Ross Miller

Laser-Enabled Advanced Packaging of Ultrathin Bare Dice in Flexible Substrates

Embedding ultrathin semiconductor dice in flexible substrates provides unique capabilities for product designers and makes products such as smart bank cards and radio-frequency identification banknotes possible. Most of the current work in this area is directed toward handling, embedding, and interconnecting the ultrathin chips. Relatively little attention is paid to another critical process step-placing the flexible and very fragile ultrathin die onto the flexible substrate reliably and in a cost-efficient manner, suitable for high throughput assembly. The presented laser-enabled technology for embedding ultrathin dice in a flexible substrate was developed at the Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND, to address this problem. The technology has been successfully demonstrated and proven for the fabrication of an RFID tag.

Noncontact Selective Laser-Assisted Placement of Thinned Semiconductor Dice

New laser-induced forward transfer (LIFT) techniques promise to be a disruptive technology by enabling high-volume placement of ultrathin bare dice. Limitations of current die-attach techniques such as pick-and-place are presented and discussed which inspired the development of this new placement method. The thermo-mechanical selective laser-assisted die transfer (tmSLADT) process is introduced as an application of the unique blistering behavior of a dynamic releasing layer when irradiated by low-energy-focused UV laser pulses. The potential for tmSLADT to be the next generation LIFT technique is demonstrated by the “touchless” transfer of 65-μm-thick silicon tiles between two substrates spaced 195 μm apart. Additionally, the advantages of an enclosed blister actuator mechanism over previously studied ablative and thermal releasing techniques are discussed. Finally, experimental results indicate that this nonoptimized die transfer process compares with, and may exceed, the placement precision of current assembly techniques.

Bimodal wireless sensing with dual-channel wide bandgap heterostructure varactors

A capacitive wireless sensing scheme is developed that utilizes an AlN/GaN-based dual-channel varactor. The dual-channel heterostructure affords two capacitance plateaus within the capacitance-voltage (CV) characteristic, owing to the two parallel two-dimensional electron gases (2DEGs) located at respective AlN/GaN interfaces. The capacitance plateaus are leveraged for the definition of two resonant states of the sensor when implemented in an inductively-coupled resonant LRC network for wireless readout. The physics-based CV model is compared with published experimental results, which serve as a basis for the sensor embodiment. The bimodal resonant sensor is befitting for a broad application space ranging from gas, electrostatic, and piezoelectric sensors to biological and chemical detection.

High Mg content wurtzite phase MgxZn1-xO epitaxial film grown via pulsed-metal organic chemical vapor deposition (PMOCVD)

We report on high quality, wurtzite MgxZn1-xO (MgZnO) epitaxial films grown via the PMOCVD method with a record high Mg content up to 51 %. A series of MgZnO films with various Mg content were grown on ZnO (~30 nm)/Al2O3(0001) and ZnO (~30 nm)/AlN (~25 nm)/Al2O3(0001) substrates. The band gap for the films estimated using UV-visible transmission spectroscopy ranges from 3.24 - 4.50 eV, corresponding to the fraction of Mg between x=0.0 to x=0.51, as determined by Rutherford backscattering spectroscopy (RBS). The cathodoluminescence (CL) measurement showed a blue shift in the spectral peak position of MgZnO, indicating an increase in Mg content. No multi-absorption edges and CL band splitting were observed, suggesting the absence of phase segregation in the as grown films. The phase purity and crystal structure of the films were further confirmed by XRD. The absence of phase separation is attributed to the fast periodic transition steps in the PMOCVD, creating a non-equilibrium system where radicals that are formed will have insufficient time to reach their energy minimum. AFM analysis of the films had decreasing surface roughness with increasing Mg content. MSM photodetector was fabricated from the films to characterize the spectral response. The devices exhibit peak response ranging between 276 - 383 nm, covering a large portion of the solar blind spectral window. Moreover, the Schottky barrier was enhanced by treating the MgZnO surface with H2O2, reducing the device’s dark current.

Polarization-mediated Debye-screening of surface potential fluctuations in dual-channel AlN/GaN high electron mobility transistors

A dual-channel AlN/GaN/AlN/GaN high electron mobility transistor (HEMT) architecture is proposed, simulated, and demonstrated that suppresses gate lag due to surface-originated trapped charge. Dual two-dimensional electron gas (2DEG) channels are utilized such that the top 2DEG serves as an equipotential that screens potential fluctuations resulting from surface trapped charge. The bottom channel serves as the transistors modulated channel. Two device modeling approaches have been performed as a means to guide the device design and to elucidate the relationship between the design and performance metrics. The modeling efforts include a self-consistent Poisson-Schrodinger solution for electrostatic simulation as well as hydrodynamic three-dimensional device modeling for three-dimensional electrostatics, steady-state, and transient simulations. Experimental results validated the HEMT design whereby homo-epitaxial growth on free-standing GaN substrates and fabrication of the same-wafer dual-channel and reces...

Temperature and pulse duration effects on the growth of MgZnO via pulsed metal organic chemical vapor deposition

The effect of substrate temperature (TS) and pulse duration (PD) on Mg incorporation, surface quality, and photoresponse properties of MgZnO films grown via PMOCVD were studied. Films grown at TS ranging from 500 to 700 °C but at identical PDs had band gaps varying from 3.38 to 3.87 eV, corresponding to Mg content between x = 0.06 and 0.27. The film with Mg content of 0.27 was the smoothest and achieved at 630 °C-optimal TS. Additionally, pulse time effect was studied by growing films at the same TS but different PDs. A film grown at PD of 12 s has incorporated ~40% higher Mg than one grown in a continuous mode (PD → ∞), indicting the cruciallity of PMOCVD to realize high Mg film. The peak response spectra of photodetectors were also varied with TS and PD, in accordance with Mg content in the films.

Quantum dot dispersions in aerogels: a new material for true volumetric color displays

The true volumetric displays project a 3D image within a cube viewable from most of its sides thus providing the ultimate physiological depth cues for countless applications. The ultra-light and highly transparent aerogels may provide the best optical medium for these displays as they can be easily fabricated in the form of a large-volume, low-scattering bulk material. On the other hand, the semiconductor nanocrystals (quantum dots, QDs) are a remarkable fluorescent material with optical properties superior to those of conventional materials. QDs dispersed in aerogels hold a promise to become the most efficient display material for volumetric 3D displays. The true volumetric displays described in the literature are built around the concept of two beams exciting the fluorescent material in their intersection. However, the optical properties of QDs are quite different from these of the fluorescent materials proposed for intersecting-beams displays and it may not be feasible to build such displays using QDs. Instead, we are proposing the use of a single focused infrared laser beam to excite a nanostructured material for volumetric color displays consisting of QDs dispersed in a transparent silica aerogel matrix. Presented are the theory and modeling results proving the feasibility of this approach.