Steps to Fabrication of MEMs

Micro Electro Mechanical System is a system of miniaturized devices and structures that can be manufactured using microfabrication techniques. It is a system of microsensors, microactuators, and other microstructures fabricated together on a common silicon substrate. A typical MEMs system consists of a microsensor that senses the environment and converts the environment variable into an electrical circuit. The microelectronics process the electrical signal and the microactuator accordingly works to produce a change in the environment.

Fabrication of MEMs device involves the basic IC fabrication methods along with the micromachining process involving the selective removal of silicon or the addition of other structural layers.

Steps of MEMs Fabrication using Bulk Micromachining:

• Step1: The first step involves the circuit design and drawing of the circuit either on a paper or on using software like PSpice or Proteus.
• Step 2: The second step involves the simulation of the circuit and modeling using CAD( Computer-Aided Design). CAD is used to design the photolithographic mask which consists of the glass plate coated with chromium pattern.
• Step 3: The third step involves photolithography. In this step, a thin film of insulating material like Silicon Dioxide is coated over the silicon substrate, and over this, an organic layer, sensitive to ultraviolet rays is deposited using spin coating technique. The photolithographic mask is then placed in contact with the organic layer.  The whole wafer is then subjected to UV radiation, allowing the pattern mask to be transferred to the organic layer. The radiation either strengthens the photoresistor weakens it. The uncovered oxide on the exposed photoresist is removed using Hydrochloric acid. The remaining photoresist is removed using hot Sulphuric acid and the resultant is an oxide pattern on the substrate, which is used as a mask.
• Step 4: The fourth step involves the removal of the unused silicon or etching. It involves the removal of a bulk of the substrate either using wet etching or dry etching. In wet etching, the substrate is immersed in a liquid solution of a chemical etchant, which etches out or removes the exposed substrate either equally in all directions(isotropic etchant) or a particular direction(anisotropic etchant). Popularly used etchants are HNA (Hydrofluoric acid, Nitric acid, and Acetic acid) and KOH(Potassium Hydroxide).
• Step 5: The fifth step involves the joining of two or more wafers to produce a multi-layered wafer or a 3 D structure. It can be done using fusion bonding which involves direct bonding between the layers or using anodic bonding.
• Step 6: The 6^th step involves assembling and integrating the MEMs device on the single silicon chip.
• Step 7: The 7^th step involves the packaging of the whole assembly to ensure protection from the outer environment, proper connection to the environment, minimum electrical interference. Commonly used packages are metal can package and ceramic window package. The chips are bonded to the surface either using a wire bonding technique or using flip-chip technology where the chips are bonded to the surface using an adhesive material that melts on heating, forming electrical connections between the chip and the substrate.

MEMs Fabrication using Surface Micromachining

• The first step involves the deposition of the temporary layer (an oxide layer or a nitride layer) on the silicon substrate using a low-pressure chemical vapor deposition technique. This layer is the sacrificial layer and provides electrical isolation.
• The second step involves the deposition of the spacer layer which can be a phosphosilicate glass, used to provide a structural base.
• The third step involves subsequent etching of the layer using the dry etching technique. Dry etching technique can be reactive ion etching where the surface to be etched is subjected to accelerating ions of the gas or vapor phase etching.
• The fourth step involves the chemical deposition of phosphorus-doped polysilicon to form the structural layer.
• The fifth step involves dry etching or removal of the structural layer to reveal the underlying layers.
• The 6th step involves the removal of the oxide layer and the spacer layer to form the required structure.
• The rest of the steps are similar to the bulk micromachining technique.

MEMs fabrication using LIGA Technique.

It is a fabrication technique that involves lithography, electroplating, and molding on a single substrate.

• 1^st step involves the deposition of a layer of Titanium or copper or Aluminum on the substrate to form a pattern.
• 2^nd step involves the deposition of a thin layer of Nickel which acts as the plating base.
• 3^rd step involves the addition of an X-ray sensitive material like PMMA (polymethyl metha acrylate).
• 4^th step involves aligning a mask over the surface and exposing the PMMA to x-ray radiation. The exposed area of PMMA is removed and the remaining one covered by the mask is left.
• 5^th step involves placing the PMMA based structure into an electroplating bath wherein the Nickel is plated on the removed PMMA areas.
• 6^th step involves the removal of the remaining PMMA layer and the plating layer, to reveal the required structure.

Advantages of MEMs technology

1. It provides an efficient solution to the need for miniaturization without any compromise on functionality or performance.
2. The cost and time of manufacturing are reduced.
3. The MEMs fabricated devices are more fast, reliable and cheaper
4. The devices can be easily integrated into systems.

Three Practical Examples of MEMs fabricated devices

• Automobile Airbag Sensor:  The pioneer application of MEMs fabricated devices was the automobile airbag sensor which consisted of an accelerometer (to measure the speed or acceleration of the car) and the control electronics unit fabricated on a single chip which can be embedded on the airbag and accordingly control the inflation of the airbag.

• BioMEMs device: A MEMs fabricated device consists of teeth like structure that has been developed by Sandia National Laboratories which has the provision to trap a red blood cell, inject it with DNA, proteins, or drugs and then release it back.

• Inkjet Printer Header: A MEMs device has been fabricated by HP which consists of an array of resistors that can be fired using microprocessor control and as the ink passes through the heated resistors, it gets vaporized to bubbles and these bubbles are forced out of the device through the nozzle, onto the paper and instantly solidify.

So I have given a basic idea about MEMs fabrication techniques. It’s quite complicated than it appears. Even there are many other techniques. if you have any queries on this topic or the electrical and electronic projects Get to know about them and do add your knowledge here.

Photo Credit:

• Bulk Micromachining Technique involving Photolithography 3.bp
• Surface Micromachining Technique by ualberta
• LIGA Technique by memsnet