OPTICAL APPLICATIONS OF DIAMOND

Rationale: 

Diamond has an exceptionally wide optical transmission window and is resistant to mechanical abrasion and chemical attack. Optical coatings are an inherent part of many optical structures. Diamond films can provide the attractive surface properties of diamond while improving optical performance.

Application Examples: 

Antireflection coatings on semiconductor photon detectors, emitters, and windows. Controlled reflection on mirrors, filters, and architectural glass. Emission pixels on field emission displays. Applications require critical control of sub-micrometer dimensions.

Coarse-grain (translucent) and fine-grain (transparent) diamond films on Corning 7059 glass.
 

Background and Current Challenges:

Highly pure single-crystalline diamond is transparent for wavelengths from approximately 0.23 mm in the ultraviolet , throughout the visible and infrared, and into the mm wave portion of the electromagnetic spectrum, except for slight absorption in the infrared (IR). The IR absorption is due to multi-phonon absorption and is small, with the largest peak corresponding to an absorption coefficient of approximately 12 cm-1 at 5 m wavelength. For most thin-film coating applications such absorption is negligible. However, not all CVD polycrystalline diamond has useful optical features. Rough surfaces and incorporation of impurities and defects can have a significant deleterious effect on polycrystalline diamond optical properties. Also, many potential optical substrates are not "friendly" to high temperature CVD processes because of thermal expansion coefficient mismatch and incompatibility with the diamond deposition plasma. These represent challenges to the application of CVD polycrystalline diamond to optical-coatings.

The challenges are being met by advances on several fronts. A low-temperature diamond deposition process has been developed which allows deposition on substrates with relatively high thermal expansion coefficients, including borosilicate glasses (for example, Pyrex  and 7059 Corning). Also, seeding and deposition methods have been advanced to allow direct deposition of optically smooth films, eliminating the need for post-processing such as polishing. Additionally, multi-layer coatings have been developed in which diamond is one (the top) layer. The use of multi-layers allows improved optical performance and also opens a significantly broadened pool of applications.

For applications requiring mirrors and filters, coatings are used to achieve high transmission or transmission for particular ranges of wavelengths. For  semiconductor substrates, which are used for a variety of applications, including light-emitting devices, photo-sensors, solar cells, and infrared windows, antireflection coatings allow an increase intransmission from approximately 50% to, in the case of multi-layer coatings, near 100%.

IR transmission through a substrate optimized for the 3 - 5 mm "atmospheric window." The material is 5 cm diameter silicon, with as-grown 400 nm thick diamond film on both sides.

Calculation (solid line) assumes an absorption-free and scatter-free diamond film. Data taken by Mr. Ingo Kleindienst

Transmission from the near UV, through the visible, to the near IR. The substrate is a 5 cm. diameter Corning 7059 coated glass substrate with a 1.07 mm thick as-grown diamond film.

Solid line is calculated for a 1.07 mm thick absorption-free diamond film with scatter produced by a 24 nm surface roughness. Data taken by Dr. Michael Ulczynski.

Raman spectrum (argon laser) for a diamond-coated borosilicate glass substrate.

Data taken by Joerg Mossbrucker.
 

Note the nearly ideal (approx. 70%) transmission observed through the diamond coated portion (Spartan helmet shape portion) of the substrate. Essentially, all transmission loss is due to reflection.

Recent presentation by Jong-kook Park.

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MNEC Publications Related to Diamond Optical Coatings include:

1. D. K. Reinhard, Diamond films as optical coatings for semiconductors, Applied Diamond Conference Proceedings, Japan, August, (1999). 
2. M. J. Ulczynski, D. K. Reinhard, and J. Asmussen, Jr., Process for depositing adherent diamond thin films, U.S. Patent, February, (1999). 
3. M. J. Ulczynski, B. Wright, and D. K. Reinhard, Diamond-coated glass substrates, Diamond and Related Materials, vol. 7, 1939-1646, (1998).
4. M. J. Ulczynski, Low-temperature deposition of transparent diamond films with a microwave cavity plasma disk reactor, Doctoral Dissertation, Electrical Engineering, Michigan State University, (1998).
5. M. J. Ulczynski, Scatter-limited optical transmission of diamond-coated glass, Applied Diamond Conference, Aug. 3-8, Edinburgh, Scotland, (1997). 
6. M. J. Ulczynski, D. K. Reinhard, and J. Asmsussen, Optical properties of diamond-coated borosilicate glass, 5th International Conference on the New Diamond Science and Technology, Sept. 8-13, Tours, France, (1996). 
7. M. J. Ulczynski, D. K. Reinhard, M. Prystajko, and J. Asmussen, Thin film diamond coatings on glass, National Institute of Standards and Technology Special Publication 885, Applications of Diamond Films and Related Materials: Third International Conference, 573-576, (1995).