Photolithography & Micromachining: Pattern Transfer & Layer Building, Study Guides, Projects, Research of Engineering

Download Photolithography & Micromachining: Pattern Transfer & Layer Building and more Study Guides, Projects, Research Engineering in PDF only on Docsity!

Processes for

Micromachining

Lecture 4 Dr. Farah Hamed

Lithography

Lithography is the technique used to transfer a computer

generated pattern onto a substrate (silicon, glass, etc.). This

pattern is subsequently used to etch an underlying thin film

(oxide, nitride, etc.) for various purposes (doping, etching, etc.).

Furthermore, photolithography uses ultraviolet (UV) light source

for purpose of patterning.

This process occurs several times during the fabrication of a

micro system device as layers build upon layers as in figures.

When building a microsystem, we must take into consideration that each layer within this system has a unique pattern. The initial process used to transfer this pattern into a layer is photolithography. The photolithography process transfers the pattern of a mask (depending on the method of exposure) to a photosensitive layer (resist). In the construction of microsystem devices a subsequent process step, usually etch or liftoff, transfers the pattern from the photosensitive layer into an underlying layer

. After the pattern transfer, the resist is usually. stripped or removed The patterned resist would identify the areas that exposed to deposition as an example building a layer of silicon dioxide above silicon substrate. Patterned photo resist is also used as a hard mask for some etch processes. The photo resist is used to protect the areas of the film that are not to be etched.

There are three basic steps to photolithography as seen in fig. 3 : 1 ) Coat - A photosensitive material (photoresist or resist) is applied to the substrate surface. This step include:- A. Cleaning: - the substrate of silicon cleaned by acetone and rinsed DI water and dried it on the heater to ensure the cleanness of the surface. B. Appling photoresist on the wafer through spin coater: - There are two types of PR (negative & positive PR), however, we used in our experiment, the positive PR. Spin coater is a device where PR applied through nozzle and its disc stabilized by vacuum, the disc rotates and spread the PR on The wafer. The speed of rotation determines the required thickness of PR which is very important to control the desired aspect ratio of the device fig. 4. C. Softbake: - After the photoresist is applied to the desired thickness, a softbake is used to remove the residual solvents of the photoresist. After the softbake, the wafer is cooled to room temperature fig. 5.

Fig. 4. Applying photoresist fig. 5. Soft bake

2 ) Expose – the mask with the coated PR suited in mask aligner device as seen in fig. 6 A. Alignment: - putting the mask in the right position is a critical issue. Due to the microscopic size of these devices, a misalignment of one micrometer (micron or 1 μm) or even smaller can destroy the entire device and all the other devices on the wafer. It is important that each layer is aligned properly and within specifications to the previous layers and subsequent layers. B. Expose: - ultraviolet (UV) light from a source travels through the mask to the resist, exposing the resist. UV light sources normally include mercury vapor lamps. The UV light hitting the resist causes a chemical reaction between the resist and the light make it soluble and ready for the step of development.

• 3 ) Develop – The exposed photoresist is subsequently dissolved with a chemical developer. The timing of this process is critical. Too long of a time leads to an "overdeveloped resist"; too little of a time leads to an "underdeveloped resist" – both of which negatively affect line width. An underdeveloped resist could prevent access to the underlying layer by leaving too much resist on the wafer. To stop the chemical reaction of the developer with the photoresist, the wafers are rinsed with (DI) water then spin-dried fig 7. 1. A post-develop hardbake is used to harden the photoresist for the subsequent process. In order to do this, the temperature of the hardbake is higher than that of the softbake after coat. The hard bake temperature for positive resist is approximately 120 °C to 140 °C fig 7. 2.

Fig. 7. 1 immersion of substrate in HF solution Fig^.^7.^2 hardbake^ at^ higher^ temperature

Wet etchants in aqueous solution offer the advantage of low-cost batch fabrication — 25 to 50 100 - mm-diameter wafers can be etched simultaneously — and can be either of the isotropic or anisotropic type. Dry etching involves the use of reactant gases, usually in a low-pressure plasma, but non plasma gas-phase etching is also used to a small degree. It can be isotropic or vertical. The equipment for dry etching is specialized and requires the plumbing of ultra-clean pipes to bring high purity reactant gases into the vacuum chamber.

Chemical-Vapor Deposition

• In contrast to sputtering, CVD is a high-temperature process, usually

performed above 300 ºC.

• The field of CVD has grown substantially, driven by the demand

within the semiconductor industry for high-quality, thin dielectric and

metal films for multilayer electrical interconnects.

• Common thin films deposited by CVD include polysilicon, silicon

oxides and nitrides, tungsten, titanium and tantalum as well as their

nitrides, and, most recently, copper and low-permittivity dielectric

insulators (εr< 3 ).

• The latter two are becoming workhorse materials for very-high-speed

electrical interconnects in integrated circuits. The deposition of

polysilicon, silicon oxides, and nitrides is routine within the MEMS

industry.

16.06.2020 16

Chemical-Vapor Deposition

• Substrate temperature, gas flows, presence of dopants, and pressure

are important process variables for all types of CVD. Power and

plasma excitation RF frequency are also important for PECVD.

**Energetic electrons excited in a high-frequency electromagnetic

field collide with gas molecules to form ions and reactive neutral

species. The mixture of electrons, ions, and neutrals is called plasma

and constitutes a phase of matter distinct from solids, liquids, or

gases. Plasma-phase operation increases the density of ions and

neutral species that can participate in a chemical reaction, be it

deposition or etching, and thus can accelerate the reaction rate.

16.06.2020 17

Deposition of Polysilicon

• Polysilicon is deposited by the pyrolysis of silane (SiH 4 ) to silicon and hydrogen in a LPCVD reactor.
• Deposition from silane in a low-temperature PECVD reactor is also possible but results in amorphous silicon.
• The deposition temperature in LPCVD, typically between 550 º and 700 ºC, affects the granular structure of the film.
• Below about 600 ºC, the thin film is completely amorphous; above about 630 ºC, it exhibits a crystalline grain structure.
• The deposition rate varies from approximately 6 nm/min at 620 ºC up to 70 nm/min at 700 ºC.
• Partial pressure and flow rate of the silane gas also affect the deposition rate. 16.06.2020 19