Donato Pasquariello

Plasma-assisted InP-to-Si low temperature wafer bonding

The applicability of wafer bonding as a tool to integrate the dissimilar material system InP-to-Si is presented and discussed with recent examples of InP-based optoelectronic devices on Si. From there, the lowering of annealing temperature in wafer bonding by plasma-assisted bonding is the essence of this review paper. Lower annealing temperatures would further launch wafer bonding as a competitive technology and enable a wider use of it. Oxygen plasma treatment has been proven to be very feasible in achieving a strong bonding already at low temperatures. It was also seen that in our experimental setups the results depended on what plasma parameters that were used, since different plasma parameters create different surface conditions.

Oxidation and Induced Damage in Oxygen Plasma In Situ Wafer Bonding

In this paper we present our in situ, oxygen plasma‐activated wafer bonding process. By keeping one wafer on the anode and the other on the cathode, we have an asymmetric plasma load on the wafers, making our bonding process interesting for low‐temperature applications where damage or defect‐sensitive active layers are bonded to less sensitive carrier wafers. As a step in optimizing the discharge parameters for plasma bonding applications, the effect of the self‐bias voltage on surface energy, oxidation rates, and damage is investigated. An optimum in surface energy was found at moderate self‐bias voltages, both at room temperature bonding and after low‐temperature annealing at 200°C. This is explained by the fact that at these voltages there is a minimum oxide thickness, which promotes the diffusion of water from the bond interface, and also by the fact that at these voltages we have the best surface cleaning conditions. Also, the surface oxide generated by the oxygen plasma seems to be reactive. With our in situ oxygen‐plasma‐activated wafer bonding process there was a major increase in surface energy for wafers bonded at moderate self‐bias volt‐ages compared to conventional wafer bonding performed in ambient air.

Surface energy as a function of self-bias voltage in oxygen plasma wafer bonding

Abstract A limitation in the use of wafer bonding has been the necessity for high-temperature annealing after contacting the wafers at room temperature. In this paper, we try to find the highest surface energy as a function of self-bias voltage in oxygen plasma-activated wafer bonding, in order to achieve a low-temperature bonding process. The bonding was performed in situ the vacuum chamber. It was found that oxygen plasma has a smoothing effect on the surface roughness, rather independent of the plasma self-bias. However, a moderate self-bias voltage proved to give the highest surface energy for the bonded wafers, both at room-temperature and after annealing at 200°C. We believe that this is due to the fact that a moderate self-bias is the most efficient in removing surface contaminants, like water and hydrocarbons. It was also found that even after annealing at higher temperatures, 480°C and 720°C, the plasma-bonded wafers showed higher surface energy values than wafers bonded in ambient air. This investigation was focused on low-effect plasmas,

Mesa‐Spacers: Enabling Nondestructive Measurement of Surface Energy in Room Temperature Wafer Bonding

In this paper, a way of measuring the surface energy of room-temperature wafer bonding is presented. The method consists of introducing well-defined defects, or spacers, at the bond interface. Here SiO 2 mesas were used as spacers. From the resulting non-bonded area, around the spacers, the value of the surface energy is obtained. Results from surface energy measurements show that there is good agreement between the razor-blade method and our mesa-spacer method. By using the mesa-spacer method, several of the problems associated with the razor-blade method are avoided. The method also allows the nondestructive measurement of the surface energy, which is interesting for applications in manufacturing of microsystems.

Evaluation of InP-to-silicon heterobonding

Abstract In this paper, we evaluate hydrophilic (HP) and hydrophobic (HB) surface pre-treatments in InP-to-Si direct wafer bonding. Surface roughness and surface chemistry was examined using atomic force microscopy and X-ray photoelectron spectroscopy, respectively. After bonding, the bonded interfaces were evaluated using infrared transmission imaging, bond-strength and current–voltage (I–V) measurements. It was found that HP surface treatment using oxygen plasma makes room temperature bonding of InP and Si very spontaneous, and results in high bond-strengths already after low-temperature annealing. This was not observed when using standard oxidising acids as HP surface treatment before bonding. HB InP and Si surfaces, also, did not prove to bond spontaneously at room temperature and the bond-strength started to increase only after annealing at about 400°C. HB bonding and annealing at 400°C was though the best choice regarding the electrical characteristics of the bonded InP/Si heterojunction.

High-speed 7× CuSi-based write-once blu-ray disc

A Cu/Si bilayer can be applied as the recording medium in a write-once Blu-ray Disc (BD-R). Recording experiments performed on BD-R stacks showed that a 1/1 ratio of Cu/Si layers prove to have the best recording results. Auger analysis points to a diffusion-driven process, primarily that of Si into Cu. By optimization of the stack, we have been able to record 1–7× BD-R 25 GB discs with bottom jitters below 6.5% using a castle-type write strategy.

Selective undercut etching of InGaAs and InGaAsP quantum wells for improved performance of long-wavelength optoelectronic devices

In this paper, the authors show how selective undercut etching of InGaAs and InGaAsP-based quantum wells (QWs) can improve the performance of InP-based optoelectronic devices. First, wet-chemical-etching characteristics are investigated. Mixtures of sulphuric and hydrogen peroxide acids are used as wet-etching solutions, and properties such as etch rates, selectivity, and anisotropy are studied in detail. Problems arising from the anisotropic nature of the etching are analyzed, and their impact on device design and performance is discussed. Second, the authors present several optoelectronic devices where selective undercut etching of InGaAs- or InGaAsP-based multiquantum wells (MQWs) improves device performance; these devices include electroabsorption modulators (EAMs), vertical-cavity semiconductors optical amplifiers (VCSOAs), and waveguide amplifier photodetectors (WAPs). Very high extinction ratios were obtained for the EAM. A selective undercut-etched VCSOA reached a record-high 17-dB fiber-to-fiber gain, and the WAP demonstrated an external quantum efficiency higher than 100%

Crystalline Defects in InP-to-Silicon Direct Wafer Bonding

InP-to-Si wafer bonding has been proposed as a way of circumventing the problems associated with lattice-mismatch in heteroepitaxial growth. Therefore, in this study the dislocation density and material degradation in InP-to-Si hydrophobic bonding are evaluated. Both interface and InP bulk defects were studied using IR-transmission, atomic force microscopy (AFM) and defect-etching. When the bonded wafers were annealed below 300°C, no volume dislocations were generated in InP. However, when annealing above 300°C, the thermal mismatch stress induced large numbers of volume dislocations in InP. It was also shown that hydrophobic InP-to-Si wafer bonding unfortunately requires high-temperature annealing to achieve sufficient bonding-strength. However, a considerably lower dislocation density was observed in InP-to-Si wafer bonding than that in InP heteroepitaxial growth on Si. Also, when the samples were annealed above 400°C, asymmetric voids emerged at the interface. These voids are associated with the nucleation of indium droplets which causes microcavities at the interface where volume dislocations can sweep-out, forming surface steps. The voids completely disappeared when channel-patterned interfaces were used.

Traveling-wave photodetectors with high power-bandwidth and gain-bandwidth product performance

Traveling-wave photodetectors (TWPDs) are an attractive way to simultaneously maximize external quantum efficiency, electrical bandwidth, and maximum unsaturated output power. We review recent advances in TWPDs. Record high-peak output voltage together with ultrahigh-speed performance has been observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal TWPDs at the wavelengths of 800 and 1300 nm. An approach to simultaneously obtain high bandwidth and high external efficiency is a traveling-wave amplifier-photodetector (TAP detector) that combines gain and absorption in either a sequential or simultaneous traveling-wave structure.

High gain, high efficiency vertical-cavity semiconductor optical amplifiers

Highly efficient long wavelength vertical-cavity semiconductor optical amplifiers (VCSOAs) are presented. A carrier confining structure was introduced by etching mesas in the active region. The carrier confinement resulted in increased efficiency and amplifier gain. The efficiency was increased by a factor of 3 as compared to previous devices made from the same active region material. 17 dB fiber-to-fiber gain was measured and the internal gain was estimated to be 24 dB. This is the highest reported amplifier gain for a long wavelength VCSOA to date.