, 2012). Even the complex systems which produces absorption curves with varied maxima and each curve have unique characteristic range and shape identifying the presence of a specific functional group.

(ii) Identification of concentration in case of unknown substance: The same spectrometry can be used in the identification of concentration of an unknown substance for which first the absorption band is chosen where the measurement of absorbance can be recorded. If the sample is already researched before, then in that case the sample’s interest absorbance spectrum is known. However, in case if its not then a double beam spectrometer is used to identify its spectrum band and where its lie. Hence in this way, a suitable absorption is chosen. Normally, most of the organic compounds gets absorbed in the visible region of ultraviolet spectrum and thus majority of biological compounds is measured by ultraviolet-visible spectrometer.

iii) Measurement of enzyme activity: When the substrate is coloured or gets absorbed in the UV visible range then the activity of the enzyme can be easily or conveniently calculated. The rate by which a substrate or a product is appearing or disappearing can be measured by a spectrometer.

2. Structural application of UV Vis spectrometer:
3. i) Identify the purity of a compound: This is one of the most significant application of spectrometer. If a compound is not showing its characteristic spectrum then the impurities present in it can be easily identified. For example: Benzene in absolute alcohol can be easily identified by this method(Oskam, 2010). As at 280nm, benzene gets absorb while alcohol spectrum is 210nm which won’t get absorb.
4. ii) Study of cis isomer and trans Isomer: The cis isomer is less elongated in comparison to trans isomer and this structural difference is visible in absorbance spectrum. The trans-isomer has higher wavelength having maximum absorption.

iii) In the determination of molecular weight: Suppose when a compound is reacted with a reagent, it gives rise to a derivative which has specific absorption band. The absorbance band of derivative which of high intensity and at a wave length where the compound does not get absorb, at this point the extinction coefficient of derivative is similar to that of the reagent. Even though, the extinction coefficient of derivative and reagent is similar, their optical density varies with the compounds of different molecular weight. Hence the molecular weight of the compound can be determined by the absorption data by using formula:

                           M= awb/OD

Where, a = absorption coefficient

             w = weight of the compound

             b = path length

              OD = optical density

1. iv) Turbidimetry: Any specific matter, in the solution or even any bacteria make the solution look turbid. This happens due to tyndall effect in which colloidal particles present in the solution scatters light & hence making solution look turbid. The particles in the solution absorbs at specific wavelength and these particles scatters incident light. Once this happens, then the radiation of wavelength which is not absorbed is passed through a suspension and the existing solution will be solely because the particles scattering light. Hence, the light which are transmitted have lower intensity compared to the incident light. This helps in measuring the intensity of the transmitted light and which will help in determining the number of particles in the solution. This phenomenon is known as turbidimetry.

Acikgoz, H. H. V., 2011. Polymers in conventional and alternative lithography for fabrication of nanostructures. European Polymer Journal, 47(11), pp. 2033-2052.

Ambadas R. Rote, P. A. K. a. R. S. B., 2012. UV-visible spectrophotometric simultaneous estimation of paracetamol and nabumetone by AUC method in combined tablet dosage form. NCBI.

Behera, G. F. A. S. S. a. S. B., 2012. UV-Visible Spectrophotometric Method Development and Validation of Assay of Paracetamol Tablet Formulation. Journal of Analytical & Bioanalytical Techniques.

Kourosh kalantar-zadeh, B. F., 2008. Nanotechnology_Enabled Sensors. Melbourne: Springer.

Minseok, D. T., 2015. Cracking-assisted photolithography for mixed-scale patterning and nanofluidic applications. Natureresearch .

Moreau, W. M., 2017. Prebake(Softbake). Springer International Publishing, pp. 329-353.

Oskam, G., 2010. Semiconductors, Metal oxides and composites: Metallization and Electrodeposition of Thin Films and Nanostructures. s.l.:The Electrochemical Society.

Ping, C. T., 2010. Alignment for Double-side Deep-exposure Lithography Tool. Researchgate.

R S Shah, R. R. S. R. B. P., 2015. UV-VISIBLE SPECTROSCOPY. INTERNATIONAL JOURNAL OF INSTITUTIONAL PHARMACY AND LIFE SCIENCES.

Sungyong Choi, J. K. P., 2010. Two step photolithography to fabricate multilevel microchannels. NCBI.

Werner, E., 2017. UV–VIS absorption spectroscopy. Sciencedirect, Volume 173, pp. 965-968.

Young, A. M., 2008. A Simulation Model to Characterize Photolithography Process of a Semiconductor Wafer Fabrication, s.l.: s.n.