A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors.

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS2) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS2based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS2. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS2interface for optimum MoS2-based device performance. The tunable band-gap of MoS2with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS2in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS2and its effect on structural and electronic properties are extensively discussed.

Piezoresistive Composite Silicon Dioxide Nanocantilever Surface Stress Sensor: Design and Optimization.

In this paper, we present the design and optimization of a rectangular piezoresistive composite silicon dioxide nanocantilever sensor. Unlike the conventional design approach, we perform the sensor optimization by not only considering its electro-mechanical response but also incorporating the impact of self-heating induced thermal drift in its terminal characteristics. Through extensive simulations first we comprehend and quantify the inaccuracies due to self-heating effect induced by the geometrical and intrinsic parameters of the piezoresistor. Then, by optimizing the ratio of electrical sensitivity to thermal sensitivity defined as the sensitivity ratio (υ) we improve the sensor performance and measurement reliability. Results show that to ensure υ ≥ 1, shorter and wider piezoresistors are better. In addition, it is observed that unlike the general belief that high doping concentration of piezoresistor reduces thermal sensitivity in piezoresistive sensors, to ensure υ ≥ 1 doping concentration (p) should be in the range: 1E18 cm-3 ≤ p ≤ 1E19 cm-3. Finally, we provide a set of design guidelines that will help NEMS engineers to optimize the performance of such sensors for chemical and biological sensing applications.

Impact of Isolation and Immobilization Layers on the Electro-Mechanical Response of Piezoresistive Nano Cantilever Sensors.

In the last decade, piezoresistive nano cantilever sensors have been extensively explored, especially for chemical and biological sensing applications. Piezoresistive cantilever sensors are multi-layer structures with different constituent materials. Performance of such sensors is a function of their geometry and constituent materials. For a fixed material set, the pre-requisite for optimizing the performance of a composite piezoresistive cantilever sensor is careful geometrical design of its constituent layers. Even though, treatise encompasses various designs of such sensors, typically for computational simplicity the functional layers i.e., the isolation and immobilization layers are neglected in the modeling stages. In this paper, we elucidate the impact of the functional layers on the electro-mechanical response of composite piezoresistive nano cantilever sensors. Systematic and detailed computations are performed using theoretical models and numerical simulations. Results show that both the isolation and immobilization layers play a critical role in governing the sensor performance. Simulation results depict that compared to a sensor with an isolation layer of thickness 100 nm, a sensor without isolation layer has 36.29% and 42.51% better deflection sensitivity and electrical sensitivity respectively. Furthermore, it is found that when an immobilization layer of thickness 40 nm is added atop the isolation layer, the deflection sensitivity and electrical sensitivity reduces by 12.98% and 15.83% respectively. Through our investigation it is shown that the isolation and immobilization layers not only play a vital role in determining the stability and electro-mechanical response of the sensor but their negligence in the design stages can be detrimental. Apart from investigating the impact of the immobilization layer thickness, to model the sensor closer to real time operational conditions, we have performed analysis to understand the impact of non-uniformity in the immobilization layer thickness and non-uniform surface stress loading on the electro-mechanical response of the sensor. Results and inferences obtained from this study will help NEMS engineers to optimize the performance of piezoresistive nano cantilever sensors and to design multi-layer cantilever platform structures for other transducers.

Concurrent Papillary Craniopharyngioma and Growth Hormone-Secreting Pituitary Adenoma: A Rare and Aggressive Collision Tumor.

Background/Objective: Collision tumors composed of craniopharyngiomas and pituitary adenomas are extremely rare. We report a collision tumor formed by a papillary craniopharyngioma and a growth hormone-secreting pituitary adenoma, which is the first report of such a tumor, to the best of our knowledge. Case Report: A 49-year-old man presented with 2 months of headaches and blurry vision. An exam demonstrated frontal bossing, enlarged jaw and hands, macroglossia, and bitemporal hemianopsia, and magnetic resonance imaging (MRI) showed a 4.1 cm sellar/suprasellar mass with mass effect on the optic chiasm. The tumor was resected twice via a craniotomy, the second time due to interval growth, with the pathology after both surgeries showing a papillary craniopharyngioma. IGF-1 was 517 ng/mL (68-225) and growth hormone suppression test was positive. Repeat MRI showed residual tumor with ongoing mass effect on the optic chiasm and radiation therapy was initiated. MRI showed interval growth of the mass and IGF-1 rose to 700 ng/mL after which the patient underwent a transsphenoidal resection of the tumor; the pathology showed a residual papillary craniopharyngioma and a PIT1 lineage adenoma with most cells expressing growth hormone. After developing numerous complications, the patient passed away. Discussion: Collision tumors of the sella are often associated with an aggressive clinical course, as they often go undiagnosed preoperatively, thus reducing the likelihood of total resection and leading to higher rates of craniopharyngioma recurrence. Conclusion: A pituitary mass with an aggressive clinical course should prompt a high index of suspicion for a sellar collision tumor, though prognosis remains poor.

A Case of Exaggerated Pituitary Hyperplasia in a Pregnant Woman.

Pituitary hyperplasia occurs as a result of an increase in pituitary cell subtypes. It can be a consequence of either a physiological or pathological condition. In our case, a 31-year-old pregnant woman at 16 weeks gestation presented with headaches and vision changes. Visual field testing demonstrated bitemporal hemianopsia, and magnetic resonance imaging (MRI) brain showed enlargement of the pituitary with compression of the optic chiasm. She was treated with cabergoline and steroids, and her symptoms improved. In a subsequent pregnancy, the patient developed similar symptoms, and with cabergoline treatment, her symptoms resolved. A postpartum MRI of her brain revealed a decrease in pituitary size back to baseline with normal pituitary hormone levels. This patient's likely diagnosis was physiologic pituitary hyperplasia. Pituitary hyperplasia can be difficult to diagnose since there are no explicit guidelines. Through deduction of imaging findings and hormonal levels, diagnosis of pituitary hyperplasia becomes a more manageable task.

A Review on Surface Stress-Based Miniaturized Piezoresistive SU-8 Polymeric Cantilever Sensors.

In the last decade, microelectromechanical systems (MEMS) SU-8 polymeric cantilevers with piezoresistive readout combined with the advances in molecular recognition techniques have found versatile applications, especially in the field of chemical and biological sensing. Compared to conventional solid-state semiconductor-based piezoresistive cantilever sensors, SU-8 polymeric cantilevers have advantages in terms of better sensitivity along with reduced material and fabrication cost. In recent times, numerous researchers have investigated their potential as a sensing platform due to high performance-to-cost ratio of SU-8 polymer-based cantilever sensors. In this article, we critically review the design, fabrication, and performance aspects of surface stress-based piezoresistive SU-8 polymeric cantilever sensors. The evolution of surface stress-based piezoresistive cantilever sensors from solid-state semiconductor materials to polymers, especially SU-8 polymer, is discussed in detail. Theoretical principles of surface stress generation and their application in cantilever sensing technology are also devised. Variants of SU-8 polymeric cantilevers with different composition of materials in cantilever stacks are explained. Furthermore, the interdependence of the material selection, geometrical design parameters, and fabrication process of piezoresistive SU-8 polymeric cantilever sensors and their cumulative impact on the sensor response are also explained in detail. In addition to the design-, fabrication-, and performance-related factors, this article also describes various challenges in engineering SU-8 polymeric cantilevers as a universal sensing platform such as temperature and moisture vulnerability. This review article would serve as a guideline for researchers to understand specifics and functionality of surface stress-based piezoresistive SU-8 cantilever sensors.