- MEMS Micro-heaters are micro-meter sized hotplates. They are composed of thin suspended membranes that have a resistive heater coil sandwiched between electrically insulating, but thermally conducting layers.
- Semiconductor device fabrication is the process used to MEMS structure like micro-heater. It is a multiple-step sequence of photographic and chemical processing steps during which the device is gradually created on a wafer made of pure semiconducting material. Silicon is the most commonly used substrate material today.
- The micro-heaters that we will work with are composed of suspended and patterned micro structures in platinum sandwiched between layers of silicon dioxide and silicon nitride.
- The platinum structures constitute a heater coil and a temperature sensor coil. The heater coil is supplied with an electrical excitation which increases the temperature of the micro-heater membrane.
- Temperature is sensed by the temperature sensor (which is an RTD), whose resistance increases with temperature in a known and repeatable way. The thermal mass of the micro-heater is kept very low and heater membrane is thermally isolated from the supporting structure.
- This technology enables high speed heating and cooling in a few milliseconds, along with accurate in-situ temperature measurement. These micro-heaters also provide excellent stability at high temperatures.
- There are 4 gold contact pads for electrical connectivity. Two of these pads are connected to the heater coil and the other two are connected to the temperature sensor coil.
- The size of the entire Micro-heater die is 4mm x 4mm. The micro-heater dies provided for experimentation are mounted on a PCB. MEMS micro-heaters have gradually gained its wide applications in combustible/ hazardous gas sensors, infrared detectors, infrared sources, thermal flow sensors, actuators, micro-thruster prototypes, polymerase chain reaction (PCR) etc.
- The micro-heater technology presents a series of advantages such as miniaturized size, low power consumption, fast response, high sensitivity and feasibility of sensor array integration.
2.1 Fabrication of micro heater
Microheater devices were fabricated in a class 100 cleanroom using lift-off. This section is the literature review of the fabrication process. Below is the detail of all the steps involved in this process.
Photolithography is the process to transfer patterns on a mask to the surface of a substrate such as silicon wafer. The steps involved in the photolithographic process are as follows
1) Wafer cleaning
- In order to get a good pattern using lithography, cleaning is the most crucial step. If the wafers are not clean, the pattern may not appear as expected. Typical contaminants that must be removed prior to photoresist coating include dust from scribing or cleaving atmospheric dust, abrasive particles, lint from wipes, photoresist residue from previous photolithography, oil etc. The well-known process used for cleaning the wafer is called RCA clean and consist of 3 steps. The wafers are first prepared by soaking them in DI water. The first step involves a 1:1:5 solution of NH4OH (ammonium hydroxide) + H2O2 (hydrogen peroxide) + H2O (water) at 75 -80 °C typically for 10 minutes. This treatment results in the formation of a thin silicon dioxide layer on the silicon surface, along with a certain degree of metallic contamination that shall be removed in subsequent steps followed by transferring the wafers into a DI water bath. The second step is a short immersion in a 1:50 solution of HF + H2O at 25 °C, in order to remove the thin oxide layer and some fraction of ionic contaminants. The third step is performed with a 1:1:6 solutions of HCl + H2O2 + H2O at 75 or 80 °C.
- The surfaces of several commonly used substrates oxidize very quickly. This surface oxide forms hydrogen bonds with water adsorbed from the air which results in poor resist adhesion on spin coating. Hexamethyldisilazane (HMDS) is one of the most commonly used adhesion promoters used to nullify this effect.
2) Photoresist application
Photoresist is a light sensitive material used for photolithography process . There are two types of photoresists
In case of positive resists, the resist is exposed with UV light wherever the underlying material is to be removed. The exposure to UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then dipped in the
developer solution, leaving windows of the bare underlying material. The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer
In case of negative resists, the resist is exposed with UV light wherever the underlying material is not to be removed. The exposure to UV light changes the chemical structure of the resist (cross-linking) resulting in polymerization of resist. The exposed resist is then dipped in the developer solution, removing all the resist except the exposed area. The mask, therefore, contains inverse of the pattern that is there on the mask. Photoresist is spin coated on the wafer at a specific speed that will determine the thickness of the resist and hence the exposure and develop time.
3) Soft Baking
- This step is used to remove any solvent from the photoresist. It is a critical step as excessive baking may affect the photoactive part of the photoresist and reduce sensitivity.
4) Mask alignment
- After prebake, the mask is aligned with the wafer to transfer the pattern on the wafer. This is not a critical step for single mask microdevices but is very critical when more than one mask is being used for fabrication.
5) Exposure and Development
- Photoresist is exposed to the UV light through the aligned mask. Depending on the tone of resist, the resist will either dissolve or polymerize upon UV exposure. The wafer is then dipped in the developer and agitated well to remove the unwanted photoresist and generate the pattern.
- A hard bake or post exposure bake is used to reduce standing waves in regular positive resist exposed on the steppers. Besides that, it also helps to thermally activate chemical processes such as image reversal.
Two techniques commonly used for making patterns of any evaporated material using lithography are wet etching and lift-Off. Since, the current work utilizes the lift-off method so it is discussed in detail
Lift-Off Method: Lift-off is a simple method for patterning deposited films that are deposited. Figure 2.1 shows the scheme used for the lift off process .
A pattern is defined on a substrate using lithography The film of interest is deposited all. After depositing the metal film, the substrate was dipped in a glass beaker filled with acetone and placed in an ultrasonicator. The beaker is left in ultrasonicator for 6-10 minutes to ensure all the resist is dissolved taking off all the metal on top and giving a pattern only on exposed substrate surface.
Depending on the type of lift-off process used, patterns can be defined with extremely high fidelity and for very fine geometries. Lift-off, for example, is the process of choice for patterning e-beam written metal lines. Because film sticks only where photoresist is cleared, the defect modes are opposite to what one might expect for etching films.