UNIT 2

INTEGRATED CIRCUIT

FABRICATION PROCESS

(PHOTOLITHOGRAPHY)

At the end of this topic, you should be able to:

• Explain photolithography process.

• Elaborate on photo mask, opaque and

transparent patterns on it.

• Explain the photo-resist layer.

• Compare between positive photo-resist and

negative photo resist.

• Identify the effects of the ultra violet light.

• Explain the sequence of photolithography

process (8 steps) based on the wafer cross

section diagram for each step.

Photolithography

Definition

• Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer.

Photoresist layer

• Photoresist is a light sensitive liquid used to form thin film.

• The main thing with photoresists is that they change chemically when exposed of light of high energy. They become either more or less acidic.

• The more acidic the resist are easier it is to remove with an alkali solution such as photoresist developer or NaOH.

• Sifat-sifat yang perlu ada pada photoresist :-

• Ia mesti melekat dengan sempurna pada permukaan substrate.

• Ketebalan resist mesti seragam pada keseluruhan permukaan substrate.

Positive and Negative Photolithography• Two types photolithography:

– Positive (more acidic when exposed)

• the resist is exposed with UV light wherever the underlying material is to be removed.

• In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer.

Positive and Negative Photolithography• Two types photolithography:

– Negative (less acidic when exposed)

• Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve.

• the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions.

Positive and Negative Photolithography

• Negative resists were popular in the early history of integrated circuit processing, but positive resist gradually became more widely used since they offer better process controllability for small geometry features.

• Positive resists are now the dominant type of resist used in VLSI fabrication processes.

Effect of exposure with ultra violet light

• When exposing a photoresist film with correct light it becomes more or less acidic (pos. or neg.) and therefore there is important to ensure that the correct areas are exposed and expose correctly (right amount of time).

Exposure• If we expose to short, the photoresist will not be acidic

enough, and there will be areas of resist remaining after development (Fig.1a).

• But there is also problems with to long exposure, where the diffraction and “focus resolution” can make the protected areas exposed (Fig.1b).

Development• The development is the process where we remove the

acidic parts of the photoresist.

• This is done with an alkali solution optimized for the photoresist.

• It is important to use correct development-time. The developer will also remove the less acidic film (but much slower).

• If the film is developed for to long time, the less acidic parts will be partially solved (fig 2a).

• Therefore, it is better to start with a short development and then additionally develop if the film is not fully developed.

Development• If the film is developed for to long time, the less acidic parts

will be partially solved (fig 2a).

• Therefore, it is better to start with a short development and then additionally develop if the film is not fully developed.

• Even when the film is fully developed, there will be some rounded edges due to both “parasitic” exposure and this “parasitic” development (fig 2b).

After developmentEcth-back (positive resist)

After development

Lift-off (negative resist)

Photolithography Process Step

Process Step Purpose

Surface Preparation

Clean and dry wafer surface

Photoresist apply(Wafer is held on a spinner chuck by vacuum and resist iscoated to uniform thickness by spin coating.)

Spin coat at thin layer ofphotoresist on surface

Softbake(Used to evaporate the coating solvent and to densifythe resist after spin coating.)

Partial evaporation of photoresistsolvent by heating

Alignment and exposure

Precise alignment of mask to wafer and exposure of _____ photoresist.

Development Removal of remained resist.

Photolithography Process Step

Process Step Purpose

Hard bake(Used to stabilize and harden the developed photoresist prior to processing steps(etching) that the resist will mask.)

Additional evaporation of solvent.

Develop inspect Inspect surface of alignment and defects

Etch Top layer of wafer is removedthrough opening in resist layer.

Strip (photoresistremoval)

Remove photoresist layer from wafer

Final inspection Surface inspection for each irregularities and other problems.

DIFFERENCES

POSITIVE PHOTORESIST NEGATIVE PHOTORESIST

Pepejal tukar kepada cecair

Proses penukaran dipanggil

sebagai fotosolubization

Digunakan untuk menghasilkan

pulau pada permukaan wafer

Cecair tukar kepada pepejal

apabila terdedah kepada cahaya

UV

Proses struktur ini dipanggil

polymesization

Digunakan untuk menghasilkan

bukaan ‘hole’ pada permukaan

wafer

DIFFERENCES