Thermal expansion sapphire

The reactor assembly was heated by electric heaters. The maximum operating temperature Is determined by the window construction. Sapphire windows (from EIMAC), brazed into Kovar sleeves, were used the sleeves were then welded directly into the stainless steel reactor housing. We found that the cell so constructed was capable of trouble-free, continuous operation at 450°C operations at somewhat higher temperatures are probably still possible but were not explored. Sapphire was chosen as a window material because it is insensitive to water vapor and is transparent in tljie wave number range of our interest (about 2400 cm to 2000 cm in these experiments). Moreover, the thermal expansion characteristics of the reactor were found to match well with those of the window fixture. 

The invention of US-A-536S088 approaches the problem of stresses generated by the mismatch of the thermal expansion coefficients of HgCdTe and silicon by including a buffer layer of sapphire. The characteristic thermal expansivity of sapphire is more similar to the thermal expansivity of HgCdTe than that of silicon. 

The hybrid circuit 10 comprises a buffer structure 16 which is comprised of a material which accommodates the difference in thermal expansion coefficients of the HgCdTe detector array 12 and the silicon read-out chip 14. The buffer layer is made of sapphire which also has good thermal conductivity properties. The buffer structure has laser drilled vias 18 which are formed in registration with unit cells of the detector array and the read-out circuit. Each of the vias is provided with indium bumps 20 at opposing ends thereof. The buffer structure is interposed between the detector array and the read-out chip. Cold weld indium bump technology is employed to couple the bumps 20 to the buffer structure. The buffer structure is further... 

The structure is bonded to a substrate 24 which is chosen to have a coefficient of thermal expansion that is selected for providing the resultant read-out chip assembly with an effective coefficient of thermal expansion that is approximately the same as an HgCdTe detector array 36. The substrate material may be GaAs (4.5-5.9 x 10"6 m/mK), CdTe, Ge (5.5-6.4 x 10"6 m/mK), and a-plane sapphire (3.5-7 5 x 10" m/mK) where the coefficients of thermal expansion are given in parentheses. The coefficients of thermal expansion for silicon, HgCdTe and epoxy are 1.2 x 10"6 m/mK, 3.8-4.5 x 1 O 6m/mK and 30-50 x 10"6 m/mK, respectively. Next, the substrate 16 is removed and aluminium pads 34a are formed. Indium bumps 34b are cold welded to corresponding indium bumps 36b. 

A large lattice mismatch between sapphire and nitrides (about 13% to AIN, 16% to GaN and 29% to InN) makes even very thin layers fully relaxed at the growth temperature. When the samples are cooled down after growth, a thermal strain is created. Such strain occurs for other materials, for example for GaAs on Si [14], and corresponds to a difference in thermal expansion between the layer and the substrate. Using thermal expansion coefficients for GaN and sapphire one can estimate that the compressive thermal strain Aa/a, which should be generated for MOCVD grown GaN on (00.1)... 

Thermal expansion of a semiconductor depends on its microstructure, i.e. stoichiometry, presence of extended defects, ffee-carrier concentration. For GaAs [24] it was shown that for samples of free-electron concentrations of about 1019 cm"3, the thermal expansion coefficient (TEC) is bigger by about 10% with respect to the semi-insulating samples. Different microstructures of samples examined in various laboratories result in a large scatter of published data even for such well known semiconductors as GaP or GaAs. For group III nitrides, compounds which have been much less examined, the situation is most probably similar, and therefore the TECs shown below should not be treated as universal values for all kinds of nitride samples. It is especially important for interpretation of thermal strains (see Datareview A 1.2) for heteroepitaxial GaN layers on sapphire and SiC. 

TABLE 6 Values of thermal expansion coefficient (TEC) for AIN, GaN and InN, together with the values for the most popular substrates sapphire, SiC and LiGaOi. 

Fig. 2. Certified values of thermal expansion coefficients for SRMs 731, 736, 737, and 739, along with tentative values for stainless steel, graphite, sapphire, and aluminum.

A cell constant close to 0.2 cm" was determined from measurements on aqueous KCl at 25 ""C and the cell constant change with temperature from the known coefficients of thermal expansion of sapphire and platinum was 0.4% over the entire temperature range (306 C-400°C). 

Adhesion to the film can be enhanced by surface treatment. The conventional method is corona discharge but more recently atmospheric pressure oxygen plasma treatments have been developed. The film can also be vacuum metallised to improve barrier performance, coated with copper for surface conductivity or with more advanced coatings such as sapphire. Mineral fillers can be used to provide higher modulus and to the control coefficient of thermal expansion in relation to particular coatings or specific applications. 

Apostolescu, D. E., Gaal, P S., and Chapman, A. S., A Proposed High Temperature Thermal Expansion Reference Material, Standard Reference Material, pp. 637-646. [Sapphire]... 

Taylor, C. T., Notcutt, M., Wong, E. K., Mann, A. G., and Blair, D. G., Measurement of the Thermal Expansion Coefficient of an All-Sapphire Optical Cavity, IEEE Trans. Instrum. Meas. 46, 183, 1977. [Sapphire]... 

---