Ion-cut Process Research Articles

(Invited) Ion-Cut from Smart to Smarter: Ulrich Gösele's Impact on Science and Technology of Ultrathin Layer Transfer

Ulrich Gösele made numerous contributions to the science and technology of semiconductor low dimensional systems, heterostructures and nanoscale structures. This paper will be based upon the personal experience of the author who worked directly with Ulrich Gösele on several fundamental and engineering subjects. In particular, the impact of Ulrich Gösele on the basic understanding and applications of the ion-cut process will be addressed.

Thermal Behavior of the Mechanical Properties of GaN throughout Hydrogen-Induced Thin Layer Transfer

We report on the thermoevolution of the mechanical properties of H-implanted bulk GaN under the ion-cut process conditions. The mechanical properties were measured using a nanoindenter equipped with a continuous stiffness measurement attachment. The data were taken in depth control mode. Based on nanoindentation analysis, we obtained a modulus of 321{plus minus}7.3GPa and a hardness of 29.6{plus minus}1GPa for the as-implanted GaN samples. After implantation, the modulus and hardness values were found to be 321{plus minus}25GPa and 29.6{plus minus}3GPa, respectively. The influence of thermal treatment on the elasto-mechanical properties of H-implanted bulk GaN is investigated and its role in ultrathin layer splitting is discussed.

Heterogeneous Integration of Compound Semiconductors

The ability to tailor compound semiconductors and to integrate them onto foreign substrates can lead to superior or novel functionalities with a potential impact on various areas in electronics, optoelectronics, spintronics, biosensing, and photovoltaics. This review provides a brief description of different approaches to achieve this heterogeneous integration, with an emphasis on the ion-cut process, also known commercially as the Smart-Cut™ process. This process combines semiconductor wafer bonding and undercutting using defect engineering by light ion implantation. Bulk-quality heterostructures frequently unattainable by direct epitaxial growth can be produced, provided that a list of technical criteria is fulfilled, thus offering an additional degree of freedom in the design and fabrication of heterogeneous and flexible devices. Ion cutting is a generic process that can be employed to split and transfer fine monocrystalline layers from various crystals. Materials and engineering issues as well as our current understanding of the underlying physics involved in its application to cleaving thin layers from freestanding GaN, InP, and GaAs wafers are presented.

On artefacts in the secondary ion mass spectrometry profiling of high fluence H+ implants in GaAs

Abstract High fluence (>1017 H/cm2) ion implantation of H in GaAs is suitable for the ion cut process, and produces H bubbles under the surface which may cause blistering. By comparing the destructive depth profiling of these implants by secondary ion mass spectrometry (SIMS) with non-destructive profiling by elastic recoil detection analysis (ERD), we demonstrate that SIMS underestimates total H content by up to a factor of 2 due to undetected H escaping from bubbles during analysis. We also show that the depth of the maximum H concentration from SIMS can be in error by 20% due to large variations in the sputter rate through the profile.

Bulk GaN Ion Cleaving

Bulk or freestanding GaN is a key material in various devices other than the blue laser diodes. However, the high cost of bulk GaN wafers severely limits the large scale exploitation of these potential technologies. In this paper, we discuss some engineering issues involved in the application of the ion-cut process to split a thin layer from 2-inch freestanding GaN. This process combines the implantation of light ions and wafer bonding and can possibly be used to reduce the cost of the fabrication of GaN-based devices by allowing the transfer of several bulk quality thin layers from the same donor wafer. To achieve this multi-layer transfer several conditions must be fulfilled. Here issues related to bulk GaN surface irregularities and wafer bowing are discussed. We also describe a method to circumvent most of these problems and achieve high quality bonding.

HF/H[sub 2]O[sub 2] Etching for Removal of Damage Layer on As-Transferred Si Layer Formed by Ion-Cut Process

When a 100 nm thick Si layer was transferred onto a bare Si wafer by the hydrogen-induced-layer-transfer process, a spongy damage layer with microvoids was formed on the transferred layer because of hydrogen blistering. The surface-to-volume ratio of the damage layer was greater than that of the layer where blistering did not occur. Therefore, the damage layer was selectively etched by an HF/H 2 O 2 mixture and completely removed (the etching rates of Si at 70°C for the bulk layer and damage layer were 0.45 and 13.8 nm/min, respectively). Consequently, a smooth, damage-free Si layer (root-mean-square roughness = 2.74 nm) was obtained.

Blistering kinetics of GaN by hydrogen implantation at high temperature

The hydrogen ion implantation condition for the ion-cut process in wurtzite-phase GaN and the associated mechanisms of surface blistering of GaN films were investigated. The hydrogen ions were implanted at 40 keV with fluences in the range of (1.5–5) × 10 17 H +/cm 2 over a wide temperature range (RT-450 °C), and at 60 keV at room temperature with fluences in the range of (3–5) × 10 17 H +/cm 2. The influences of the ion fluence, implant temperature and the post-implantation annealing conditions on blistering process were studied. Optical microscopy, field emission scanning electron microscopy (FE-SEM), high resolution X-ray diffraction (HRXRD), Rutherford backscattering spectrometry/channeling (RBS/C), and cross-sectional transmission electron microscopy (XTEM) were used to investigate splitting kinetics, and the optimum conditions for achieving blistering only after post-implantation annealing were determined for the GaN ion-cut process. The optimum fluence for the GaN blistering by hydrogen implantation and subsequent low temperature annealing (250–350 °C) was in the (2–3) × 10 17 H +/cm 2 range within the implantation temperature window of 150–300 °C with activation energies in the range of 1.0–1.6 eV depending on the implanted ion fluence and implant temperature.

InP Layer Transfer with Masked Implantation

InP layer transfer with masked implantation was investigated to eliminate ion-implantation induced damage involved in the ion-cut process. InP donor wafers were selectively implanted with hydrogen through a mask at a dose of 8.5 X 10 16 ions/cm 2 at 160 keV. The layers which were subsequently mechanically exfoliated were characterized by large pyramidal protrusions on the surface, associated with the unimplanted regions. This undesirable morphology was bypassed through the inclusion of a selective etch-stop layer. The resulting structures possessed flat surfaces suitable for further bonding. This process enables the transfer of finished devices, unaffected by ion-implantation, onto a variety of desirable substrates.

High performance germanium photodetectors integrated on submicron silicon waveguides by low temperature wafer bonding

We demonstrate germanium photodetectors integrated on submicron silicon waveguides fabricated with a low temperature ( <or=400 degrees C) wafer bonding and ion-cut process. The devices shows a low dark current of approximately 100 nA, a fiber accessed responsivity of > 0.4 A/W and an estimated quantum efficiency of above 90%.

Characterization of bonded silicon-on-aluminum-nitride wafers with RBS, TEM and HRXRD techniques

Bonded silicon-on-insulator (SOI) materials via the Smart-cut TM (or ion-cut) process attract increasing interests due to their great capability and versatility in fabricating novel virtual substrates. Previously we reported the successful fabrication of silicon-on-aluminum-nitride (SOAN) structure with smart-cut process [C.L. Men, Z. Xu, Z. An, X.Y. Xie, M. Zhang, C.L. Lin, Physica B 324 (2002) 229]. In this work, we characterize the bonded SOAN structure with Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM) together with Kikuchi line diffraction patterns, and high resolution X-ray diffraction (HRXRD) techniques. It is found that, after smart-cut process, the top Si layer preserves a rather good single crystalline quality, while strain is induced in top Si and can be altered in the annealing step. The rotational misalignment during bonding step can be evaluated by Kikuchi line patterns. Our discussions here may be common for all bonded virtual substrates via smart-cut technology.

Effects of hydrogen implantation temperature on InP surface blistering

We have investigated the effects of hydrogen implantation temperature on the ion-cut process of InP by examining the correlation between surface blistering and the ion induced damage, hydrogen distribution, and strain. Using Rutherford backscattering spectrometry, elastic recoil detection, and x-ray diffraction, it was found that both the point defects induced by the hydrogen implantation and the in-plane compressive stress were necessary for hydrogen trapping and H-platelet nucleation and growth. The control of implantation temperature is crucial for creating sufficient defects and strain to induce surface blistering or layer exfoliation.

High crystalline-quality III-V layer transfer onto Si substrate

In this study, an approach combining ion cutting and selective chemical etch for the transfer of high crystalline-quality III-V layers on SiO2∕Si substrate has been investigated. This layer transfer scheme takes advantage of the ion-cutting process by conserving III-V substrates for reuse and simultaneously improving the transferred layer quality and surface condition without using chemical and mechanical polishing. The relocation of the ion-implantation damage maximum enables the transfer of relatively defect-free InP-based layers onto a Si substrate coated with an oxide layer and results in structures ready for further optoelectronic device fabrication or epitaxial growth.

Proton Implantation Mechanism in GaN Layer Transfer by Using the Ion-Cut Process

The proton implantation condition for the ion-cut process in wurtzite-phase GaN and the associated mechanisms of surface blistering of GaN films were investigated. Etch-pit density (EPD) measurements, photoluminescence (PL) spectroscopy, and high resolution X-ray diraction (HRXRD) were used to investigate the crystalline quality of the as-grown GaN epi-wafers. The proton beam was implanted at 60 keV with fluences in the range of 3 5 ◊ 10 17 H + /cm 2 at room temperature. The influences of the crystallinity of the GaN wafers, the ion fluence and the post-implantation annealing conditions (200 450 C) on the blistering process were studied. Optical microscopy, field emission scanning electron microscopy (FE-SEM), HRXRD, Rutherford backscattering Spectrometry (RBS)/channeling (RBS/C), and cross-sectional transmission electron microscopy (XTEM) were used to investigate splitting kinetics, and the optimum conditions for achieving exfoliation only after post-implantation annealing were determined for the GaN ion-cut process. Our results suggest that the ion-cut process is sensitive to both the implant fluence and the annealing conditions, however, so far as the measured crystal quality is concerned, against our expectations, there was no notable influence on the blistering property. The optimum post-implantation annealing temperature was less than 350 C, and low temperature splitting is of importance for layer transfer between dissimilar materials with very dierent thermal expansion coecients.

Fabrication of InP/SiO2/Si Substrate using Ion-Cutting Process and Selective Chemical Etching

In this study, an InP layer was transferred onto a Si substrate coated with a thermal oxide, through a process combining ion-cutting process and selective chemical etching. Compared with conventional ion-cutting of bulk InP wafers, this layer transfer scheme not only takes advantage of ion- cutting by saving the remaining substrates for reuse, but also takes advantage of selective etching to improve the transferred surface conditions without using the chemical and mechanical polishing. An InP/InGaAs/InP heterostructure initially grown by MOCVD was implanted with H+ ions. The implanted heterostructure was bonded to a Si wafer coated with a thermal SiO2 layer. Upon subsequent annealing, the bonded structure exfoliated at the depth around the hydrogen projected range located in the InP substrate. Atomic force microscopy showed that after selective chemical etchings on the as-transferred structure, a final structure of InP/SiO2/Si was obtained with a relatively smooth surface.

Double-flip transfer of indium phosphide layers via adhesive wafer bonding and ion-cutting process

A twice-transferred layer of indium phosphide (InP) onto a host substrate by means of ion cutting and adhesive wafer bonding was demonstrated to be suitable for device integration. A thin layer was first transferred onto a glass handle through the ion-cutting process by hydrogen implantation and adhesive bonding. The InP/adhesive/glass structure was then bonded to a host substrate using benzocyclobutene, followed by thermal separation of the temporary bond. The surface of the resulting InP layer is that of the original substrate, demonstrating the feasibility of prefabricated device layer transfer without etching sacrificial donors. The electrical properties of the transferred layer are also presented.

Microwave Activation of Exfoliation in Ion–cut Silicon Layer Transfer

AbstractMicrowave heating is used to initiate the ion-cut process for transfer of coherent silicon-layers onto insulator substrates. Hydrogen and boron co-implanted silicon was bonded to an insulative substrate before processing inside a 2.45 GHz, 1300 W cavity applicator microwave system. Sample temperatures measured using a pyrometer were comparable to previous ion – cut studies. Selected samples were further annealed to repair any damage created in the ion implant process. Rutherford backscattering spectrometry and selective area electron diffraction patterns show high crystallinity in transferred layers. RUMP simulation of backscattering spectra and cross-sectional transmission electron microscopy demonstrate that thicknesses of the transferred layers are comparable to previous ion-cut exfoliation techniques. Surface quality as characterized by an atomic force microscope compares well with previous ion-cut studies. Hall measurements were used to characterize electrical properties of transferred layers. The mobility and carrier density of microwave activated ion – cut silicon on insulator processed samples compares well with previous annealing techniques.

Characterising ion-cut in GaAs by Rutherford backscattering spectroscopy

The influence of ion species on the ion-cut, or smart-cut© process in GaAs has been investigated in this paper. Semi-insulating GaAs, implanted with H2+, He+ and Ne+, were studied by Rutherford backscattering spectroscopy (RBS) in channelling mode, to investigate the lattice disorder produced by implanting different ion species. The RBS spectra show that surface dechannelling is enhanced where blisters are present. The samples were also studied by Nomarski microscopy and atomic force microscopy to investigate the effect of the ion species on the blistering process. It was found that the ion-cut process can be achieved at lower doses by using the implantation of He alone rather than H. Our results indicate that the implantation of Ne is not a suitable alternative to H or He implantation.

Hydrogen/Deuterium-defect complexes involved in the ion cutting of Si (0 0 1) at the sub-100 nm scale

We studied the thermal evolution of H/D-stabilized defects aiming at understanding the microscopic mechanisms leading to crystalline silicon surface blistering and exfoliation. The critical hydrogen-defect complexes involved in this phenomenon have been identified by Raman spectroscopy analysis. A plausible mechanism for the ion-cut process emerging from our analyses is proposed. Surprising but instructive effects observed under isotope substitution are also discussed.

Microwave-cut silicon layer transfer

Microwave heating is used to initiate exfoliation of silicon layers in conjunction with the ion-cut process for transfer of silicon layers onto insulator or heterogeneous layered substrates. Samples were processed inside a 2.45 GHz, 1300 W cavity applicator microwave system for time durations as low as 12 s. This is a significant decrease in exfoliation incubation times. Sample temperatures measured by pyrometry were within previous published ranges. Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy were used to determine layer thickness and crystallinity. Surface quality was measured by using atomic force microscopy. Hall measurements were used to characterize electrical properties as a function of postcut anneal time and temperature.

Silicon layer transfer using plasma hydrogenation

In this work, we demonstrate a novel approach for the transfer of Si layers onto handle wafers, induced by plasma hydrogenation. In the conventional ion-cut process, hydrogen ion implantation is used to initiate layer delamination at a desired depth, which leads to ion damage in the transferred layer. In this study, we investigated the use of plasma hydrogenation to achieve high-quality layer transfer. To place hydrogen atoms introduced during plasma hydrogenation at a specific depth, a uniform trapping layer for H atoms must be prepared in the substrate before hydrogenation. The hydrogenated Si wafer was then bonded to another Si wafer coated with a thermal oxide, followed by thermal annealing to induce Si layer transfer. Cross-section transmission electron microscopy showed that the transferred Si layer was relatively free of lattice damage. The H trapping during plasma hydrogenation, and the subsequent layer delamination mechanism, are discussed. These results show direct evidence of the feasibility of using plasma hydrogenation to transfer relatively defect-free Si layers.