Nanofabrication Processes

Due to the size of the structures made during nanofabrication, the semiconductor wafers are highly vulnerable to foreign objects, such as dust. The waveguide structures being fabricated see a width of 2 microns. Interference by a piece of dust of similar width results in device fabrication failure if left on the wafer before depositing an oxide layer. For this reason, cleanrooms and their procedures are designed to eliminate dust particles and organic material that may invade the device fabrication process. Still, caution must be taken at each step to ensure that there is no dust on one’s sample. The wafer should be observed under a microscope before each etching and depositing procedure and cleaned using the appropriate method, depending on the previous and next steps.

            If the wafer does not have a layer of photoresist on it, the cleaning procedure is to dip the wafer into a beaker of acetone, then isopropanol and then DI water. The typical duration of this is for about 1 minute each. If there is a layer of old photoresist on the wafer, this may need to be increased to five minutes each. If old photoresist remains, a flood UV exposure using the mask aligner and development may proceed a second solvent wash. The RIE machine is also used for removing photoresist on wafers if used on an O2 descum process.

Masks are templates used in the mask aligner to create patterns on wafers. These plates must also be cleaned after using, since they will make contact with wafers that have photoresist on them. This is especially critical for masks that have small patterns (<20 microns). After finishing use, soak in acetone, isopropanol and DI water for 5 minutes or longer. If after inspecting the mask on a microscope there is still photoresist remaining, an ultrasonic bath for 20 minutes or more or a flood exposure on the contact side of the mask can be attempted, however it likely will not be needed if it is cleaned continually after use for each day.

            If the wafer being used requires a custom epitaxial structure, the first step in the fabrication process is to grow these layers. A common method for research applications is molecular beam epitaxy (MBE). MBE machines are used primarily for research due to their accuracy but slow growth rate. MOCVD, on the other hand is used for simpler epitaxial structures and mass production.

PECVD is used to deposit oxides such as SiO2, SiNx and others. If using small wafers, using a larger carrier wafer is common practice. When creating a PECVD deposit recipe, the gas mixture and temperature are selected.

The E-beam evaporator is used to deposit metals such as gold, aluminum, chrome, platinum and other materials such as germanium.

E-Beam Evaporator

F E-Beam Evaporator

Reactive Ion Etching (RIE) is used for etching oxides and deposits on wafers using a chemical plasma that is charged with an electromagnetic field and under a strong vacuum. This is termed a dry etching process. For RIE etching recipes, the pressure, chemical and RF power are chosen. The RIE is also used to remove photoresist and organic material using an O2 clean process.

RIE: Wafers Loaded

The ICP (or Inductively Coupled Plasma) tool can etch many materials including SiO2, SiNx, Cr, GaAs and AlGaAs. ICP etch recipes are designed using a selected pressure, RF and ICP power, etchant gas and temperature. When using the ICP, run a chamber cleaning process with O2 and Argon with a dummy wafer loaded. After a cleaning run, the desired etch process should be run with a dummy wafer first before loading the desired wafer. For smaller wafers, thermal conducting paste can be applied between the wafer and a larger carrier wafer. For deep wafer etches on semiconductor material such as GaAs, the edges of the wafer will be etched more. To avoid this, other wafers can be placed aside it.

Acid Etching is a wet etch process, unlike RIE and ICP. This is performed at a bench using a blend of chemicals to etch the semiconductor wafer itself, typically. Heavier protective gear is worn during this process to prevent contact with some of the most dangerous chemicals used in a nanofabrication facility. Hydroflouric acid, a deadly neurotoxin is one chemical that is used frequently in a nanofabrication facility [37]. One use of an acid etch recipe used is an AlGaAs-selective etch.

Photolithography is a technique used in semiconductor device fabrication. First, a light-sensitive layer called photoresist is added to a semiconductor wafer. Depending on the type of resist (positive or negative), this layer can be removed using developer after applying UV light. A mask is template used to apply UV light only to a desired region or shape on the wafer. After this is done, etching can be performed exclusively to the parts of the wafer without a photo-sensitive layer.

            The spinner is the machine that is used to apply photoresist to a wafer. The wafer is first held on a vacuum arm and photoresist is applied. The vacuum arm is then spun at a desired spin rate and duration. This creates a uniform film of photoresist on the wafer. After running on the spinner, the wafer should sit on a hot plate for a specified time and temperature.

Photoresist applied to a wafer on a spinner

Particularly for non-circular wafers, photoresist can build up along the edges, creating an uneven surface. This is problematic for the following steps, so the photoresist is removed from the edges and underside of the wafer using a swab and acetone.

            At this point, the wafer is ready to be loaded into the mask aligner, along with the mask template mentioned earlier.

Wafer and Mask in Mask Aligner

If there is already a pattern on the wafer, the wafer position in the mask aligner can be adjusted to ensure alignment. It is recommended to include an alignment feature on the mask die, such as a veneer mark, especially if alignment is critical to that fabrication step. After aligning as needed, the UV light exposure time is selected and applied to the wafer. The wafer is dipped in developer for a specified time, then in DI water, and gently blow dried with a nitrogen gun.

Developed Veneer Marks used in Alignment

Lift-off photoresist is used when creating a metal feature on a wafer. In this process, normal photoresist can be applied over the lift-off resist (before putting on a hot plate). After running the wafer in the spinner, lift-off resist needs to be removed from the edges using tweezers. Lift-off resist will react differently to acetone, so for edge bead removal, a separate solution needs to be used. After performing photolithography and metal depositions, the lift-off resist may need to be developed using yet another type of solution. For LOR 20-3, it is recommended that the wafer sit on a hot plate at 80 degrees C in the lift-off developer solution for 12 hours. It also recommends a wash in cool lift-off developer and isopropanol after sitting on the hot plate. Refer to data sheets for specific instructions for chemicals.

Lift-Off Resist

Electron beam Lithography (EBL) performs the same role as the mask aligner, but with much higher precision due to the smaller electron wavelength. Photoresist still needs to be applied before using an EBL machine. Instead of using a mask, it follows the pattern on a GDSII file. An EBL can be used for all lithography steps in fact, though it is much slower, so it is used only for steps with a narrow feature that is not achievable on a mask aligner or stepper. An EBL machine also contains an SEM, scanning electron microscope and can be used for that function as well.

            The SEM or Scanning Electron Microscope is used for examining structures that are too small for an optical microscope. This is achieved using an electron beam, which excites conductive materials. SEM is necessary for inspecting etch qualities and for making accurate measurements of component sizes. The SEM operates by directing an electron gun at the sample. The charged atoms release electrons, producing the signal that is detected for the image.

SEM Image of MMI Coupler after SiNx Passivation

One tool that is useful for measuring the height profile on a wafer is the Dektak tool. This is often used after an etching process. This gives the height profile along a line on the wafer.

Dektak Height Measurement

The ellipsometer is used for measuring the thickness of a film deposit. Unlike the Dektak, the ellipsometer is able to measure multiple layers. However, it is not useful for precise positioning or height profiles over a distance on the wafer. The ellipsometer is typically only used for uniform deposits on wafers that have not been exposed to etching or photolithography, unless the shapes are very large.