Jesmin Haq

Perpendicular spin transfer torque magnetic random access memories with high spin torque efficiency and thermal stability for embedded applications (invited)

Magnetic random access memories based on the spin transfer torque phenomenon (STT-MRAMs) have become one of the leading candidates for next generation memory applications. Among the many attractive features of this technology are its potential for high speed and endurance, read signal margin, low power consumption, scalability, and non-volatility. In this paper, we discuss our recent results on perpendicular STT-MRAM stack designs that show STT efficiency higher than 5 kBT/μA, energy barriers higher than 100 kBT at room temperature for sub-40 nm diameter devices, and tunnel magnetoresistance higher than 150%. We use both single device data and results from 8 Mb array to demonstrate data retention sufficient for automotive applications. Moreover, we also demonstrate for the first time thermal stability up to 400 °C exceeding the requirement of Si CMOS back-end processing, thus opening the realm of non-volatile embedded memory to STT-MRAM technology.

Demonstration of fully functional 8Mb perpendicular STT-MRAM chips with sub-5ns writing for non-volatile embedded memories

We present major breakthroughs in MTJ design for STT-MRAM applications allowing reliable write for pulse lengths down to 1.5ns, data retention up to 125°C for 10 years and full compatibility with BEOL process up to 400°C for 1 hour. We have successfully integrated the novel structure onto an 8Mbit test chip. We demonstrate writing of every single cell in the array using sub-5ns pulses over a wide temperature range without using any error correction. We also show that sensing times of 4ns are sufficient to read every data cell. The inherent scalability of the design makes it a prime candidate for universal embedded non-volatile memories down to the 28nm node and beyond.

Temporary bond-debond process for manufacture of flexible electronics: Impact of adhesive and carrier properties on performance

Manufacturing of microelectronics on flexible substrates is challenged by difficulties in maintaining alignment and conformity of the substrate through deposition, patterning, and etch processes. To address these difficulties, a temporary bond-debond method has been developed for effective automated handling of flexible substrate systems during electronics fabrication. The flexible substrate is temporarily bonded to a rigid carrier, which provides structural support and suppresses bending during processing. The photolithographic alignment of the bonded system is strongly dependent upon the viscoelastic properties of the bonding adhesive. An additional challenge is to control the stress developed during processing; these stresses evolve predominately through thermomechanical property mismatches between the carrier and flexible substrate. To investigate the role of the thermomechanical properties of the carrier and adhesive, the stress, and subsequent bowing of bonded systems (flexible substrate-adhesive-ca...

Fully functional perpendicular STT-MRAM macro embedded in 40 nm logic for energy-efficient IOT applications

We present for the first time a fully functional 40 nm perpendicular STT-MRAM macro (1 Mb, ×32/×64 IO) embedded into a foundry standard CMOS logic platform. We achieved target design specifications of 20 ns read access time and 20-100 ns write cycle time without redundancy repair at standard core and IO voltages. The full 1 Mb macro can be switched reliably with write pulse as short as 6 ns, which results in full-chip write power of ~ 3.2 μW/Mbps at ×64. This is the lowest eNVM write power reported at a full-chip level and about three orders of magnitude smaller than that of eFLASH. The 0.5 Mbit high-density bitcell array also demonstrates good Rp distribution and 100 % STT switching. Our results demonstrate superior power-area-feature attributes of perpendicular STT-MRAM as a best-in-class unified eNVM solution for Internet-of-Things (IOT) applications at 40 nm as well as the scalability of these advantages to 28 nm and beyond.

Temporary bond—debond technology for high‐performance transistors on flexible substrates

Abstract— A processing technology based upon a temporary bond—debond approach has been developed that enables direct fabrication of high-performance electronic devices on flexible substrates. This technique facilitates processing of flexible plastic and metal-foil substrates through automated standard semiconductor and flat-panel tool sets without tool modification. The key to processing with these tool sets is rigidifying the flexible substrates through temporary bonding to carriers that can be handled in a similar manner as silicon wafers or glass substrates in conventional electronics manufacturing. To demonstrate the power of this processing technology, amorphous-silicon thin-film-transistor (a-Si:H TFT) backplanes designed for electrophoretic displays (EPDs) were fabricated using a low-temperature process (180°C) on bonded-plastic and metal-foil substrates. The electrical characteristics of the TFTs fabricated on flexible substrates are found to be consistent with those processed with identical conditions on rigid silicon wafers. These TFTs on plastic exhibit a field-effect mobility of 0.77 cm2/V-sec, on/off current ratio >109 at Vds = 10 V, sub-threshold swing of 365 mV/dec, threshold voltage of 0.49 V, and leakage current lower than 2 pA/μm gate width. After full TFT-array fabrication on the bonded substrate and subsequent debonding, the flexible substrate retains its original flexibility; this enables bending of the EPD display without loss in performance.

Solving the paradox of the inconsistent size dependence of thermal stability at device and chip-level in perpendicular STT-MRAM

Current understanding of thermal stability of perpendicular STT-MRAM based on device-level data suggests that the thermal stability factor A is almost independent of device diameter above ~30nm. Here we report that contrary to this conventional wisdom, chip-level data retention exhibits substantial size dependence for diameters between 55 and 100 nm. We show that the method widely used to measure A is inaccurate for devices larger than ~30 nm, leading to significant underestimation of the size dependence. We derive an improved model, allowing us to reconcile the size dependence of A measured at device and chip level.

Achieving Sub-ns switching of STT-MRAM for future embedded LLC applications through improvement of nucleation and propagation switching mechanisms

We present recent advances in writing speed of pSTT_MRAM which demonstrate its potential as a candidate for replacement of LCC cache for advanced technology nodes as well as applications where non-volatility may be needed. In this paper we explore the feasibility of sub-ns switching of devices and their characterization using comprehensive time resolved electrical measurement of the reversal mechanism. We show that the switching mechanism can be described as a simple nucleation followed by propagation model that can be characterized statistically. We further demonstrate that after optimization of the Magnetic Tunnel Junction (MTJ) stack, single devices can be switched reliably using write pulse length down to 750ps while preserving functionality and data retention @ 125°C. Results of the integration at array level on an 8MB test vehicle are also presented allowing full array writing using 3ns pulses without ECC and demonstrated data retention of 10 years (1ppm) at 125°C.

Direct Fabrication of a-Si:H Thin Film Transistor Arrays on Flexible Plastic Film and Metal Foil Substrates: Critical Challenges and Enabling Solutions

In this paper we describe solutions to effectively address critical challenges in direct fabrication of amorphous silicon thin film transistor (TFTs) arrays for active matrix flexible displays. For both metal foil and plastic flexible substrates a manufacturable handling protocol in automated display-scale equipment is required. We have successfully demonstrated a temporary bonding protocol that required development of new enabling materials, tools and processes. For metal foil substrates, the principal challenges are planarization and electrical isolation, and management of stress (CTE mismatch) during TFT fabrication. For plastic substrates, the principal challenges are dimensional instability management in conjunction with manufacturing-ready temporary adhesives. Solutions required a systems-level approach to address the challenges of the substrates and their handling simultaneously.

Demonstration of an MgO based anti-fuse OTP design integrated with a fully functional STT-MRAM at the Mbit level

STT-MRAM technology has been attracting renewed attention since the embedability of a working STT-MRAM design has been demonstrated [1]. In this paper we expand on the versatility of STT-MRAM by demonstrating the conversion of a standard STT-MRAM cell to a One Time Programmable (OTP) anti-fuse cell. Both designs are integrated at the Mbit level on a single chip using the same magnetic stack, processing and CMOS cell design. A single BEOL mask change can convert an STT-MRAM device to an OTP design by simply reducing its size. The increased resistance yields larger voltage drop across the device, due to the voltage divider effect in the 1T-1MTJ cell and is sufficient to trigger reliable dielectric breakdown of the oxide tunnel barrier, effectively shorting the device. In this paper we demonstrate the seamless integration of an OTP based on STT-MRAM and 100% programming and reading yield at the Mbit level.