What is RF SOI?

SOI stands for Semiconductor on Insulator technology; RF SOI is the use of this technology for high-frequency radio frequency applications. Compared to the bulk CMOS technology in which the transistor is fabricated directly over a silicon substrate, RF SOI technology uses a thin layer of silicon placed on top of an insulating layer made of silicon dioxide. This results in a transistor with better electrical characteristics like lower parasitic capacitance, lower power consumption, increased circuit speed, better isolation and better signal integrity. This technology is mainly used to develop high-performance RF Switches, but can also be used to develop RF amplifiers and passive components, making it ideal for high-density, mixed signal ICs.

One of the key advantages of RF SOI Technology over Bulk-CMOS is that it removes the short-channel effects, provides better isolation between the devices by providing full dielectric isolation of the devices, and provides resistance to latch-up. Thus, leading to faster switching speeds, lower power consumption, and improved device performance.

Junction Capacitance in Bulk Substrate vs SOI technology

Key Electrical Characteristics of RF SOI

The typical performance parameters of RF SOI are given in the table below.

These performance parameters are typical for RF-SOI technology and can change based on specific device designs and applications. 

RF SOI Device Structure 

The SOI wafer consists of a substrate layer, usually a high-resistive silicon substrate responsible for providing the mechanical support to the device. Just above it there’s the insulated layer (buried oxide layer) which allows for the vertical isolation from the substrate, and also is the most evident distinction between SOI and bulk silicon methods. The buried oxide helps reduce the parasitic capacitance and make the device size smaller. The SOI-based devices use silicon dioxide or sapphire as the electrical insulator above the silicon substrate.

A thin layer of silicon film (Silicon Channel) is placed on the top of buried oxide, which is used to make the transistor. The full dielectric isolation of the device helps in reducing the parasitic capacitance and improving the performance of the device. 

SOI Wafer and its cross-section 

Types of SOI Devices

Based on the structure, and thickness of the silicon and insulating layer, SOI devices are characterized into two types: Partially Depleted SOI (PD-SOI) devices, and Fully Depleted SOI (FD-SOI) devices.

What is FD-SOI?

In FD-SOI devices, the silicon channel above the Buried Oxide (BOX) layer is thin, usually 5-20 nm thick (1/4 of the gate length) and the BOX layer is 5-50 nm thick. Due to this, the entire channel region is fully depleted of charge carriers when the device is in operation which leads to better electrostatic control of the gate. This full depletion eliminates the floating body effects (when a portion of the silicon body in a transistor is electrically isolated from the substrate, it is called “floating body”, and when this floating body holds an unknown voltage due to charge accumulation, it leads to the “floating body effect”) and results in more stable threshold voltage, reduced variability and improved performance.  

What is PD-SOI?

In PD-SOI, the silicon film above the Buried Oxide (BOX) layer is relatively thick, usually between 50 and 90 nm and the BOX layer is typically 100 – 200 nm thick.  Due to this, only a portion of the silicon layer is depleted of charge carriers (electron or holes) when the device is turned on. As a result, it still has a neutral region that can store charges which leads to a higher parasitic capacitance as compared to FD-SOI devices and also leads to floating body effects. 

While FD-SOI offers superior performance in many aspects, PD-SOI remains relevant because it can be more  cost-effective option in certain applications where the performance benefits of FD-SOI aren’t necessary, while still providing improvements over traditional bulk silicon particularly in terms of leakage current and improved radiation resistance. Thus, PD-SOI acts as a middle ground between the bulk silicon and high-performance FD-SOI. 

Fabrication of SOI

The SiO2-based SOI wafers can be produced by several methods, and processes like SIMOX, Wafer bonding and Seed Methods. 

SIMOX (Separation by Implantation of Oxygen): This method uses an oxygen ion beam implantation process followed by high-temperature annealing. It creates a buried SiO2 layer beneath the silicon surface. A major advantage of this process is that it allows precise control of the buried oxide (BOX) thickness and depth, resulting in high-quality SOI wafers. 

Wafer Bonding 

 This process involves directly bonding the oxidized silicon with a second substrate. A major portion of the second substrate is then removed, leaving a thin silicon layer on top. This process produces high-quality SOI wafers with precise control over the thickness of the silicon layer.   

One of the prominent examples of a wafer bonding process is the Smart Cut method developed by the French firm Soitech which uses the technique of ion implantation followed by controlled exfoliation to determine the thickness of the uppermost silicon layer.

NanoCleave: This technology has been developed by Silicon Genesis Corporation and separates the silicon via stress at the interface of silicon and silicon-germanium alloy. This method allows for precise control of the top silicon layer’s thickness and can be used to produce high-quality SOI wafers that are suitable for advanced applications. 

Applications of RF SOI Technology

RF SOI technology is used extensively in high-frequency and high-performance applications like Wireless communication systems, radar systems, satellite and space communications etc. One of the main applications of RF SOI technology is RF switches.

In the cellular world it is used to develop the Antenna switch and Tuner in a Front-End module. In automotive radar and Automotive Driver Assistance Systems (ADAS), it boosts radar performance by providing high linearity and efficient RF signal management. In satellite systems, it is used in the transceivers and other RF components due to its reliability and ability to operate in harsh environments. It is also used in medical devices like imaging systems, MRI and implantable medical devices due to its low power consumption.