Detecting defects that reduce breakdown voltage using machine learning and optical profilometry.

Semiconductor wafer manufacturing relies on the precise control of various performance metrics to ensure the quality and reliability of integrated circuits. In particular, GaN has properties that are advantageous for high voltage and high frequency power devices; however, defects in the substrate growth and manufacturing are preventing vertical devices from performing optimally. This paper explores the application of machine learning techniques utilizing data obtained from optical profilometry as input variables to predict the probability of a wafer meeting performance metrics, specifically the breakdown voltage (Vbk). By incorporating machine learning techniques, it is possible to reliably predict performance metrics that cause devices to fail at low voltage. For diodes that fail at a higher (but still below theoretical) breakdown voltage, alternative inspection methods or a combination of several experimental techniques may be necessary.

Methodology and implementation of a tunable deep-ultraviolet laser source for photoemission electron microscopy.

Photoemission electron microscopy (PEEM) is a unique and powerful tool for studying the electronic properties of materials and surfaces. However, it requires intense and well-controlled light sources with photon energies ranging from the UV to soft X-rays for achieving high spatial resolution and image contrast. Traditionally, many PEEMs were installed at synchrotron light sources to access intense and tunable soft X-rays. More recently, the maturation of solid-state lasers has opened a new avenue for laboratory-based PEEMs using laser-based UV light at lower photon energies. Here, we report on the characteristics of a laser-based UV light source that was recently integrated with a PEEM instrument. The system consists of a high repetition rate, tunable wavelength laser coupled to a harmonics generation module, which generates deep-UV radiation from 192 nm to 210 nm. We comment on the spectral characteristics and overall laser system stability, as well as on the effects of space charge within the PEEM microscope at high UV laser fluxes. Further, we show an example of imaging on gallium nitride, where the higher UV photon energy and flux of the laser provides considerably improved image quality, compared to a conventional light source. These results demonstrate the capabilities of laser-based UV light sources for advancing laboratory-based PEEMs.

Using machine learning with optical profilometry for GaN wafer screening.

To improve the manufacturing process of GaN wafers, inexpensive wafer screening techniques are required to both provide feedback to the manufacturing process and prevent fabrication on low quality or defective wafers, thus reducing costs resulting from wasted processing effort. Many of the wafer scale characterization techniques-including optical profilometry-produce difficult to interpret results, while models using classical programming techniques require laborious translation of the human-generated data interpretation methodology. Alternatively, machine learning techniques are effective at producing such models if sufficient data is available. For this research project, we fabricated over 6000 vertical PiN GaN diodes across 10 wafers. Using low resolution wafer scale optical profilometry data taken before fabrication, we successfully trained four different machine learning models. All models predict device pass and fail with 70-75% accuracy, and the wafer yield can be predicted within 15% error on the majority of wafers.

Optimizing performance and yield of vertical GaN diodes using wafer scale optical techniques.

To improve the manufacturing of vertical GaN devices for power electronics applications, the effects of defects in GaN substrates need to be better understood. Many non-destructive techniques including photoluminescence, Raman spectroscopy and optical profilometry, can be used to detect defects in the substrate and epitaxial layers. Raman spectroscopy was used to identify points of high crystal stress and non-uniform conductivity in a substrate, while optical profilometry was used to identify bumps and pits in a substrate which could cause catastrophic device failures. The effect of the defects was studied using vertical P-i-N diodes with a single zone junction termination extention (JTE) edge termination and isolation, which were formed via nitrogen implantation. Diodes were fabricated on and off of sample abnormalities to study their effects. From electrical measurements, it was discovered that the devices could consistently block voltages over 1000 V (near the theoretical value of the epitaxial layer design), and the forward bias behavior could consistently produce on-resistance below 2 mΩ cm2, which is an excellent value considering DC biasing was used and no substrate thinning was performed. It was found that high crystal stress increased the probability of device failure from 6 to 20%, while an inhomogeneous carrier concentration had little effect on reverse bias behavior, and slightly (~ 3%) increased the on-resistance (Ron). Optical profilometry was able to detect regions of high surface roughness, bumps, and pits; in which, the majority of the defects detected were benign. However a large bump in the termination region of the JTE or a deep pit can induce a low voltage catastrophic failure, and increased crystal stress detected by the Raman correlated to the optical profilometry with associated surface topography.

Patient Satisfaction With Medical Student Participation in a Longitudinal Integrated Clerkship: A Controlled Trial.

PURPOSE: To determine whether longitudinal student involvement improves patient satisfaction with care. METHOD: The authors conducted a satisfaction survey of patients followed by 10 University of Minnesota Medical School students enrolled in 2016-2017 in the Veterans Affairs Longitudinal Undergraduate Medical Education (VALUE) program, a longitudinal integrated clerkship at the Minneapolis Veterans Health Care System. Students were embedded in an ambulatory practice with primary preceptors who assigned students a panel of 14 to 32 patients to follow longitudinally in inpatient and outpatient settings. Control patients, matched on disease severity, were chosen from the preceptor's panel. Two to five months after the students completed the VALUE program, the authors conducted a phone survey of the VALUE and control patients using a validated, customized questionnaire. RESULTS: Results are reported from 97 VALUE patients (63% response rate) and 72 controls (47% response rate) who had similar baseline characteristics. Compared with control patients, VALUE patients reported greater satisfaction with explanations provided by their health care provider, their provider's knowledge of their personal history, and their provider's looking out for their best interests (P < .05). Patients in the VALUE panel selected the top category more often than control patients for overall satisfaction with their health care (65% vs 43%, P < .05). CONCLUSIONS: The results of this controlled trial demonstrate that VALUE student longitudinal participation in patient care improves patient satisfaction and patient-perceived quality of health care for VALUE patients compared with controls matched by primary care provider and disease severity. These findings may have implications outside the Veterans Administration population.

Room Temperature Skyrmions in Strain-Engineered FeGe thin films.

Skyrmions hold great promise for low-energy consumption and stable high density information storage, and stabilization of the skyrmion lattice (SkX) phase at or above room temperature is greatly desired for practical use. The topological Hall effect can be used to identify candidate systems above room temperature, a challenging regime for direct observation by Lorentz electron microscopy. Atomically ordered FeGe thin films are grown epitaxially on Ge(111) substrates with ~ 4 % tensile strain. Magnetic characterization reveals enhancement of Curie temperature to 350 K due to strain, well above the bulk value of 278 K. Strong topological Hall effect was observed between 10 K and 330 K, with a significant increase in magnitude observed at 330 K. The increase in magnitude occurs just below the Curie temperature, a similar relative temperature position as the onset of Skx phase in bulk FeGe. The results suggest that strained FeGe films may host a SkX phase above room temperature when significant tensile strain is applied.

The Role of Dedicated Musculoskeletal Urgent Care Centers in Reducing Cost and Improving Access to Orthopaedic Care.

OBJECTIVES: Over the past few years, the United States has seen the rapid growth of dedicated musculoskeletal urgent care centers owned and operated by individual orthopaedic practices. In June of 2014, our practice opened the first dedicated orthopaedic urgent care in the region staffed by physician assistants and supervised by orthopaedic surgeons. Our hypothesis is that such centers can safely improve orthopaedic care for ambulatory orthopaedic injuries, decrease volume for overburdened emergency departments (EDs), reduce wait times and significantly decrease the cost of care while improving access to orthopaedic specialists. DESIGN: Retrospective review. SETTING: Level 2 trauma center and physician-owned orthopaedic urgent care. PATIENTS: Consecutive series of patients seen in the hospital ED (n = 87,629) and orthopaedic urgent care (n = 12,722). INTERVENTION: None. OUTCOMES: ED wait time, total visit time, time until being seen by provider, time until consultation with orthopaedic surgeon, total visit charges, and effect on orthopaedic practice revenue. RESULTS: During the 12 months of study, 12,722 patients were treated in our urgent care. The average urgent care wait time until being seen by a provider was 17 minutes compared with 45 minutes in hospital ED. Total visit time was 43 minutes in the urgent care and 156 minutes in the hospital ED. Time to being seen by an orthopaedic specialist was 1.2 days for urgent care patients compared with 3.4 days for ED patients. The average charge for an urgent care visit was $461 compared with $8150 in hospital ED. During the course of study, urgent care treatment reduced charges to health care system by $97,819,458. Hospital ED orthopaedic volume did decrease as expected but total ED patient volume remained the same. There was no measureable effect on hospital ED wait times. Hospital surgical case volume did not change over the period of study and the orthopaedic census remained stable. Urgent care construction, marketing, administration, imaging, and labor costs totaled $1,664,445. Urgent care revenue from evaluation and management, imaging, durable medical equipment, and casting totaled $2,577,707. Practice revenue from follow-up care of patients who entered practice through the urgent care totaled $7,657,998. CONCLUSION: Dedicated musculoskeletal urgent care clinics operated by orthopaedic surgery practices can be extremely beneficial to patients, physicians, and the health care system. They clearly improve access to care, whereas significantly decreasing overall health care costs for patients with ambulatory orthopaedic conditions and injuries. In addition, they can be financially beneficial to both patients and orthopaedic surgeons alike without cannibalizing local hospital surgical volumes. LEVEL OF EVIDENCE: Therapeutic Level III.

Technique for the dry transfer of epitaxial graphene onto arbitrary substrates.

To make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO(2), GaN and Al(2)O(3) substrates using a thermal release tape. Subsequent Hall effect measurements illustrated that minimal degradation in the carrier mobility was induced following the transfer process in lithographically patterned devices. Correspondingly, a large drop in the carrier concentration was observed following the transfer process, supporting the notion that a gradient in the carrier density is present in C-face EG, with lower values being observed in layers further removed from the SiC interface. X-ray photoemission spectra collected from EG films attached to the transfer tape revealed the presence of atomic Si within the EG layers, which may indicate the identity of the unknown intrinsic dopant in EG. Finally, this transfer process is shown to enable EG films amenable for use in device fabrication on arbitrary substrates and films that are deemed most beneficial to carrier transport, as flexible electronic devices or optically transparent contacts.