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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.

10 comments on “(Synthetic) Diamonds Are a Designer’s Best Friend

  1. bjcoppa
    June 5, 2013

    Very interesting read and nice summary of this topic. TriQuint is one of the leaders in GaN-on-diamond. It is a viable tech pathway for high-power, high-frequency devices although costs are too high for most commercial applications. However, diamond over SiC or Si offers a number of performance advantages that the US military is willing to pay for under defense contracts which TQ can support as opposed to non-defense contractors in this market.

  2. Brad Albing
    June 6, 2013

    Steve – here's what I don't understand. How can diamond be such a good electrical insulator (implying that the elctrons are tightly bound to the atoms – shells are complete in the lattice structure) but it's such a good thermal conductor (which I assumed meant the electrons were free to wiggle about vigorously). Thoughts?

  3. Steve Taranovich
    June 6, 2013

    You're right Brad, it almost sounds too good to be true and as if you can't have both good insulating and thermal properties in the same element. But here is why:

    Think of it this way—Diamonds are a form of carbon. In diamonds, each carbon atom is the same distance to it's neighboring carbon atom and these covalent bonds (Sharing electrons between two atoms) are very strong. There are no free electrons in diamond—hence no current can flow in it even under high voltages. It's hard to get those electrons free.

    Diamond is a good heat conductor (Five times better than copper) because of those strong covalent bonds between the carbon atoms. In diamond, thermal conductivity is caused by high frequency lattice vibrations of the crystal which transfers energy. There are two elements that affect the efficiency of the heat transfer:

    1 Strong coupling between the atom. Diamond surely has that.

    2 The propagating waves in the crystal and how the waves are scattered due to crystal imperfections. In diamond there are virtually no crystal imperfections, hence not much scattering so that heat easily transfers across the diamond structure.

     

     

     

  4. Brad Albing
    June 6, 2013

    OK, so the electrical nonconductivity part I understood pretty well regarding the covalent bonds. The thermal part due to the lattice vibrations is what I apparently forgot from my chemistry or physics courses.

  5. BBolliger
    June 7, 2013

    Diamond is indeed being evaluated by military contractors to thermally manage their high-power RF devices. But diamond is not so expensive that it cannot economically fit into some commercial applications as well. Today for example, GaN RF power-amplifier suppliers and planar optical IC suppliers for communications infrastructure applications ship devices with diamond heat spreaders; the increase in performance and efficiency more than pays for the cost of the diamond. But it's true that it is the system cost reduction that pays for the cost of the diamond, and thus the cost analysis has to go beyond just comparing costs at the individual device level.

  6. DEREK.KOONCE
    June 11, 2013

    So, if they transferred the GaN semiconductor onto the diamond substrate, does this mean the GaN is a planar device? And no electrical connection through the bottom of the device?

    My understanding the vertical structure uses the bottom of the substrate as the drain (in the case of a MOSFET). Also, a planar device resistance is not a low as a vertical one – for the same area.

  7. Brad Albing
    June 11, 2013

    @Derek – re [6/11/2013 4:32:35 PM] – let's hope one of the other people who know way more about semiconductor fabrication can speak to this – beyond my ken.

  8. bjcoppa
    June 19, 2013

    Most device applications do not require what diamond has to offer in terms of physical and electrical properties to the level making up for the cost. SiC will suffice for the majority of applications in the mean time. There has been a ton of research into the growth of this material at the university level over the last decade but most companies have not taken that research to the next level and commercialized it. It's like defense-related applications will lead the way when that happens.

  9. Netcrawl
    June 26, 2013

    @analoging yes you're right! therer have been huge research works on this material in many labs, its has these uniques properties, it has the highest known thermal conductivity, and also the highest known resistance to thermal shock, and in terms of application, this one has lot of potentials- the most exciting is in defense industry.

  10. Brad Albing
    June 26, 2013

    We'll see if we can get more material published on all these related topics (substrates, heat conductivity, diamond, methods of bonding to these diffenent materials….).

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