Corinne E. Packard

Hot nanoindentation in inert environments.

An instrument capable of performing nanoindentation at temperatures up to 500 degrees C in inert atmospheres, including partial vacuum and gas near atmospheric pressures, is described. Technical issues associated with the technique (such as drift and noise) and the instrument (such as tip erosion and radiative heating of the transducer) are identified and addressed. Based on these considerations, preferred operation conditions are identified for testing on various materials. As a proof-of-concept demonstration, the hardness and elastic modulus of three materials are measured: fused silica (nonoxidizing), aluminum, and copper (both oxidizing). In all cases, the properties match reasonably well with published data acquired by more conventional test methods.

Hardening of a metallic glass during cyclic loading in the elastic range

Although fatigue failure is well documented in metallic glasses, the mechanism responsible for damage accumulation during cyclic loading below the yield point remains elusive. This letter describes a high-resolution nanomechanical study of an Fe-based bulk metallic glass subjected to cyclic loading in the nominal elastic range. An increase in the yield load was observed with an increasing number of subyield loading cycles, providing a clean documentation of kinematic irreversibility in very small volumes of material that experience no shear bands either prior to or during cyclic loading.

Cyclic hardening of metallic glasses under Hertzian contacts: Experiments and STZ dynamics simulations

A combined program of experiments and simulations is used to study the problem of cyclic indentation loading on metallic glasses. The experiments use a spherical nanoindenter tip to study shear band formation in three glasses (two based on Pd and one on Fe), after subjecting the glass to cycles of load in the nominal elastic range. In all three glasses, such elastic cycles lead to significant increases in the load required to subsequently trigger the first shear band. This cyclic hardening occurs progressively over several cycles, but eventually saturates. The effect requires cycles of sufficient amplitude and is not induced by sustained loading alone. The simulations employed a new shear transformation zone (STZ) dynamics code to reveal the local STZ operations that occur beneath an indenter during cycling. These results reveal a plausible mechanism for the observed cyclic hardening: local regions of confined microplasticity can develop progressively over several cycles, without being detectable in the global load–displacement response. It is inferred that significant structural change must attend such microplasticity, leading to hardening of the glass.

Contact-Printed Microelectromechanical Systems

2010 WILEY-VCH Verlag Gmb Standard photolithography-based methods for fabricating microelectromechanical systems (MEMS) present several drawbacks including incompatibility with flexible substrates and limitations to wafer-sized device arrays. In addition, it is difficult to translate the favorable economic scaling seen in the capital equipment-intensive microelectronics industry to the manufacture of MEMS since additional specialized processes are required and wafer volume is comparatively small. Herein we describe a new method for rapid fabrication of metallic MEMS that breaks the paradigm of lithographic processing using an economically and dimensionally scalable, large-area microcontact printing method to define 3D electromechanical structures. This technique relies on an organic molecular release film to aid in the transfer of a metal membrane via kinetically controlled adhesion to a viscoelastic stamp. We demonstrate the fabrication of MEMS bridge structures and characterize their performance as variable capacitors. Flexible, paper-thin device arrays produced by this method may enable such applications as pressure sensing skins for aerodynamics, phased array detectors for acoustic imaging, and novel adaptive-texture display applications. The methods and tools used in the mature field of microelectronics fabrication have enabled fabrication of today’s MEMS structures with micrometer-scale features of submicrometer precision, using process sequences that can readily integrate MEMS with measurement and control circuits. However, together with the benefits of using the established processing technologies, MEMS fabricated within the existing silicon microelectronics-based framework also inherit the limitations of the present techniques including expensive per-chip processing costs of MEMS devices, limited maximum size and form-factor, and a materials set restricted to the conventional microelectronic materials. These standard processing techniques impede integration of MEMS technologies in applications that go beyond single chip or single sensor use and are particularly restrictive when one considers expanding the use of MEMS into large area or flexible substrate applications. No established market for large area MEMS has yet developed; however, promising applications include sensor skins for humans and vehicles, phased array pressure sensors, adaptive-texture surfaces, and incorporation of arrayed MEMS devices with other large area electronics. In such applications, compatibility of the MEMS technology with flexible substrates is highly desirable. If MEMS are fabricated directly on the flexible sheets, such as polymeric substrates, the elevated-temperature processing (as is typical for thermal growth of oxides and the deposition of polysilicon in conventional MEMS processing) must be avoided to prevent substrate damage. An alternative, low-temperature approach in which structures fabricated on silicon wafers are bonded to a flexible sheet and then released from the silicon by fracturing small supports or by etching a sacrificial layer, has been demonstrated for silicon electronics, but has not been applied to MEMS fabrication. The technological push to move to flexible, large-area applications while avoiding the drawbacks of conventional MEMS processing motivates development of new MEMS fabrication techniques which do not rely on photolithography or other solvent-processing, and can be performed at near room temperature, to avoid mechanical stresses and substrate damage. We demonstrate in this study a newMEMS fabrication technique using microcontact printing in atmospheric conditions to transfer continuous metal films over a relief structure, forming suspended metal membranes of sub-micrometer thickness that serve as mobile mechanical elements in capacitive MEMS devices. Our technology has the ability to form metallic MEMS structures without requiring elevated-temperature processing, high pressure, or wet chemical or aggressive plasma release etches. Simplicity and scalability of the demonstrated technique can create a paradigm shift in the design and fabrication of integrated MEMS devices. Compatibility of the technique with low temperature processing on flexible polymeric or metal foil substrates enables us to envision a complete method for rapid, near-room-temperature fabrication of flexible, large-area, integrated microor optoelectronic/MEMS circuits. TheMEMS structures are formed by the contact lift-off transfer (Contact-Transfer) technique, which enables us to pick up a thin metallic membrane from a donor transfer pad when the membrane is contacted by a viscoelastic stamp, such as polydimethylsiloxane (PDMS). The metallic membranes are first prepared by evaporating a thin metal film onto a donor transfer pad, which has been pre-coated with an organic molecular release layer prior to metal deposition. The surface of the PDMS stamp is placed in contact with the planar metallic membrane then rapidly peeled off, picking up the metal film (Fig. 1). During the rapid removal of the viscoelastic PDMS stamp, the weak adhesion energy to the metal is increased sufficiently to effect pick up, due to the kinetically controlled adhesion characteristic of elastomers. The PDMS stamp is molded with 20-mm-scale ridges using a siliconmaster grating, so that only some of the stamp area adheres to the metal film when the two are brought in contact. However, when the stamp and the donor pad are separated,

Development of a Visual Inspection Data Collection Tool for Evaluation of Fielded PV Module Condition

A visual inspection data collection tool for the evaluation of fielded photovoltaic (PV) modules has been developed to facilitate describing the condition of PV modules with regard to field performance. The proposed data collection tool consists of 14 sections, each documenting the appearance or properties of a part of the module. This report instructs on how to use the collection tool and defines each attribute to ensure reliable and valid data collection. This tool has been evaluated through the inspection of over 60 PV modules produced by more than 20 manufacturers and fielded at two different sites for varying periods of time. Aggregated data from such a single data collection tool has the potential to enable longitudinal studies of module condition over time, technology evolution, and field location for the enhancement of module reliability models.

Controlled exfoliation of (100) GaAs-based devices by spalling fracture

The importance of exfoliation techniques increases as the semiconductor industry progresses toward thinner devices as a way to reduce material costs and improve performance. The controlled spalling technique is a recently developed substrate removal process that utilizes the physics of fracture to create wafer cleavage parallel to the surface at a precise depth. In this letter, we apply principles of linear elastic fracture mechanics to predict the process conditions needed to exfoliate (100) GaAs of a desired thickness. Spalling can be initiated in a controllable manner, by depositing a stressor film of a residual stress value just below the threshold value to induce a spontaneous spall. Experimental data show process window requirements to controllably spall (100) GaAs. Additionally, experimental spall depth in (100) GaAs compares well to spalling mechanics predictions when the effects of wafer thickness and modulus are considered. To test spalled material quality, III-V single junction photovoltaic devices are lifted off of a (100)-GaAs substrate by spalling methods and electrical characteristics are recorded. No degradation is observed in the spalled device, illustrating the potential of this method to rapidly produce thin, high quality devices.

Electrically tunable organic vertical-cavity surface-emitting laser

An electrically tunable organic vertical-cavity surface-emitting laser (VCSEL) is demonstrated and characterized. A lasing wavelength tunability of Δλ = 10 nm with 6 V actuation is shown for a red laser emission tuned between λ = 637 nm and λ = 628 nm. Wavelength tuning of the VCSEL structure is enabled by electrostatic deflection of a reflective flexible membrane that is suspended over an air gap and a dielectric mirror, forming a 3λ lasing cavity. The lasing gain medium consists of an evaporated organic thin film coated on a reflective membrane, which is then additively placed over a patterned substrate containing the dielectric mirror to fabricate an array of air-gap-VCSEL structures, each 100 μm in diameter. Beyond the electrostatic actuation of these tunable lasers, the VCSEL array geometry also has the potential to be used as pressure sensors with an all-optical remote excitation and readout and a pressure sensitivity of 64 Pa/nm in the demonstrated configuration.

Development of GaInP Solar Cells Grown by Hydride Vapor Phase Epitaxy

We demonstrate the growth of homojunction GaInP solar cells by dynamic hydride vapor phase epitaxy for the first time. Simple unpassivated n-on-p structures grown in an inverted configuration with gold back reflectors were analyzed. Short wavelength performance varied strongly with emitter thickness, since collection in the emitter was limited by the lack of surface passivation. Collection in the base increased strongly with decreasing doping density, in the range 1 × 10¹⁶ − 5 × 10¹⁷ cm⁻³. Optical modeling indicated that, in our best device, doped ∼1 × 10¹⁶ cm⁻³, almost 94% of photons that passed through the emitter were collected. Modeling also indicated that the majority of collection occurs in the depletion region with this design, suggesting that nonradiative recombination there might limit device performance. In agreement with this observation, the experimental dark <italic>J–V</italic> curve exhibited an ideality factor near <italic>n</italic> = 2. Thus, limitation of deep level carrier traps in the material is a path to improved performance. Preliminary experiments indicate that a reduced V/III ratio, which potentially affects the density of these presumed traps, improves cell performance. With reduced V/III ratio, we demonstrate a ∼13% efficient GaInP cell measured under the 1-sun AM1.5G spectrum. This cell had an antireflective coating, but no front surface passivation.

Engineering controlled spalling in (100)-oriented GaAs for wafer reuse

Controlled spalling offers a way to cleave thin, single-crystal films or devices from wafers, particularly if the fracture planes in the material are oriented parallel to the wafer surface. Unfortunately, misalignment between the favored fracture planes and the wafer surface preferred for photovoltaic growth in (100)-oriented GaAs produces a highly faceted surface when subject to controlled spalling. This highly faceted cleavage surface is problematic in several ways: (1) it can result in large variations of spall depth due to unstable crack propagation; (2) it may introduce defects into the device zone or underlying substrate; and (3) it consumes many microns of material outside of the device zone. We present the ways in which we have engineered controlled spalling for (100)-oriented GaAs to minimize these effects. We expand the operational window for controlled spalling to avoid spontaneous spalling, find no evidence of dislocation activity in the spalled film or the parent wafer, and reduce facet height and facet height irregularity. Resolving these issues provides a viable path forward for reducing III-V device cost through the controlled spalling of (100)-oriented GaAs devices and subsequent wafer reuse when these processes are combined with a high-throughput growth method such as Hydride Vapor Phase Epitaxy.

A novel way to characterize Metal-Insulator-Metal devices via nanoindentation

Metal-Insulator-Metal (MIM) devices are crucial components for applications ranging from optical rectennas for harvesting sunlight to infrared detectors. To date, the relationship between materials properties and device performance in MIM devices is not fully understood, partly due to the difficulty in making and reproducing reliable devices. One configuration that is popular due to its simplicity and ease of fabrication is the point-contact diode where a metal tip serves as one of the metals in the MIM device. The intrinsic advantage of the point-contact configuration is that it is possible to achieve very small contact areas for the device thereby allowing very high-frequency operation. In this study, precise control over the contact area and penetration depth of an electrically conductive tip into a metal/insulator combination is achieved using a nanoindenter with in-situ electrical contact resistance measurement capabilities. A diamond probe tip, doped (degeneratively) with boron for conductivity, serves as the point contact and second ‘metal’ (b-Diamond) of the MIM diode. The base layer consists of Nb/Nb2O5 thin films on Si substrates and serves as the first metal /insulator combination of the MIM structure. The current-voltage response of the diodes is measured under a range of conditions to assess the validity and repeatability of the technique. Additionally, we compare the results of this technique to those acquired using a bent-wire approach and find that Nb/Nb2O5/b-Diamond MIM devices show an excellent asymmetry (60–300) and nonlinearity values (∼6–9. This technique shows great promise for screening metal-insulator combinations for performance without the uncertainty that stems from a typical bent-wire point-contact.