Difference between thick film deposition vs thin film deposition
This page compares thick film deposition vs thin film deposition and describes difference between thick film deposition and thin film deposition.
In thick film deposition,conductor pattern is printed before resistance deposition has taken place on dielectric substrate(e.g. alumina). This method is suitable at low microwave frequencies. The RF circuits for example parallel coupled and open circuit stub filters, RF couplers, RF attenuators, power dividers are manufactured using thick film deposition.
In thin film deposition chemical etching is done to form components. This method is more suitable at high frequencies. In other words, in the case of thin film hybrid circuits; metallization is deposited by evaporation and desired pattern is developed using photolithographic techniques.
At microwave frequencies, thin film is preferred over thick film technique to minimize loss.
Think Film Deposition Method
Following two methods are adopted to manufacture thin film MICs.
1. Thick film patterns are deposited or printed as conductive, resistive or insulating layers. They are fired on to ceramic substrate usually alumina or sometimes quartz. This method is mostly used in MIC manufacturing.
2. A printed circuit technique is used to etch desired pattern in the copper cladding of usually polyolefin substrate. This is very common PCB fabrication method.
Each method is much simple and have less requirement of both the equipment and environment compare to thin film technology.
The details of fabrication steps in thick film deposition are as follows:
• A metal e.g. Au paste is prepared and kept in refrigerator.
• The artwork for circuit design is prepared and photographic processing is used to obtain positive transparency.
• Fine stainless steel(or polyester mesh) is tightly stretched over a rigid frame fitting into screen printer. This screen is coated with a suitable photoresist layer.
• The +ve transparency is held in intimate contact with the coated surface of the screen.
• Exposure of standard UV light and wash bake processes will make a screen with apertures for the required circuit. All other areas are made opaque with durable photoresist layer.
• The screen is then placed in a printed jig. The microwave substrate is placed below the aperture region of the screen firmly by vacuum suction.
• A few millimeters of paste is placed on the screen.
• After optimizing the parameters the aperture region of the screen is wet and a wet deposite of paste is transferred on to the substrate.
• The wet circuit paste is left horizontally to settle in a clean toom for about 15-20 minutes and then dried at nearly 100 degree C for about 20 mins using an infrared drying machine.
• The deposit is fired at 900 to 1000 degreeC to form a metallic substance.
• Finally after firing the circuit, either laser trimming or an etch back process is conducted to achieve the precise definition of the circuit.
Thin Film Deposition Method
Following steps are followed in thin film deposition method.
• STEP1: Substrates are cleaned to a very high specification.
• STEP2: Evaporation or sputtering- a thin layer of less than 0.1 µm of chromium on the surface of the substrate.
• STEP3: Evaporation or sputtering- a very thin layer of size less than 0.5 µm Cu or Au of similar thickness on to this layer of step-2.
• STEP4: Electroplating the bulk conductor Cu of approx. 5µm thickness on to layer as in step-3.
The step-2 and step-3 are used for mechanical and electrical foundation layers. The circuits are defined using photolithographic techniques. The very thin layers are produced by magnetron sputtering where in combined E and H fields produce ionization of the Cu charge. The ions are attracted towards the substrate used as anode.
Hybrid MIC vs. MMIC
• Thick film hybrid MIC support frequency upto 10 GHz.
• Thin film hybrid MIC support frequency upto 100GHz. Ideal for mm wave applications.
• MMIC support mm wave applications similar to thin film.
The undesired parasitic effects are minimized in MMIC due to absence of wire bonding and embedment of active components within a PCB.
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