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(Synthetic) Diamonds Are a Designer’s Best Friend

With more and more functionality being packed into an analog IC, and even the desire to incorporate power elements (MOSFETs, IGBTs, etc.) onto the die, we need to start thinking outside the box for ways to remove the excess heat that will be inevitable and damaging to the die as well as degrading the reliability of the circuitry.

Since synthetic diamond is an ideal material for a range of thermal management applications, and has a thermal conductivity four times higher than copper, designers need to take a much closer look at ways to integrate this very efficient, heat transferring material.

Element Six is a company that has developed a range of polycrystalline diamond and synthetic diamond composite materials that can be used in a wide range of thermal management applications.

As a bonus, the synthetic diamond material is an electric insulator.

Various synthetic diamond processes exist. We use Element Six as an example of what is available to the designer:

CVD (chemical vapor deposition) diamond solutions
One of the first applications of synthetic diamond was as a heat sink for sensitive electronic components used in the telecommunications industry. CVD diamond process
Element Six has Diafilm 200, the world's first synthetic diamond thermal material that can offer thermal conductivity in excess of 2000W/mK. This top-end grade of CVD diamond material is very useful to advanced designers who work with high power or high power density devices and require extreme performance for their thermal packaging needs. SCD (silicon cemented diamond) thermal solutions
Element Six SCD Thermal is part of a new family of synthetic diamond-silicon carbide composites that are being developed to provide solutions in a wide range of industrial applications. SCD Thermal combines synthetic diamond particles with silicon carbide to create a range of low-cost materials that have excellent thermal and mechanical properties, making them ideal for use in the thermal management of electronic systems. Synthetic diamond as a semiconductor material
Synthetic diamond has long been recognized as a semiconductor material that can detect many different types of radiation, from UV and X-rays. Element Six has the ability to make synthetic diamond material of the size, quality, and consistency required for advanced detection applications, such as in high energy physics research at the Large Hadron Collider (LHC) project at CERN (Conseil Européen pour la Recherche Nucléaire).

Element Six, working in collaboration with the CERN RD42 Group, developed the synthesis technology so that the synthetic diamond material produced could meet the needs of high-energy physics experiments. The work within Element Six produced electronic grade material with properties optimized for use in the LHC.

Synthetic diamond is ideal for these types of extreme monitoring and detection applications. Silicon detectors cannot be used over long periods because of radiation damage, whereas synthetic diamond is able to survive the harsh radiation environment for long periods.

Synthetic diamond's molecular structure makes it the world's most versatile super-material.

Synthetic diamond's molecular structure makes it the world's most versatile super-material.

Synthetic diamond properties by Element Six
The characteristics of synthetic diamond include:

  • The broadest electromagnetic transmission spectrum of any material
  • A wide electronic band gap (it carries very low current even under high voltages)
  • The ability to combine/dope with boron and take on a similar electrical conductivity to metal
  • The highest known thermal conductivity
  • The highest known resistance to thermal shock
  • Low thermal expansion
  • Low dielectric constant and loss
  • High electrical carrier mobility
  • A very low coefficient of friction
  • Chemical and biochemical inertness
  • Excellent electrical insulator properties

Optical properties
Synthetic diamond has the widest spectral band of any known material — extending from ultraviolet to far infrared and the millimeter-wave microwave band. Coupled with its mechanical and thermal properties, this makes it the ideal “window” material for many industrial, R&D, defense, and laser applications, particularly in the production of laser optics where synthetic diamond provides optimum exit windows for CO2 lasers, such as those used in automotive cutting applications. Electronic properties
Synthetic diamond also has a number of exciting electrical properties such as a low dielectric constant and loss, a high electrical carrier mobility, and a wide electronic band gap (it allows very low current even under high voltages), all of which allow its use in advanced healthcare applications. These include therapy for eye cancer sufferers where synthetic diamond-based radiation detectors ensure the delivery of the correct dosage to target just the cancer-affected tissue, and not healthy tissue around it.

Quantum physicists at Harvard University are currently using the highest purity synthetic diamond from Element Six to develop synthetic diamond-based quantum computer technology that could enable faster data processing and secure communication.

Latest industry news
As a possible future integration of power elements onto an integrated analog circuit, TriQuint Semiconductor has managed to reduce device temperature and increase thermal conductivity by using GaN-on-diamond wafers. This technology should enable new generations of RF power amplifiers (PAs) that reduce size or increase power output by a factor of three compared to today's GaN (gallium nitride) solutions.

The feasibility of GaN-on-diamond HEMT devices has been proven with the successful transfer of a semiconductor epitaxial overlay onto a synthetic diamond substrate.

The feasibility of GaN-on-diamond HEMT devices has been proven with the successful transfer of a semiconductor epitaxial overlay onto a synthetic diamond substrate.

The company successfully transferred a semiconductor epitaxial overlay onto a synthetic diamond substrate, preserving key GaN crystalline layers. GaN semiconductors can now potentially benefit from the high thermal conductivity of the diamond substrate and low thermal resistance at the boundary between the GaN and diamond materials. This achievement proves the feasibility of GaN-on-diamond HEMT (high electron mobility transistor) devices.

So it seems that TriQuint has achieved the goal of a three-fold improvement in heat dissipation while preserving RF functionality, which was the primary objective of the Defense Advanced Research Projects Agency's (DARPA's) Near Junction Thermal Transport (NJTT) program. NJTT focuses on device thermal resistance “near the junction” of the transistor. Thermal resistance inside device structures can be responsible for more than 50 percent of normal operational-temperature increases. In its research, TriQuint has shown that GaN RF devices can operate at a much higher power density and in smaller sizes by leveraging thermal-management techniques.

This can ultimately pave the way to integration of a GaN device onto a silicon die with a synthetic diamond substrate as a first step to higher level, efficient integration of the power element on a silicon or CMOS die.

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