Thick Film vs. Thin Film Deposition: A Comparison

This article explores the differences between thick film deposition and thin film deposition techniques, outlining their processes, advantages, and applications.

Thick Film Deposition

In thick film deposition, a conductor pattern is printed onto a dielectric substrate (e.g., alumina) before the resistance deposition takes place. This method is well-suited for applications at lower microwave frequencies.

RF circuits, such as parallel-coupled and open-circuit stub filters, RF couplers, RF attenuators, and power dividers, are often manufactured using thick film deposition techniques.

Thin Film Deposition

In contrast, thin film deposition utilizes chemical etching to form components. This makes it more suitable for high-frequency applications.

For thin film hybrid circuits, metallization is deposited through evaporation, and the desired pattern is developed using photolithographic techniques. At microwave frequencies, thin film is generally preferred over thick film to minimize loss.

Thin Film Deposition Methods

Two primary methods are employed for manufacturing thin film MICs (Microwave Integrated Circuits):

1. Thick Film Pattern Deposition: Conductive, resistive, or insulating layers are deposited or printed, then fired onto a ceramic substrate (usually alumina or sometimes quartz). This is a common method in MIC manufacturing.
2. Printed Circuit Technique: A printed circuit technique is used to etch the desired pattern in the copper cladding of a substrate, typically polyolefin. This is a standard PCB (Printed Circuit Board) fabrication method.

Both methods are simpler and require less sophisticated equipment and controlled environments compared to thin film technology.

Fabrication Steps in Thick Film Deposition

The following steps are involved in thick film deposition:

• A metal paste (e.g., Au) is prepared and stored in a refrigerator.
• Artwork for the circuit design is created, and photographic processing is used to produce a positive transparency.
• A fine stainless steel (or polyester mesh) screen is tightly stretched over a rigid frame that fits into a screen printer. This screen is coated with a suitable photoresist layer.
• The positive transparency is held in close contact with the coated surface of the screen.
• Exposure to standard UV light and wash bake processes creates a screen with apertures for the required circuit. All other areas are made opaque with a durable photoresist layer.
• The screen is placed in a printed jig. The microwave substrate is positioned below the aperture region of the screen, held firmly by vacuum suction.
• A small amount of paste is placed on the screen.
• After optimizing the parameters, the aperture region of the screen is wet, and a wet deposit of paste is transferred onto the substrate.
• The wet circuit paste is left horizontally to settle in a clean room for approximately 15-20 minutes.
• The substrate is then dried at around 100 degrees Celsius for about 20 minutes using an infrared drying machine.
• The deposit is fired at 900 to 1000 degrees Celsius to form a metallic substance.
• Finally, after firing the circuit, either laser trimming or an etch-back process is performed to achieve the precise definition of the circuit.

Fabrication Steps in Thin Film Deposition

The following steps are typically followed in the thin film deposition method:

• STEP 1: Substrates are cleaned to a very high specification.
• STEP 2: Evaporation or sputtering deposits a thin layer (less than 0.1 µm) of chromium on the surface of the substrate.
• STEP 3: Evaporation or sputtering deposits a very thin layer (less than 0.5 µm) of Cu or Au with similar thickness on top of the chromium layer from step 2.
• STEP 4: Electroplating deposits a bulk conductor layer of Cu (approximately 5 µm thickness) onto the layer from step 3.

Steps 2 and 3 are used to create mechanical and electrical foundation layers. The circuits are defined using photolithographic techniques. The very thin layers are produced by magnetron sputtering, where combined electric (E) and magnetic (H) fields ionize the Cu charge. The ions are attracted towards the substrate, which acts as the anode.

Hybrid MIC vs. MMIC (Monolithic Microwave Integrated Circuit)

• Thick film hybrid MICs support frequencies up to 10 GHz.
• Thin film hybrid MICs support frequencies up to 100 GHz, making them ideal for millimeter-wave applications.
• MMICs also support millimeter-wave applications, similar to thin films.

Undesired parasitic effects are minimized in MMICs due to the absence of wire bonding and the embedment of active components within a PCB.