Shallow trench isolation

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Scaling of isolation with transistor size. Isolation pitch is the sum of the transistor width and the trench isolation distance. As the isolation pitch shrinks, the narrow channel width effect becomes more apparent. Isolation pitch vs design rule.PNG
Scaling of isolation with transistor size. Isolation pitch is the sum of the transistor width and the trench isolation distance. As the isolation pitch shrinks, the narrow channel width effect becomes more apparent.
The shallow trench isolation fabrication process of modern integrated circuits in cross-sections. Shallow trench isolation process.svg
The shallow trench isolation fabrication process of modern integrated circuits in cross-sections.

Shallow trench isolation (STI), also known as box isolation technique, is an integrated circuit feature which prevents electric current leakage between adjacent semiconductor device components. STI is generally used on CMOS process technology nodes of 250 nanometers and smaller. Older CMOS technologies and non-MOS technologies commonly use isolation based on LOCOS. [1]

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Electric current flow of electric charge

An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas (plasma).

In electronics, leakage may refer to a gradual loss of energy from a charged capacitor. It is primarily caused by electronic devices attached to the capacitors, such as transistors or diodes, which conduct a small amount of current even when they are turned off. Even though this off current is an order of magnitude less than the current through the device when it is on, the current still slowly discharges the capacitor. Another contributor to leakage from a capacitor is from the undesired imperfection of some dielectric materials used in capacitors, also known as dielectric leakage. It is a result of the dielectric material not being a perfect insulator and having some non-zero conductivity, allowing a leakage current to flow, slowly discharging the capacitor.

Contents

STI is created early during the semiconductor device fabrication process, before transistors are formed. The key steps of the STI process involve etching a pattern of trenches in the silicon, depositing one or more dielectric materials (such as silicon dioxide) to fill the trenches, and removing the excess dielectric using a technique such as chemical-mechanical planarization.

Certain semiconductor fabrication technologies also include deep trench isolation, a related feature often found in analog integrated circuits.

The effect of the trench edge has given rise to what has recently been termed the "reverse narrow channel effect" [2] or "inverse narrow width effect". [3] Basically, due to the electric field enhancement at the edge, it is easier to form a conducting channel (by inversion) at a lower voltage. The threshold voltage is effectively reduced for a narrower transistor width. [4] [5] The main concern for electronic devices is the resulting subthreshold leakage current, which is substantially larger after the threshold voltage reduction.

Electric field spatial distribution of vectors representing the force applied to a charged test particle

An electric field is a vector field surrounding an electric charge that exerts force on other charges, attracting or repelling them. Mathematically the electric field is a vector field that associates to each point in space the force, called the Coulomb force, that would be experienced per unit of charge by an infinitesimal test charge at that point. The units of the electric field in the SI system are newtons per coulomb (N/C), or volts per meter (V/m). Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature.

Threshold voltage

The threshold voltage, commonly abbreviated as Vth, of a field-effect transistor (FET) is the minimum gate-to-source voltage VGS (th) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency.

Process flow

Lithography printing process

Lithography is a method of printing originally based on the immiscibility of oil and water. The printing is from a stone or a metal plate with a smooth surface. It was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works. Lithography can be used to print text or artwork onto paper or other suitable material.

Reactive-ion etching

Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The plasma is generated under low pressure (vacuum) by an electromagnetic field. High-energy ions from the plasma attack the wafer surface and react with it.

See also

Related Research Articles

MOSFET transistor used for amplifying or switching electronic signals

The metal-oxide-semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, whose voltage determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect transistor.

NMOS logic implements logic gates and other digital circuits

N-type metal-oxide-semiconductor logic uses n-type field-effect transistors (MOSFETs) to implement logic gates and other digital circuits. These nMOS transistors operate by creating an inversion layer in a p-type transistor body. This inversion layer, called the n-channel, can conduct electrons between n-type "source" and "drain" terminals. The n-channel is created by applying voltage to the third terminal, called the gate. Like other MOSFETs, nMOS transistors have four modes of operation: cut-off, triode, saturation, and velocity saturation.

CMOS technology for constructing integrated circuits

Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors, data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1963 while working for Fairchild Semiconductor.

BiCMOS is an evolved semiconductor technology that integrates two formerly separate semiconductor technologies, those of the bipolar junction transistor and the CMOS transistor, in a single integrated circuit device.

Silicon on insulator (SOI) technology refers to the use of a layered silicon–insulator–silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance, thereby improving performance. SOI-based devices differ from conventional silicon-built devices in that the silicon junction is above an electrical insulator, typically silicon dioxide or sapphire. The choice of insulator depends largely on intended application, with sapphire being used for high-performance radio frequency (RF) and radiation-sensitive applications, and silicon dioxide for diminished short channel effects in microelectronics devices. The insulating layer and topmost silicon layer also vary widely with application.

Subthreshold conduction

Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages below the threshold voltage. The terminology for various degrees of inversion is described in Tsividis.

The term high-κ dielectric refers to a material with a high dielectric constant κ. High-κ dielectrics are used in semiconductor manufacturing processes where they are usually used to replace a silicon dioxide gate dielectric or another dielectric layer of a device. The implementation of high-κ gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components, colloquially referred to as extending Moore's Law.

Front end of line

The front-end-of-line (FEOL) is the first portion of IC fabrication where the individual devices are patterned in the semiconductor. FEOL generally covers everything up to the deposition of metal interconnect layers.

Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a MOS transistor, the switching characteristics of the transistor can be permanently changed. Hot-carrier injection is one of the mechanisms that adversely affects the reliability of semiconductors of solid-state devices.

Multigate device

A multigate device or multiple-gate field-effect transistor (MuGFET) refers to a MOSFET that incorporates more than one gate into a single device. The multiple gates may be controlled by a single gate electrode, wherein the multiple gate surfaces act electrically as a single gate, or by independent gate electrodes. A multigate device employing independent gate electrodes is sometimes called a multiple-independent-gate field-effect transistor (MIGFET).

Multi-threshold CMOS (MTCMOS) is a variation of CMOS chip technology which has transistors with multiple threshold voltages (Vth) in order to optimize delay or power. The Vth of a MOSFET is the gate voltage where an inversion layer forms at the interface between the insulating layer (oxide) and the substrate (body) of the transistor. Low Vth devices switch faster, and are therefore useful on critical delay paths to minimize clock periods. The penalty is that low Vth devices have substantially higher static leakage power. High Vth devices are used on non-critical paths to reduce static leakage power without incurring a delay penalty. Typical high Vth devices reduce static leakage by 10 times compared with low Vth devices.

LOCOS, short for LOCal Oxidation of Silicon, is a microfabrication process where silicon dioxide is formed in selected areas on a silicon wafer having the Si-SiO2 interface at a lower point than the rest of the silicon surface.

PMOS logic p-type MOSFETs to implement logic gates

P-type metal-oxide-semiconductor logic uses p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) to implement logic gates and other digital circuits. PMOS transistors operate by creating an inversion layer in an n-type transistor body. This inversion layer, called the p-channel, can conduct holes between p-type "source" and "drain" terminals.

Lau Wai Shing, also known as Wai Shing Lau, is a Hong Kong electrical engineer and materials scientist. He worked on both Si-based and III-V based microelectronics.

The MASTAR is an analytical model of Metal-Oxide Semiconductor Field-Effect Transistors, developed using the voltage-doping transformation (VDT) technique. MASTAR offers good accuracy and continuity in current and its derivatives in all operation regimes of the MOSFET devices. The model has been successfully used in CAD/EDA simulation tools.

The subthreshold slope is a feature of a MOSFET's current–voltage characteristic.

Polysilicon depletion effect is the phenomenon in which unwanted variation of threshold voltage of the MOSFET devices using polysilicon as gate material is observed, leading to unpredicted behaviour of the electronic circuit. Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells.

Tunnel field-effect transistor

The tunnel field-effect transistor (TFET) is an experimental type of transistor. Even though its structure is very similar to a metal-oxide-semiconductor field-effect transistor (MOSFET), the fundamental switching mechanism differs, making this device a promising candidate for low power electronics. TFETs switch by modulating quantum tunneling through a barrier instead of modulating thermionic emission over a barrier as in traditional MOSFETs. Because of this, TFETs are not limited by the thermal Maxwell–Boltzmann tail of carriers, which limits MOSFET drain current subthreshold swing to about 60 mV/decade of current at room temperature. The concept was proposed by Chang et al while working at IBM. Joerg Appenzeller and his colleagues at IBM were the first to demonstrate that current swings below the MOSFET’s 60-mV-per-decade limit were possible. In 2004, they reported they had created a tunnel transistor with a carbon nanotube channel and a subthreshold swing of just 40 mV per decade.

The field-effect transistor (FET) is an electronic device which uses an electric field to control the flow of current. This is achieved by the application of a voltage to the gate terminal, which in turn alters the conductivity between the drain and source terminals.

References

  1. Quirk, Michael & Julian Serda (2001). Semiconductor Manufacturing Technology: Instructor's Manual Archived September 28, 2007, at the Wayback Machine ., p. 25.
  2. Jung, Jong-Wan; Kim, Jong-Min; Son, Jeong-Hwan; Lee, Youngjong (30 April 2000). "Dependence of Subthreshold Hump and Reverse Narrow Channel Effect on the Gate Length by Suppression of Transient Enhanced Diffusion at Trench Isolation Edge". Japanese Journal of Applied Physics. 39 (Part 1, No. 4B): 2136–2140. doi:10.1143/JJAP.39.2136.
  3. A. Chatterjee et al., IEDM 1996.(conference announcement) , doi:10.1109/VLSIT.1996.507831 Missing or empty |title= (help)
  4. Pretet, J; Ioannou, D; Subba, N; Cristoloveanu, S; Maszara, W; Raynaud, C (November 2002). "Narrow-channel effects and their impact on the static and floating-body characteristics of STI- and LOCOS-isolated SOI MOSFETs". Solid-State Electronics. 46 (11): 1699–1707. doi:10.1016/S0038-1101(02)00147-8.
  5. Lee, Yung-Huei; Linton, Tom; Wu, Ken; Mielke, Neal (May 2001). "Effect of trench edge on pMOSFET reliability". Microelectronics Reliability. 41 (5): 689–696. doi:10.1016/S0026-2714(01)00002-6.