photo lithography for MEMS and developing micro-structures. the sequence for developing devices in clean room

1. Photo lithography sequence for developing micro
structure & RIE (REACTIVE ION ETCHING)
AHMED M. ABDELGAWAD, Arab academy for science and
Technology
Ahmed.sayes@yahoo.com
XNEM-program

2. 1- Theory
Lithography is the technique used to transfer a computer generated pattern onto a substrate
(silicon, glass, GaAs, 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.
Fig. 1 shows all steps of photolithography.
This process occurs several times during
the fabrication of a micro system device
as layers build upon layers as in fig. 2.
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
Fig. 1 lithography process flow
Fig. 2 micro device built by micromachining technique

3. mask for some etch processes. The photo resist is used to protect the areas of the film that are
not to be etched.
2- Procedures
Fig. 3. Steps of photolithography
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

4. 2) Expose – the mask with the coated PR suited in mask aligner device as seen in fig. 6 .1, 6 .2.
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 1μm) 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.
Fig .6 .1 fig .6 . 2
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.

5. 4) Deposition of silicon dioxide by PECVD:-
At this step, we deposited a layer of silicon dioxide above the developed surface to cover
both of exposed parts of substrate and the remaining parts of resist. Details about the
PECVD discussed in another report.
5) Wet etching: - removes the material through a chemical reaction between a liquid etchant and the layer
to be etched which differs from Dry etching removes the material through a chemical reaction and/or a
physical interaction between etchant gasses and the exposed layer. Table .1 shows some of the thin films
used in the construction of microsystems devices, the etch process used (wet or dry), and the etchants for
each type of film.
However, there is a special stage designed for handling such these dangerous chemical
solutions and there a special safety precautions must be considered as; glasses, mask,
gloves and so on…
Table .1 various etching solutions
Through this stage an organic solvent has been used to chemically etch the remained resist (sacrificial layer) and
form the structure as illustrated in fig .8
Fig .7 .1 immersion of substrate in HF solution fig .7 .2 hardbake at higher temperature

6. 6) DRIE (DEEP REACTIVE ION ETCHING):-
This device introduces an alternative way for etching; the difference here is that
this technique more expensive and complicated, however, it gives higher range of
control for etching process due to high number of variables which could be
controlled. the process work as in the sputtering where photo resist be on the surface
of substrate, and the regions to be etched be exposed through the photo resist and
inside the material. The process of etching in this type mainly be a physical process
but controlling the factors as:- 1- the intensity of plasma through( applied voltage or
frequency in case of RF sputtering) or 2- the pressure of the medium make a
combination between chemical and physical etching.
This process is effective to get straight engravings or making etching apart from the
crystalinity of the material. Thus, it can be used to achieve designs with high aspect
ratio. Finally, it gives the opportunity to get full control over the process of etching
principles of operation shown in fig .9

7. Fig .9 A) shows the total structure for DRIE which consisted of handler part and process part, B) the internal components
in the process part and the factors influence etching process.
A
B