Dielectric materials for semiconductor chips
Little funda about dielectric material:
Dielectric material characterize with very low electrical conductivity (one millionth of a mho / cm), in which an electric field can be sustained with a minimal leakage. It can store electrical energy/charge.
The electrically sensitive molecules inside dielectric material called polar molecules align by a pattern whenever external electric voltage is applied. The polar molecules align with the field so that the positive charges accumulate on one face of the dielectric and the negative charges on the other face.
Dry air is a good example of a dielectric material but is a poor dielectric. Most of the dielectric materials are solid such as porcelain (ceramic), glass, mica, plastics, and the oxides of various metals. Some liquids and gases can also good dielectric properties, vaccum and distilled water are also dielectric materials.
Dielectric material is popularly used in capacitors to reduce the size of the capacitor. The ceramic and plastic film capacitors are good examples. Another major application of Dielectric materials is in semiconductor chips to insulate transistors from each other. Dielectric materials are also used in various other electrical and electronic components.
Why the dielectric material is used in capacitor?
Dielectric material offers mechanical strength
By factor x the capacitance is increased
The "dielectric breakdown", or maximum electric field before conduction between the plates starts, can be higher than for air, allowing high voltage ratings
Separation of the plate can be small, which also increases capacitance
Dielectric strength may be defined as the maximum potential gradient to which a material can be subjected without insulating breakdown, that is
Where DS is the dielectric strength in kV/mm, VB the breakdown voltage, and d the thickness.
Dielectric constant is the ratio of the amount of stored electrical energy when a potential is applied, relative to the permittivity of a vacuum.
Due to improvement in dielectric material characteristics used in semiconductor devices, the performance of semiconductor device such as power consumption, speed, and size are improved. Also semiconductor chips can integrate high capacitance capacitors inside the chip to save the circuit from using external decoupling capacitors between the power and the ground planes.
These decoupling capacitors will reduce the transient voltage on the voltage supply, which are caused by the current spikes that occur when the transistors on the semiconductor circuit switch on or off.
Since in the mid-1990s, the microelectronics industry has innovated high- and low-k dielectrics (k is the dielectric constant of a material) for continuing reduction of both horizontal and vertical dimensions of integrated circuits (Ics). Due to use of low K material the gate leakage current and heat dissipation can be brought down. Low K materials offer lower propagation delay, and lower cross talk enabling devices to operate at higher frequencies i.e in the range of giga hertz.
Low K dielectric materials:
In both the vertical and horizontal dimensions the reduction in spacing of metal interconnects has created the need for low-k materials that serve as interlevel dielectrics to offset the increase in signal propagation time between transistors, known as RC delay (R is metal wire resistance and C is interlevel dielectric capacitance). To fulfill these requirements at 32nm and lesser IC fabrication nodes, innovation in dielectric materials is must if the device density of Ics has to continue at Moore's Law rate. Low K materials are used in multi level interconnects, interlayer dielectrics, and for passivation layers.
Some of the examples of low K dielectric material are, Nanopourous Silica, Hydrogensilsesquioxanes (HSQ), Teflon-AF (Polytetrafluoethylene or PTFE), Silicon Oxyflouride (FSG). The present trend now is using dielectric with K of less than 2.
High K dielectric materials:
The High K dielectric materials are needed for the storage capacitors, and nonvolatile static memory devices. Wherever high capacitance is required the high-K material is used.
High K dielectric material is used between gate and the silicon in CMOS transistors to increase the capacitance of the metal and silicon.
Conventional materials such as thermal and chemical vapour deposition (CVD) SiO2 are being replaced with new materials such as high k gate dielectrics, and carbon doped SiO2 for low k interlevel dielectrics.
Some of the latest high k dielectric materials include:
--- SiNx with 4 < k < 10
--- Ta2O5, Al2O3, ZrO2, and HfO2 with 10 < k < 100
--- PZT with k<100
To find dielectric constant for thousands of chemical compounds and elements visit the site: http://www.clippercontrols.com/info/dielectric_constants.html
Author: Srinivasa Reddy N