R.A.M. Wolters

Universiteit Twente, Enschede, Overijssel, Netherlands

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Publications (90)112.71 Total impact

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    ABSTRACT: This paper describes the potential of tunable strain in field-effect transistors to boost per- formance of digital logic. Voltage-controlled strain can be imposed on a semiconductor body by the integration of a piezoelectric material improving transistor performance. In this paper, we derive the relations governing the subthreshold swing in such devices to improve the understanding. Using these relations and considering the mechanical and technological boundary conditions, we discuss possible device architectures that employ this principle. Further, we review the recently published experimental and modeling results of this device, and give analytical estimates of the power consumption.
    05/2015; 3(3):149. DOI:10.1109/JEDS.2015.2409303
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    ABSTRACT: In this work, we investigated an approach of hot-wire assisted ALD (HWALD), utilizing a hot (up to 2000 °C) tungsten (W) wire. Tungsten films were deposited by this method using alternating pulses of WF6 gas and atomic hydrogen (at-H). The latter was generated by catalytic dissociation of molecular hydrogen (H2) upon the hot-wire. The W films were grown on a 100-nm thick thermal SiO2. The growth process was monitored in real time by an in-situ spectroscopic ellipsometer (SE). The real-time SE monitoring revealed the coexistence of three processes: CVD, etching, and ALD of the W film. WF6 could back-stream diffuse to the hot-wire, resulting in WF6 decomposition and generation of a flux of fluorine (F). The latter caused etching of the grown W film and the filament, and provided extra tungsten supply, which might cause CVD. Higher pressure and higher carrier gas flow rate were found to largely suppress the back-stream diffusion of WF6, which efficiently limited CVD. By controlling the dose of WF6 and process pressure, the etching had also been minimized. X-ray photoelectron spectroscopy of optimized HWALD grown W revealed 99 at% of W; concentrations of oxygen and fluorine were lower than 1%, below the detection limit.
    Physica Status Solidi (A) Applications and Materials 05/2015; DOI:10.1002/pssa.201532305 · 1.62 Impact Factor
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    ABSTRACT: This work discusses the design, finite element method modeling (FEM), fabrication and characterization of a silicon-based, catalytic micro calorimetric sensor. The sensing area is comprised of two titanium silicide (TiSi2) -polysilicon (poly-Si) resistive temperature sensors symmetrically positioned relative to a poly-Si heater on which an oxidation promoting catalyst is deposited. The resistive structures are located on a suspended, thereby, thermally isolating, low mechanical stress membrane and integrated into a glass flow channel. The micro-calorimetric sensor is applied for measuring propane and hydrogen concentrations in air. An approach to optimization of thermal and fluidic design of the microsensor is presented based on developed models: (i) a 3D thermo-electric analysis of the suspended heater, and (ii) a 2D thermo-chemical analysis of the catalytic oxidation of propane in the flow channel. Influence of the design and material of the membrane on the power consumption and temperature distribution across the sensing area are analyzed. A relationship between the thermal design of the sensor, reaction conditions and its operation as a thermal actuator and sensor of reaction heats are discussed. Various thermo-electrical characterizations (electrical, infrared surface imaging and transient thermal response measurements) in the context of microcalorimetric sensing are performed. Microsensors with a 50×50 μm2 sensing area consume ca. 12 mW at an operational temperature of 350◦C. Thermal imaging with an infrared camera indicates local heating with a temperature gradient across the active area estimated to be 4◦C μm−1 (at ca. 500◦C). The heating and cooling times are found to be ca. 1 msec and 8 msec, respectively. Temperature vs power curves are determined for both stationary and constant flow conditions of various gases. Based on the experimental and modeling results we envision that these microsensors can be successfully used for calorimetric sensing and analysis of gaseous samples.
    Sensors and Actuators B Chemical 09/2014; DOI:10.1016/j.snb.2014.08.077 · 4.29 Impact Factor
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    Buket Kaleli · R.J.E. Hueting · M.D. Nguyen · R.A.M. Wolters
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    ABSTRACT: Earlier theoretical reports predicted that the usage of a piezoelectric stressor layer around the FinFET, i.e., the PiezoFET, offers a great potential for steep subthreshold slope devices. For the first time, we analyzed the practical realization of such PiezoFETs comprising a piezoelectric stressor layer, lead–zirconate–titanate (PZT), and aluminum–nitride (AlN) deposited on n-type silicon FinFETs. A high-piezoelectric response in the range of 100 pm/V has been obtained for the PZT PiezoFET evidencing the converse piezoelectric effect in the device. The piezoelectric response for the AlN device was much less (13 pm/V) as expected. Underlying device properties, such as subthreshold swing (SS) and low-field electron mobility have been significantly affected by the presence of the PZT stressor. A 20%–50% change in the mobility and a change in the SS (about 5 mV/decade) have been observed. The change can be attributed to the strain induced reduction of the interface trap density at the ${rm Si}/{rm SiO}_{2}$ interface. This strain is partly formed by the bias over the piezoelectric layer, which indicates the converse piezoelectric effect related tunable strain in both the silicon channel and gate oxide.
    IEEE Transactions on Electron Devices 06/2014; 61(6):1929-1935. DOI:10.1109/TED.2014.2316164 · 2.36 Impact Factor
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    B. Kaleli · M.D. Nguyen · J. Schmitz · R.A.M. Wolters · R.J.E. Hueting
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    ABSTRACT: We realized metal-ferroelectric-metal (MFM) capacitors comprising high-quality ferroelectric lead zirconate titanate (Pb(Zr0.52Ti0.48)O3 or PZT) thin films on an LaNiO3/poly-Si/titanium nitride (TiN)/SiO2 integrated on a 100 mm Si wafer. Promising effective piezoelectric coefficient and remnant polarization of 53 pm/V and 19.2 μC/cm2, respectively, are obtained for the 100 nm-PZT/20 nm-LNO stack. Further analysis of the samples indicates the presence of a passive layer, possibly near the Ti/PZT interface at the top electrode. A leakage current model has been used to explain the obtained current density–electric field curves. In this model, diffusion limited transport has been assumed in which the injection is interface-controlled. Based on the capacitance and the leakage current measurements, the thickness and dielectric constant values of the passive layer are estimated to be 2.1 nm and 23, respectively. The observed apparent low barrier height value of 0.32 eV is attributed to ferroelectric polarization related phenomena. A good agreement between measurement and leakage current model is obtained.
    Microelectronic Engineering 05/2014; 119:16-19. DOI:10.1016/j.mee.2014.02.012 · 1.34 Impact Factor
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    B Kaleli · M D Nguyen · J Schmitz · R A M Wolters · R J E Hueting
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    ABSTRACT: Ferroelectric films attract high interest due to their applications in high density capacitors [1], sensors and micromechanical systems [2], and nonvolatile memories [3]. Recently, there has been a growing number of investigations on novel steep-subthreshold devices where thin ferroelectric or piezoelectric layers are employed [4-6]. The integration of ferroelectric/piezoelectric action in low power devices, such as switches, requires the reduction of the Pb(Zr0.52Ti0.48)O3 (PZT) thickness. In addition, uniform films on larger substrates are desired in practice. In this work, we focus on the investigation of planar sub-100 nm thick PZT films on 100 mm Si substrates with the use of LaNiO3 (LNO) buffer layers and an encapsulated-TiN bottom electrode down to a total stack thickness of 25 nm (20 nm LNO, 5 nm TiN). TiN is a widely employed gate material in advanced Si technology. Here it is used as electrode for the PZT thin film capacitors with the motivation of steep-subthreshold device application [6]. Test structures were realized on 100 mm Si wafers to characterize the thin PZT/LNO stacks. A schematic cross section is shown in Fig. 1. Fig. 2 shows the 3D upward response of the PZT. The piezoelectric d33,f coefficient was measured to be 53 pm/V. A high d33,f value favors the application of the PZT films as a stressor [6]. Furthermore, the polarization hysteresis (P-E) loops of PZT thin film capacitors with an LNO layer thickness of 10 nm and 20 nm were measured and plotted in Fig. 3. A remnant polarization Pr value of 19 µC/cm 2 is obtained. The electrical performance of the PZT layer in terms of leakage current and capacitance is also of major importance for device applications. High-frequency capacitance-electric field (C-E) and temperature dependent leakage current density-field (J-E) measurements were performed. Fig. 4 shows the dielectric constant-electric field curves, which were calculated from the corresponding C-E curves. We observe typical butterfly shaped dielectric constant curves indicating dielectric response variation with the dc-bias due to the domain reorientation process [7]. The center of the hysteresis loops shifts towards a negative bias and this can be explained by the presence of a passive layer; which most likely has been formed at the film/electrode interfaces during the fabrication [8]. Employing capacitance and leakage current measurements, the effective thickness of the passive layer and its relative dielectric constant are estimated to be 2.1 nm and 23, respectively. The Schottky barrier height (SBH) can be extracted from temperature dependent current measurements. For this purpose, we used the diffusion theory [9,10] applicable to Schottky contacts described by Eq. (1). As a result, we obtained the SBH at the Ti/PZT interface of qφB= 0.32 eV. Fig. 5 shows the measurements and the theoretical fit of the current density as a function of the applied electric field. In conclusion, despite the aggressive dimensions of the LNO and TiN layer we obtained good ferroelectric and piezoelectric properties and a low leakage current density which is important for future low-power applications. Our analysis of sub-100 nm PZT layers also shows that the leakage current can be described with a diffusion-based rather than a pure Schottky emission model.
    Micro and Nano Engineering, London; 09/2013
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    B. Kaleli · T. van Hemert · R.J.E. Hueting · R.A.M. Wolters
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    ABSTRACT: Metal induced strain in the channel region of silicon (Si) fin-field effect transistor (FinFET) devices has been characterized using Raman spectroscopy. The strain originates from the difference in thermal expansion coefficient of Si and titanium-nitride. The Raman map of the device region is used to determine strain in the channel after preparing the device with the focused ion beam milling. Using the Raman peak shift relative to that of relaxed Si, compressive strain values up to –0.88% have been obtained for a 5 nm wide silicon fin. The strain is found to increase with reducing fin width though it scales less than previously reported results from holographic interferometry. In addition, finite-element method (FEM) simulations have been utilized to analyze the amount of strain generated after thermal processing. It is shown that obtained FEM simulated strain values are in good agreement with the calculated strain values obtained from Raman spectroscopy.
    Thin Solid Films 08/2013; 531:57-61. DOI:10.1016/j.tsf.2012.12.094 · 2.13 Impact Factor
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    Hao van Bui · Alexey Y. Kovalgin · Jurriaan Schmitz · Rob A. M. Wolters
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    ABSTRACT: Using low pressure atomic layer deposition, ultra-thin continuous TiN films were prepared. The temperature coefficient of resistance (TCR), resistivity and field effect properties of these films were investigated. With decreasing film thickness, a positive-to-negative transition of TCR and a steep increase of resistivity were observed. This is attributed to the metal-semimetal transition of the TiN films. We demonstrate appreciable field-induced current modulation up to 11% in a 0.65 nm TiN film. The field effect is remarkably independent of temperature. A polarity asymmetry of the current-voltage characteristics is found, attributed to the interplay between different types of the carriers.
    Applied Physics Letters 07/2013; 103(5):051904. DOI:10.1063/1.4817007 · 3.52 Impact Factor
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    H. Van Bui · A.Y. Kovalgin · R.A.M. Wolters
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    ABSTRACT: This work reports on the determination and comparison of the resistivity of ultra-thin atomic layer deposited titanium nitride films in the thickness range 0.65–20 nm using spectroscopic ellipsometry and electrical test structures. We found that for films thicker than 4 nm, the resistivity values obtained by the two techniques are in good agreement. However, below 4 nm, the comparison shows an increasing difference with decreasing film thickness. A difference with a factor of 3 was found at 1.8 nm and increased up to hundreds at 0.65 nm. We attribute this significant difference to the electron scattering effects at grain boundaries and interfaces which can not be fully taken into account by spectroscopic ellipsometry measurements.
    Applied Surface Science 03/2013; 269:45-49. DOI:10.1016/j.apsusc.2012.09.074 · 2.54 Impact Factor
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    H Van Bui · A Y Kovalgin · A A I Aarnink · R A M Wolters
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    ABSTRACT: We present the generation of atomic hydrogen made by the dissociation of molecular hydrogen upon collision with a tungsten (W) filament kept at a high temperature (T ≈ 1600–1900°C). We demonstrate the ability to create atomic hydrogen and to introduce it in short pulses in experiments on etching of tellurium (Te) films. We further utilize the generated atomic hydrogen (H) to explore its impact on surface reactions in the TiCl4/NH3 precursor system. Atomic hydrogen is introduced in pulses additionally to TiCl4 and NH3 with different pulse sequences. For the TiCl4/NH3/H sequence, there is no influence on the process compared to the ALD without H-pulses. The growth rate remains at 0.02 nm/cycle and the oxygen (residual gas) content - at 3–5 at%. For the TiCl4/H/NH3 pulse sequence, the growth rate decreases to 0.01 nm/cycle and the oxygen content increases to 30–35 at%. Only TiCl4/H pulses result in no growth after the formation of approximately one monolayer. Similar effect occurs after introducing NH3 via the hot filament, pointing to the decomposition of NH3 and the formation of atomic hydrogen.
    Journal of The Electrochemical Society 02/2013; 2(4):P149-P155. DOI:10.1149/2.015304jss · 2.86 Impact Factor
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    H. Van Bui · A. Y. Kovalgin · R. A. M. Wolters
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    ABSTRACT: This work reports on the initial growth of atomic layer deposited titanium nitride thin films on SiO2 substrate at temperatures of 350–425°C and process pressures of 2.6–3.2 × 10−2 mbar. We used spectroscopic ellipsometry for in situ monitoring the growth, atomic force microscopy and electrical measurements for further film characterization. We demonstrate that the growth obeys Stranski – Krastanov mode with an initial 2D growth followed by a 3D island formation. The growth of the islands eventually leads to coalescence. We found the 2D–3D transition to be independent of temperature whereas the coalescence of the 3D islands is strongly affected by temperature. The former takes place as the film thickness reaches 0.69 ± 0.1 nm, which is equivalent to 3 monolayers of TiN. The latter occurs at a thickness of 2.5 ± 0.1 nm for 350°C and at a thickness of 3.5 ± 0.1 nm for 425°C.
    Journal of The Electrochemical Society 10/2012; 1(6):P285-P290. DOI:10.1149/2.020206jss · 2.86 Impact Factor
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    E.J. Faber · R.A.M. Wolters · Jurriaan Schmitz
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    ABSTRACT: We present a novel method for determining the temperature budget of the process side of silicon substrates and chips, based on well-known silicide formation reactions of metal-Si systems and (four-point probe) resistance measurements. In this paper, we focus on the Pd-Si system that is most temperature sensitive in the range from 100°C to 200°C. A variety of test structures is introduced to exploit the specific properties of the diffusion-limited reaction between Pd and Si. Among others, this resulted in gap-based layouts that facilitate an extension of the temperature range to 350°C. Designs and measurement results are presented, indicating the practicality and the robustness of the proposed technique.
    IEEE Transactions on Semiconductor Manufacturing 08/2012; 25(3):339-345. DOI:10.1109/TSM.2012.2202793 · 0.98 Impact Factor
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    ABSTRACT: Test structures for the electrical characterization of ultrathin conductive films are presented based on electrodes on which the ultrathin film is deposited. Two different designs are discussed: a novel design with buried electrodes and a conventional design with electrodes at the surface. This paper includes test structure design and fabrication, and the electrical characterization of atomic layer deposition TiN films down to 4 nm. We demonstrate that the novel test structures provide the same results as the conventional structures, and have the advantage of broader materials choice (i.e., conductor-dielectric combination). The proposed structures can be used successfully to characterize sub-10 nm films.
    IEEE Transactions on Semiconductor Manufacturing 05/2012; 25(2):178-184. DOI:10.1109/TSM.2011.2181674 · 0.98 Impact Factor
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    A.Y. Kovalgin · Natalie Tiggelman · R.A.M. Wolters
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    ABSTRACT: The parasitic factors that strongly influence the measurement accuracy of cross-bridge Kelvin resistors have been extensively discussed during the last few decades. The minimum value of specific contact resistance that can be accurately extracted has been estimated. In this paper, we present an analytical model to account for the actual current flow across the contact and propose an area-correction method for a reliable extraction of specific contact resistance. The model is experimentally verified for low-resistivity (close-to-ideal) metal-to-metal contacts. The minimum contact resistance is determined by the dimensions of the two-metal stack in the area of contact and sheet resistances of the metals used.
    IEEE Transactions on Electron Devices 02/2012; 59(2):426-432. DOI:10.1109/TED.2011.2174365 · 2.36 Impact Factor
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    Erik J. Faber · Rob A. M. Wolters · Jurriaan Schmitz
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    ABSTRACT: We present a novel method for determining the temperature budget of the process side of silicon substrates and chips, based on well-known silicide formation reactions of metal–Si systems and (four-point probe) resistance measurements. In this paper, we focus on the Pd–Si system that is most temperature sensitive in the range from 100 °C to 200 °C. A variety of test structures is introduced to exploit the specific properties of the diffusion-limited reaction between Pd and Si. Among others, this resulted in gap-based layouts that facilitate an extension of the temperature range to 350 °C. Designs and measurement results are presented, indicating the practicality and the robustness of the proposed technique.
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    ABSTRACT: A new microfabricated device for heating and sensing in gases is presented. It is based on the resistive heating of a micro- or nano-metric hollow cylinder of titanium nitride, and measurement of its (temperature-dependent) resistance. This article presents the fabrication and temperature calibration of the device, and illustrates its function as flow meter and thermal conductivity meter. A temperature of 280 °C is achieved at a power consumption of only 5.5 μW, orders of magnitude less than existing commercial hotplate devices. The thermal time constant can be as low as 60-120 microseconds.
    Solid-State Device Research Conference (ESSDERC), 2012 Proceedings of the European; 01/2012
  • E. Vereshchagina · R.A.M. Wolters · J.G.E. Gardeniers
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    ABSTRACT: In this work we present a low-cost, low-power, small sample volume microcalorimetric sensor for the measurement of reaction heats. The polysilicon-based microcalorimetric sensor combines several advantages: (i) complementary metal oxide semiconductor technology (CMOS) for future integration; (ii) elements of silicon micromachining (MEMS) to control thermal performance; (iii) heterogeneous catalysts for selective detection and analysis of individual gas compounds; and (iv) microfluidics for optimized control over the reaction conditions.
    Sensors and Actuators A Physical 10/2011; 169(2-2):308-316. DOI:10.1016/j.sna.2011.02.010 · 1.94 Impact Factor
  • E Vereshchagina · R A M Wolters · J G E Gardeniers
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    ABSTRACT: Thin films of titanium silicide (TiSi2) formed on heavily boron-doped polycrystalline silicon (poly-Si/B+) were applied for the first time for resistive temperature sensing. The temperature sensors exhibited a high-temperature coefficient of resistance of 3.8 × 10−3 °C−1, a linear dependence of resistance on temperature and an excellent thermal and electrical stability up to 800 °C. This work discusses the fabrication method and the morphological and electrical characterization of the TiSi2/poly-Si thin film resistors throughout the stages of its formation.
    Journal of Micromechanics and Microengineering 09/2011; 21(10):105022. DOI:10.1088/0960-1317/21/10/105022 · 1.73 Impact Factor
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    A W Groenland · R A M Wolters · A Y Kovalgin · J Schmitz
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    ABSTRACT: In this work, metal-insulator-metal (MIM) and metal-insulator-silicon (MIS) capacitors are studied using titanium nitride (TiN) as the electrode material. The effect of structural defects on the electrical properties on MIS and MIM capacitors is studied for various electrode configurations. In the MIM capacitors the bottom electrode is a patterned 100 nm TiN layer (called BE type 1), deposited via sputtering, while MIS capacitors have a flat bottom electrode (called BE type 2-silicon substrate). A high quality 50-100 nm thick SiO2 layer, made by inductively-coupled plasma CVD at 150 degrees C, is deposited as a dielectric on top of both types of bottom electrodes. BE type 1 (MIM) capacitors have a varying from low to high concentration of structural defects in the SiO2 layer. BE type 2 (MIS) capacitors have a low concentration of structural defects and are used as a reference. Two sets of each capacitor design are fabricated with the TiN top electrode deposited either via physical vapour deposition (PVD, i.e., sputtering) or atomic layer deposition (ALD). The MIM and MIS capacitors are electrically characterized in terms of the leakage current at an electric field of 0.1 MV/cm (I leak) and for different structural defect concentrations. It is shown that the structural defects only show up in the electrical characteristics of BE type 1 capacitors with an ALD TiN-based top electrode. This is due to the excellent step coverage of the ALD process. This work clearly demonstrates the sensitivity to process-induced structural defects, when ALD is used as a step in process integration of conductors on insulation materials.
    Journal of Nanoscience and Nanotechnology 09/2011; 11(9):8368-73. DOI:10.1166/jnn.2011.5036 · 1.34 Impact Factor
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    H Van Bui · A A I Aarnink · A Y Kovalgin · R A M Wolters
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    ABSTRACT: We report the electrical resistivity of atomic layer deposited TiN thin films in the thickness range 2.5-20 nm. The measurements were carried out using the circular transfer length method structures. For the films with thickness in the range of 10-20 nm, the measurements exhibited linear current-voltage (I-V) curves. The sheet resistance R(sh) was determined, and the resistivity was calculated. A value of 120 microohms-cm was obtained for a 20 nm TiN layer. With decreasing film thickness, the resistivity slightly increased and reached 135 microohms-cm for a 10 nm film. However, the measurements on 2.5-5.0 nm thick films revealed non-linear I-V characteristics, implying the dependence of the measured resistance, and therefore the resistivity, of the layers on applied voltage. The influence of the native oxidation due to the exposure of the films to air was taken into account. To fully eliminate this oxidation, a highly-resistive amorphous silicon layer was deposited directly after the ALD of TiN. The electrical measurements on the passivated 2.5- and 3.5 nm TiN layers then exhibited linear I-V characteristics. A resistivity of 400 and 310 microohms-cm was obtained for a 2.5- and 3.5 nm TiN film, respectively.
    Journal of Nanoscience and Nanotechnology 09/2011; 11(9):8120-5. DOI:10.1166/jnn.2011.5069 · 1.34 Impact Factor

Publication Stats

929 Citations
112.71 Total Impact Points

Institutions

  • 2005–2015
    • Universiteit Twente
      • • Institute for Nanotechnology (MESA+)
      • • Department of Semiconductor Components (SC)
      Enschede, Overijssel, Netherlands
  • 2011
    • NXP Semiconductors
      Eindhoven, North Brabant, Netherlands