FABRICATION STEPS OF NANO SIZED MATERIAL USING PHOTO LITHOGRAPHY

Rimsha Zia

FABRICATION STEPS OF NANO SIZED MATERIAL USING PHOTO LITHOGRAPHY

LITHOGRAPHY AND PHOTOLITHOGRAPHY:

Nanolithography is a top down technique for the fabrication of nanomaterials at nanoscale in nanotechnology. It is the branch of nanotechnology gives the study and the fabricating of nanoscale structures which means that the size of those structure must be of approximately 100nm. With the help of this art nanoelectronics and nanophotonic devices and systems can be fabricated.

 

Before going to this technique let me explain you some major terms used in this article. You must have some questions in your mind that what is nanotechnology and Top down approach etc.??

Nanotechnology: A technology deals with the dimensions at nanoscale. And one nano meter is equal to 1*10e-9.

Nanoscience: it is the application of nanotechnology

In the world of nanotechnology, we have two approaches for manufacturing of materials

  1. Top down approach
  2. Bottom up approach

Top down approach: deals with the slicing and successive cutting of a bilk material to get the nano sized structures or nano sized material.

Bottom up approach: refers to fabricating the material by combining atom by atom, molecules by molecule or cluster by cluster.

Moving back to our topic which is the fabrication of nanomaterial using the lithography technique which hold the top down approach technology. A word lithography drive from two Latin words lithos and graphy which means stone and printing (to write) respectively. This referred to making the objects from stones or inking the stone and then pushing them onto the paper (like getting the printed image of something on a paper). It depends upon the fact that water grease is repellant to each other. This technique is discovered by a German author in 1796.

Later, this technique got famed and in 20th century it become the important technique having the remarkable capabilities in the field of art. Many lithography techniques are now developed using the different system, lenses, exposure radiations sources like X rays, photons, ions and neutral atoms too.

There are many types of lithography such as:

  1. Photo lithography
  2. X-ray lithography
  3. E beam lithography
  4. Dip pen lithography
  5. Thermal lithography

 

 From the above types, photolithography is the most important and most frequently technique used for the fabrication of nano scale materials.

Photolithography:

 Photolithography derived from Latin words which derived from these following words : photo means light, litho means stone where as graphy means writing. Joseph Niepce was the first person who takes the first photograph using the bitumen of Judea on a pewter plate in 1826. Later, in 1935 and 1940 louis developed the first negative photoresist and Otto sues developed the first positive photoresist respectively.

Description:

The pattern will be generated photographically on silicon wafer (substrate) i.e. with the help of light pattern will be formed on substrate.

Following are the steps required:

  1. Wafer cleaning
  2. Photoresist
  3. Soft baking
  4. Mask alignment
  5. Exposure
  6. Development
  7. Hard baking

 

Steps Explanation:

First step of the technique is to wash the substrate from the appropriate solvent to remove the impurities or any other matter which deforms the final product. Those appropriate solvents can be acetone or methanol. Dry the substrate in oven for approximately 10 min at 120-150 ◦C. Stabilize the substrate in stabilizer t 115-120 ◦C.

After the removal of impurities, deposit a layer of silicon dioxide on the surface of substrate which acts as a barrier.

By using the high spin coaters and adjusting its RPM with respect to time, spin the substrate which therefore gives a uniform layer of silicon dioxide on the substrate.

Photoresist is an organic polymer which changes the chemical structure when it exposed to some ultra-violent light. It contains a light sensitive substance which means that it would allow image transfer on printed circuit board. We have two types of photoresist one is positive and the other one is negative photoresist.

In positive photoresist we get the exact same result which means when it exposed to ultra-violent light. therefore, the result will be exact copy of the original design   but in negative photoresist you will get the negative image of dummy mask (photoresist). Means that exposure to the light resist to polymerize and thus be more difficult to dissolve. Developer removes the unexposed resist (part on which UV light did no work done). This type of negative photographic pattern will be defined as negative photoresist

Set the substrate on the hotplate or in oven by adjusting its temperature and time. This makes the photoresist non-sticky. It also hardens the amorphous solid and removes the evaporative solvents from coating. Keep it remember that you must be careful while baking, overbaking will harm the sensitizer which causes problem at the end.

After this step photo mask is then aligned with substrate or silicon wafer. Photomask is a square glass plate with a patterned emulsion of metal film on one side.in easy words you can define this as dummy mask or the shape you want to fabricate.

After this alignment, substrate is exposed to UV light for the exposure. There are three primary exposure methods which are

  1. Contact
  2. Proximity
  3. Projection

 

 

Contact:

The contact exposure setup is mentioned in descending order below:

  1. Light source
  2. Optical system
  3. Mask
  4. Photoresist
  5. Silicon wafer

Proximity:

The only difference between contact and proximity is that there is a gap between photoresist and silicon wafer.

Projection:

In projection we have two differences that is we have two gaps and two optical systems.

The sequence of required system is as follows:

  1. Light source
  2. Optical system

Gap

  1. Mask

Gap

  1. Optical system
  2. photoresist
  3. silicon wafer

There alignment setup makes them different from one another

After this the next step is exposure using photo resist developer which is an alkaline solution. This developer removes the photoresist layer.

Hard baking:

This is the last step which is not necessary, but I recommend hard baking. Hard baking improves the quality of your product. this improves adhesion of the photoresist to the substrate surface. For hard baking the required temperature is 120 to 150 degrees centigrade.

Applications:

Name plate and number plate, IC designing, electronic board, also in sensors  etc.

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