Substrate Mounting Research Articles

Optical in-situ temperature management for high-quality ZnO molecular beam epitaxy

Temperature management of the substrate is of great importance for molecular beam epitaxy as well as for other thin film deposition techniques. In this work, we compare different in-situ methods for simultaneous measurement of substrate temperature and growth rate for wide band semiconductors such as ZnO and GaN. The different analysis techniques investigated are emissivity-corrected pyrometry as well as band gap thermometry and reflectivity-based growth-rate determination. Combining in-situ and ex-situ measurements with simulation data, we demonstrate characteristic behaviors of common substrate mounting techniques for Al2O3 and ZnO substrates. Moreover, we investigate the performance of reflectometry-based growth rate determination during ZnO homoepitaxy.

Flight Qualification and Circuit Development of Sensor Front-End Electronics for PACS/Hershel at Liquid Helium Temperature

In the framework of the development of the European Space Agency's Herschel Space Observatory (HSO), IMEC designed the cold-readout electronics (CRE) for the PACS instrument. Key specifications for this circuit were high linearity, low power consumption, high uniformity, and very low noise at an operating temperature of 4.2K (liquid helium temperature, LHT). To ensure high production yields and uniformity, relatively easy availability of the technology, and portability of the design, the circuit was implemented in a standard CMOS technology. The circuits are functional at room temperature, which allows screening prior to integration and qualification and has an important impact on the production yield and time. The circuit was mounted on an Al2O3 substrate for optimum electrical performance. On the same substrate, bias signal generation, short-circuit protection circuitry, and decoupling capacitors for the power lines were integrated. This led to a relatively complex substrate containing over 30 passives and one die, integrated by means of conducting and nonconducting glue and nearly 80 wire bonds. Because the detector arrays will be cooled down to 4.2K prior to launch, reliability and launch-survivability of the mounted substrate at this temperature and in this harsh environment had to be demonstrated. For this purpose, the quality and associated reliability of every assembly step is verified during substrate mounting. This included verification of the compatibility of the bond material, optimization of the bond yield, and temperature cycling (between room temperature and LHT) of the devices. Other tests on qualification models would focus on circuit functionality under proton and gamma irradiation, cryogenic vibration tests to demonstrate launch survivability, and exhaustive temperature cycling to qualify the assembly procedure. We present in this paper the complete integration and qualification of the developed circuits, including assembly and verification during production of the flight models and qualification of the assembly method on the qualification models.

MBE growth and device processing of MWIR HgCdTe on large area Si substrates

The traditional substrate of choice for HgCdTe material growth has been lattice matched bulk CdZnTe material. However, as larger array sizes are required for future devices, it is evident that current size limitations of bulk substrates will become an issue and therefore large area Si substrates will become a requirement for HgCdTe growth in order to maintain the cost-efficiency of future systems. As a result, traditional substrate mounting methods that use chemical compounds to adhere the substrate to the substrate holder may pose significant technical challenges to the growth and fabrication of HgCdTe on large area Si substrates. For these reasons, non-contact (indium-free) substrate mounting was used to grow mid-wave infrared (MWIR) HgCdTe material on 3″ CdTe/Si substrates. In order to maintain a constant tepilayer temperature during HgCdTe nucleation, reflection high-energy electron diffraction (RHEED) was implemented to develop a substrate temperature ramping profile for HgCdTe nucleation. The layers were characterized ex-situ using Fourier transform infrared (FTIR) and etch pit density measurements to determine structural characteristics. Dislocation densities typically measured in the 9 106 cm−2 to 1 107 cm−2 range and showed a strong correlation between ramping profile and Cd composition, indicating the uniqueness of the ramping profiles. Hall and photoconductive decay measurements were used to characterize the electrical properties of the layers. Additionally, both single element and 32 32 photovoltaic devices were fabricated from these layers. A RA value of 1.8 106-cm2 measured at −40 mV was obtained for MWIR material, which is comparable to HgCdTe grown on bulk CdZnTe substrates.

A discussion of modelling idealised ablative materials with particular reference to fire testing

Abstract An investigation is made of a mathematical model of an idealised ablative material of finite thickness, which is mounted horizontally and exposed to a constant, uniform heat flux. The effect of a substrate or sample mounting is also investigated. Numerical and approximate solutions are developed for a perfectly conducting and a perfectly insulating substrate. These two extreme cases represent idealised upper and lower bounds for the mass loss rate of an ablative material mounted on a real substrate. Three ablation regimes (thermally thick, thermally non-thin and thermally thin) are identified and quantified in terms of the dimensionless parameter l l c , where l is the initial thickness of the sample and lc is a critical thickness defined in terms of the difference in ablation temperature from ambient temperature, incident heat flux and thermal conductivity. The approximate solutions are valid for the thermally thin and non-thin regimes. It is observed in the numerical solutions that the substrate or sample holder has negligible effect on the mass loss rate in the thermally thick regime. However, the sample holder may have a large effect on the mass loss rate for the thermally thin and non-thin regimes.

Improved substrate temperature stability during molecular beam epitaxy growth using indium free mounting of small substrates of various shapes

Large substrate surface temperature decreases are observed during molecular beam epitaxy growth onto small indium bonded substrates, due to coating of the molybdenum block. These large temperature transients, along with other difficulties associated with indium bonding (e.g., potential substrate surface damage, unwanted indium on the back of the substrate) make indium free substrate mounting desirable, however, indium free mounting systems have previously been restricted to whole standard‐sized round wafers. In this study we compare the temperature characteristics of new indium free modular substrate mounting blocks, which can accommodate substrates of various sizes (up to 3 inch diameter) and shapes, with traditional indium mounting of small substrates. Although the new modular indium free holders have a large molybdenum surface area exposed to the molecular beams, we find that the substrate surface temperature transients during growth are reduced to nearly negligible levels versus similar growth on indium bonded substrates, due to the effective thermal isolation of the substrate from the molybdenum. The real effects of these temperature differences observed by in situ pyrometry were confirmed by ex situ deep level transient spectroscopy (DLTS) measurements of homoepitaxial GaAs films, which show a two‐to‐three order of magnitude trap density increase in films grown on indium bonded substrates versus films grown on substrates mounted without indium bonding. This DLTS result is consistent with a real growth temperature difference between the two mounting techniques as was measured in situ by pyrometry.

Wafer-scale temperature mapping for molecular beam epitaxy and chemical beam epitaxy

A technique for the measurement of the variation in surface temperature across GaAs and InP wafers under normal molecular beam epitaxy and chemical beam epitaxy growth conditions is described. It is based on measuring heat treatment induced changes in the thickness of an epitaxial layer. The thickness map of a given slice, generated by scanning reflectance spectrometry, is compared with that obtained after a subsequent heat treatment in the deposition system. The thickness difference map obtained by subtracting the two data files is a measure of the evaporative loss of material due to the additional heat treatment. Since the rate of loss of material is exponentially dependent on the surface temperature (activation energy ∼3.7 eV), small temperature variations (<0.2 °C) can be readily measured. The high spatial resolution (<200 μm point-to-point), the whole wafer mapping capability and the use of a differential technique to eliminate thickness variations due to nonthermal causes, enables the effects of substrate mounting and back surface preparation to be clearly discerned.

Analysis of novel wideband ridge-loaded finline including finite metallisation thickness and substrate mounting grooves

A full wave analysis of a wideband ridge-loaded finline including finite metallisation thickness and substrate mounting grooves has been carried out for the first time. The method is based on the spectral analysis and boundary integration approach (SAABIA). The dispersion characteristics are presented for some practical wideband ridge-loaded finline structures (RFSs). The results show that the singlemode bandwidth of RFSs may be extended by adjusting the dimensions of the ridges

MBE as a production technology for HEMT LSIs

Molecular beam epitaxy (MBE) is widely used to produce high quality epitaxial layers for advanced heterostructure devices. High electron mobility transistors (HEMTs) based on selectively-doped AlGaAs/GaAs heterojunction structures are one of the most successful applications of MBE. This paper reviews MBE techniques developed in our laboratories for HEMT LSI fabrication. To meet material requirements for submicron-gate AlGaAs/GaAs HEMT LSIs, we designed a multiwafer MBE system that enables good uniformity and high throughput. Epitaxial growth is done simultaneously on three 3-inch wafers mounted in a 7.5-inch holder. The variations of layer thickness and carrier concentration over the holder area were less than ±1%. One-touch substrate mounting without indium solder was developed for the t.5-inch holder. In-situ monitoring of growth rates by RHEED instensity oscillation with an automated growth system was introduced to control epitaxial parameters precisely. The density of oval defects due to Ga sources was stably controlled to less than 10 cm -2 and carrier depletion at the substrate-epitaxial layer interface was significantly reduced. These excellent material characteristics make it possible to develop high-performance HEMT logic and memory LSI circuits.

Finlines in rectangular and circular waveguide housings including substrate mounting and bending effects-finite element analysis

The finite-element method is used to derive the dispersion characteristics and field components of dominant and higher order modes in finlines. The method is accurate and covers the metallization thickness, substrate mounting grooves, bending of the substrate, and arbitrary cross sections. Results for structures already obtained with other methods have been found to agree well with available data. As a new contribution, the effect of substrate bending on the propagation constant is studied. The dispersion characteristics for the dominant and higher order modes for bilateral finlines in circular waveguide housing are calculated. The field plots for all the modes are given. >

Theoretical and experimental characterization of nonsymmetrically shielded coplanar waveguides for millimeter-wave circuits

A quasi-planar structure, the nonsymmetrically shielded coplanar waveguide (NSCPW), is proposed as a quasi-TEM transmission line with advantageous characteristics for millimeter-wave circuit applications. Advantages in terms of broadband behavior and ease of machining, as well as device mounting and substrate mounting, are pointed out. An experimental method is developed which allows the evaluation of the transmission line spectrum in a very wide frequency band (15:1) with a single transmission measurement. The experimentally evaluated propagation characteristics of the dominant and higher order modes are shown to be in excellent agreement with the theoretical predictions based on the generalized transverse resonance technique. This method is also used for an extensive characterization of the structure in terms of characteristic impedance and useful frequency band.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A comparison of the step coverage of aluminum coatings produced by two sputter magnetron systems and a dual-beam ion system

We metallized features 0.6 μm deep on silicon integrated circuits using three different deposition systems to examine step coverage problems. These problems can be mitigated by changing the substrate mounting geometry, and/or by bombarding the coating with argon during deposition. The effects of changing the energy and the current density of the bombarding argon flux were investigated using a dual-beam ion system. Our results show that the best coatings from the standpoint of step coverage and electrical resistivity occur for extremely low energy (70 eV) argon bombardment. Porous irregular coatings with high resistivities result from argon bombardment at higher energies and higher current densities. These results are comparable with the results achieved using a d.c. magnetron system, where a substrate bias on the order of -200 to -250 V is necessary to obtain a similar degree of coating planarization.

A Unified Hybrid-Mode Analysis for Planar Transmission Lines with Multilayer Isotropic/Anisotropic Substrates

A unified hybrid-mode analysis is presented for determining the propagation characteristics of multiconductor, multilayer planar transmission lines. The analysis employs the conservation of complex power technique, and the emphasis is on numerical efficiency and simplicity. Numerical results, for finline and microstrip configurations, aim at the clarification of the effects of the metalization thickness, dielectric anisotropy, and substrate mounting grooves.

Molecular Beam Epitaxial Growth Over Multiple Wafers Using an Indium-Free Mounting Technique

ABSTRACTAn indium-free substrate mounting technique was developed for a 190-mm diameter substrate holder on which three 3-inch GaAs substrates are mounted. Reduction of the heat conduction between the substrates and the holder kept the temperature variation in the substrate within ±5° C. Highly uniform epitaxial layers were grown with low residual impurity concentrations using this technique. The variations in layer thickness and carrier concentration of Si-doped GaAs and AlGaAs epitaxial layers were within ±1% over the substrate holder. High mobility of a two-dimensional electron gas in a selectively doped GaAs/N-AlGaAs heterostructure was obtained uniformly over three 3-inch wafers. Since the holder was designed to hold the wafers without stress, no stress-related degradation of the surface morphology was observed.

An indium-free mount for GaAs substrate heating during molecular beam epitaxial growth

The substrate mounting system for a Varian GEN/II molecular beam epitaxy (MBE) system was modified to allow the growth of device quality GaAs and AlGaAs epitaxial layers on GaAs substrates without the use of indium bonding. The indium-free holders are compatible and interchangeable with the standard molybdenum blocks that use indium bonding. The wafer is heated by direct radiation from the unmodified spiral filaments with no intermediate absorbers or backside wafer coatings. A commercial pyrometer with a silicon detector was used to measure the absolute temperature. The silicon detector is sensitive to radiation from 0.8 to 1.1 μm, which is primarily above the band gap energy of GaAs at substrate temperatures above 550 °C. For photon energies above the band gap the spectral emittance of GaAs is nearly independent of the substrate doping, thickness, and temperature. With this pyrometer a maximum temperature difference of 3 °C was measured over the central two-thirds of a 2 in. diam semi-insulating GaAs wafer. In contrast, the maximum temperature difference on n+GaAs substrates was significantly larger (30 °C over the central two-thirds of the wafer) and showed the same pattern as the filament configuration. This large difference is explained by a simple model which takes into account the smaller thermal conductivity and larger total emittance of n+GaAs compared to undoped GaAs.

Hybrid tantalum chip capacitors. A new encapsulated tantalum chip capacitor designed especially for direct microcircuit substrate mounting : S. A. Bell and W. J. Hyland. Proceedings ISHM Microelectronics Symposium, October–November (1972), p. 4-A-1-1

Hybrid tantalum chip capacitors. A new encapsulated tantalum chip capacitor designed especially for direct microcircuit substrate mounting : S. A. Bell and W. J. Hyland. Proceedings ISHM Microelectronics Symposium, October–November (1972), p. 4-A-1-1

Fabrication of a Lithium Tantalate Temperature-Stabilized Optical Modulator

LiTaO(3) optical modulators have been developed using the technique of self-compensation for therma stability. By using two crystals in cascade, with optic axes 180 degrees apart, and a half-wave plate between them with axis at 45 degrees , the birefringence of each crystal is equal but opposite and there is no net change in birefringence with temperature. Aside from thermal stability, the optical modulator described damps out mechanical resonances caused by the piezoelectric properties of the crystals, and its thin-film substrate mounting is applicable to wideband modulation. Experimental measurements show that 95% modulation depth can be attained over a 25 degrees C temperature range with virtually no change in the amplitude or phase of the modulated signal.