D.J. Gravesteijn

Evidence of the Thermo-Electric Thomson Effect and Influence on the Program Conditions and Cell Optimization in Phase-Change Memory Cells

We present physical and electrical evidence of the Thomson thermo-electric effect in line-type phase-change memory cells. This causes a shift of the molten zone during RESET programming towards the anode contact, and as a consequence the phase change material (PCM) design at the contact area has a significant influence on the program conditions. First statistical studies showed a reduction of minimum Reset currents by ~5% and Set voltages by -28% when PCM extensions around the anode are used instead of fine line contacts. This Thomson effect remains important with further cell scaling.

RF Characterization and Analytical Modelling of Through Silicon Vias and Coplanar Waveguides for 3D Integration

High-aspect ratio (12.5) through silicon vias (TSV) made in a silicon interposer have been electrically characterized in the direct current (dc) and microwave regimes for 3D interconnect applications. The vias were micro-machined in silicon, insulated, and filled with copper employing a bottom-up copper electroplating technique in a “via-first” approach. DC via resistance measurements show good agreement with the theoretical expected value (~ 16 mΩ) . Radio-frequency (RF) measurements up to 50 GHz have been performed on coplanar waveguides located on the back-side of the wafers and connected to the front-side with TSVs. The S-parameters indicate clearly the beneficial impact of double sided ground planes of the RF signals. The via resistance extracted from impedance measurements is in good agreement with dc values, while the inductance (53 pH) and capacitance (2.4 pF) of the TSV are much lower than conventional wire bonding, which makes the use of TSV very promising for 3D integration. An advanced analytical model is proposed for the interconnect system with vias and lines and shows very good agreement with the experimental data with a limited number of fitting parameters. This work gives a proof of concept for high aspect ratio TSV manufacturing and new insights to improve 3D interconnect modeling for systems-in-package applications in the microwave regime.

Gate-workfunction engineering using poly-(Si,Ge) for high-performance 0.18 /spl mu/m CMOS technology

We show that poly-SiGe can be readily integrated as a gate material into an existing CMOS technology to achieve significant increase in the transistor performance. In order to preserve the standard salicidation scheme, a buffer poly-Si layer is introduced in the gate stack. PMOST channel profiles are optimized to account for the change of the gate workfunction. High-performance CMOS 0.18 /spl mu/m devices are manufactured using p- and n-type poly-Si/Si/sub 0.8/Ge/sub 0.2/ gates.

Gate current and oxide reliability in p/sup +/ poly MOS capacitors with poly-Si and poly-Ge/sub 0.3/Si/sub 0.7/ gate material

Fowler-Nordheim (FN) tunnel current and oxide reliability of PRiLOS capacitors with a p/sup +/ polycrystalline silicon (poly-Si) and polycrystalline germanium-silicon (poly-Ge/sub 0.3/Si/sub 0.7/) gate on 5.6-nm thick gate oxides have been compared. It is shown that the FN current depends on the gate material and the bias polarity. The tunneling barrier heights, /spl phi//sub B/, have been determined from FN-plots. The larger barrier height for negative bias, compared to positive bias, suggests that electron injection takes place from the valence band of the gate. This barrier height for the GeSi gate is 0.4 eV lower than for the Si gate due to the higher valence band edge position. Charge-to-breakdown (Q/sub bd/) measurements show improved oxide reliability of the GeSi gate on of PMOS capacitors with 5.6 nm thick gate oxide. We confirm that workfunction engineering in deep submicron MOS technologies using poly-GeSi gates is possible without limiting effects of the gate currents and oxide reliability.

Evolution of cell resistance, threshold voltage and crystallization temperature during cycling of line-cell phase-change random access memory

Doped SbTe phase change (PRAM) line cells produced by e-beam lithography were cycled 100 million times. During cell cycling the evolution of many cell properties were monitored, in particular the crystalline and amorphous resistance, amorphous resistance drift exponent, time-dependent threshold voltage, threshold voltage as a function of RESET pulse height, crystallization temperature, and activation energy of crystal growth. The power of the present approach is that all these properties were measured simultaneously during the life of single cells. The evolution of the cell properties can be summarized by (i) an initialization phase characterized by settle-in effect of the material surrounding the programmable region, (ii) a usable life phase where initially the cell properties remain fairly constant until after ∼5 × 105 cycles decomposition of the programmed region caused degradation of the cell properties, and (iii) finally an end of life phase where the cell is stuck in the SET state after typically 10...

The influence of resistance drift on measurements of the activation energy of conduction for phase-change material in random access memory line cells

Temporal drift of the amorphous resistance in phase-change random access memory (PRAM) is a temperature accelerated process. Increasing the temperature will speed up the drift process which is shown to affect measurements of the activation energy of conduction (Ea, slope of log(R) versus 1/kT). Doped SbTe phase change (PRAM) line cells were brought to the amorphous state and were subjected to annealing experiments. First, it is shown that when the temperature is increased by a fixed rate, the resistance does not follow a unique function of temperature but depends on the heating rate. This can be attributed to resistance drift taking place during the ramp. Upon cooling, the drift process freezes and only then physically relevant, i.e., time independent, values for Ea can be obtained, because of the absence of additional drift. The observed increase in resistance as a function of annealing history (for various frozen-in drift levels) is modeled and well-reproduced using a trap limited band transport model. ...

Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

Growth Rate Determination through Automated TEM Image Analysis : Crystallization Studies of Doped SbTe Phase-Change Thin Films

A computer-controlled procedure is outlined here that first determines the position of the amorphous-crystalline interface in an image. Subsequently, from a time series of these images, the velocity of the crystal growth front is quantified. The procedure presented here can be useful for a wide range of applications, and we apply the new approach to determine growth rates in a so-called fast-growth-type phase-change material. The growth rate (without nucleation) of this material is of interest for comparison with identical material used in phase-change random access memory cells. Crystal growth rates in the amorphous phase-change layers have been measured at various temperatures using in situ heating in a transmission electron microscope. Doped SbTe films (20 nm thick) were deposited on silicon nitride membranes, and samples with and without silicon oxide capping layer were studied. The activation energy for growth was found to be 3.0 eV. The samples without capping layer exhibit a nucleation rate that is an order of magnitude higher than the samples with a silicon oxide capping layer. This difference can be attributed to the partial oxidation of the phase-change layer in air. However, the growth rates of the samples with and without capping are quite comparable.

Void growth modeling upon electromigration stressing in narrow copper lines

A simple three-dimensional void growth model is presented that can be used to simulate the resistance behavior in narrow copper lines upon thermo-electrical stressing. The output of the model is compared with experimental results obtained from electromigration tests carried out on single damascene copper lines encapsulated by a physical vapor deposition tantalum nitride–tantalum barrier. The electromigration resistance profiles are found to depend on different line and barrier parameters. The simulations yield a better understanding of the physical phenomena responsible for changes in the resistance profiles. The effect of a void cutting a copper line is seen as an asymptotic increase or “jump” in the measured resistance profile. At that moment, the barrier shunts the current and the void does not necessarily induce a catastrophic failure. Therefore, more voids can be formed in the line upon electromigration (EM) stress; every void spanning the line initiates a “jump” in the resistance profile. The descri...

Gas phase sensing of alcohols by Metal Organic Framework – polymer composite materials

Affinity layers play a crucial role in chemical sensors for the selective and sensitive detection of analytes. Here, we report the use of composite affinity layers containing Metal Organic Frameworks (MOFs) in a polymeric matrix for sensing purposes. Nanoparticles of NH2-MIL-53(Al) were dispersed in a Matrimid polymer matrix with different weight ratios (0–100 wt %) and drop-casted on planar capacitive transducer devices. These coated devices were electrically analyzed using impedance spectroscopy and investigated for their sensing properties toward the detection of a series of alcohols and water in the gas phase. The measurements indicated a reversible and reproducible response in all devices. Sensor devices containing 40 wt % NH2-MIL-53(Al) in Matrimid showed a maximum response for methanol and water. The sensor response time slowed down with increasing MOF concentration until 40 wt %. The half time of saturation response (τ0.5) increased by ∼1.75 times for the 40 wt % composition compared to devices coated with Matrimid only. This is attributed to polymer rigidification near the MOF/polymer interface. Higher MOF loadings (≥50 wt %) resulted in brittle coatings with a response similar to the 100 wt % MOF coating. Cross-sensitivity studies showed the ability to kinetically distinguish between the different alcohols with a faster response for methanol and water compared to ethanol and 2-propanol. The observed higher affinity of the pure Matrimid polymer toward methanol compared to water allows also for a higher uptake of methanol in the composite matrices. Also, as indicated by the sensing studies with a mixture of water and methanol, the methanol uptake is independent of the presence of water up to 6000 ppm of water. The NH2-MIL-53(Al) MOFs dispersed in the Matrimid matrix show a sensitive and reversible capacitive response, even in the presence of water. By tuning the precise compositions, the affinity kinetics and overall affinity can be tuned, showing the promise of this type of chemical sensors.