Photolithography and etching are key techniques in microfabrication. They allow us to create tiny patterns and structures on materials like silicon. These processes are essential for making microchips, sensors, and other small devices.

Photolithography uses light to transfer patterns onto materials. Etching then removes specific parts of the material to create the desired structures. Together, they enable the precise shaping of microscopic components in modern technology.

Photolithography

Photoresist Application and Properties

• Photoresist is a light-sensitive material used to create patterns on a substrate
• Applied as a thin layer on the substrate surface through spin coating
• Two types of photoresist: positive (exposed areas become soluble) and negative (exposed areas become insoluble)
• Photoresist properties include resolution, sensitivity, adhesion, and etch resistance

Mask Design and UV Exposure

• Mask is a template containing the desired pattern to be transferred onto the photoresist
• Typically made of glass or quartz with opaque patterns (chrome or iron oxide)
• UV light is exposed through the mask onto the photoresist-coated substrate
• Exposure time and intensity depend on the photoresist type and thickness

Development and Pattern Transfer

• After UV exposure, the photoresist undergoes a development process
• For positive photoresist, the exposed areas are dissolved in a developer solution (tetramethylammonium hydroxide (TMAH))
• For negative photoresist, the unexposed areas are dissolved in the developer
• The remaining photoresist pattern acts as a protective layer for subsequent etching or deposition processes
• The pattern is transferred from the photoresist to the underlying substrate

Wet Etching

Isotropic Etching

• Wet etching involves the use of liquid chemical etchants to remove material from a substrate
• Isotropic etching occurs when the etch rate is equal in all directions
• Results in rounded or undercut etch profiles
• Commonly used isotropic etchants include hydrofluoric acid (HF) for silicon dioxide and nitric acid (HNO3) for metals

Anisotropic Etching

• Anisotropic etching occurs when the etch rate varies with crystal orientation or direction
• Produces more vertical and controlled etch profiles compared to isotropic etching
• Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are commonly used anisotropic etchants for silicon
• Anisotropic etching is essential for creating high aspect ratio structures (deep trenches or holes)

Dry Etching

Reactive Ion Etching (RIE)

• Dry etching uses gaseous etchants instead of liquid chemicals
• Reactive Ion Etching (RIE) is a common dry etching technique that combines chemical and physical etching mechanisms
• In RIE, a plasma is generated by applying a strong RF electromagnetic field to a low-pressure gas
• The plasma contains reactive species (ions and radicals) that chemically react with the substrate material
• The ions are accelerated towards the substrate by an electric field, resulting in physical sputtering and anisotropic etching
• RIE allows for better control over etch directionality and selectivity compared to wet etching

Deep Reactive Ion Etching (DRIE)

• Deep Reactive Ion Etching (DRIE) is an advanced dry etching technique used to create deep, high aspect ratio structures in silicon
• DRIE involves alternating cycles of etching and passivation steps (Bosch process)
• During the etching step, SF6 plasma is used to isotropically etch the silicon
• In the passivation step, a fluorocarbon polymer (C4F8) is deposited on the sidewalls to protect them from further etching
• The alternating cycles enable the creation of deep, vertical structures with aspect ratios greater than 20:1
• DRIE is widely used in the fabrication of MEMS devices, such as accelerometers, gyroscopes, and microfluidic channels