Voltage Coefficient Of Capacitance Research Articles

Enhanced Voltage Linearity of HfO2 Metal-Insulator-Metal Capacitors by H2O Prepulsing Treatment on Bottom Electrode

The effects of H2O prepulsing treatments on the bottom Pt electrode of metal-insulator-metal (MIM) capacitors prior to HfO2 atomic layer deposition (ALD) were investigated for the first time. We demonstrated a MIM capacitor with capacitance density of 9 fF/μm2, quadratic voltage coefficient of capacitance (VCC) α of 499 ppm/V2 and low leakage current density of 5×10-8 A/cm2 at 1V. X-ray photoelectron spectroscopy (XPS) results reveal that the surfaces of the bottom Pt electrode were partially hydroxylated, which is considered as able to reduce the oxygen vacancies.

High-Performance Metal–Insulator–Metal Capacitors With $\hbox{HfTiO}/\hbox{Y}_{2}\hbox{O}_{3}$ Stacked Dielectric

The HfTiO/Y2O3 stacked dielectric is proposed as the dielectric of metal-insulator-metal (MIM) capacitor. Y2O3 is more thermodynamically stable than HfTiO as contacting with TaN electrode. Interfacial layer thickness and leakage current density can be reduced by inserting a thin Y2O3 layer between HfTiO and TaN. The negative quadruple voltage coefficient of capacitance (VCC-α) of Y2O3 cancels out the positive VCC-α of HfTiO to achieve low VCC-α. The MIM capacitor structure with HfTiO/Y2O3 dielectric shows a capacitance density that is higher than 11 fF/μm2 and VCC-α that is lower than 1222 ppm/V2. The leakage currents at -1 and -2 V are 6.4 and 14 nA/cm2, respectively. These results suggest that the HfTiO/Y2O3 stacked dielectric is a promising candidate for MIM capacitors.

금속유기분해법을 사용한 Zr0.7Sn0.3TiO4박막 제조 및 유전특성

Zr0.7Sn0.3TiO4 (ZST) thin films were fabricated by metal-organic decomposition, and their dielectric properties were investigated in order to evaluate their potential use in passive capacitors for rf and analog/mixed signal integrated circuits. The ZST thin film annealed at the temperature of 800°C showed a dielectric constant of 27.3 and a dielectric loss of 0.011. The capacitor using the ZST film had quadratic and linear voltage coefficient of capacitance (VCC) of -65 ppm/V and -35 ppm/V at 100 kHz, respectively. It also exhibited a good temperature coefficient of capacitance (TCC) value of -32 ppm/°C at 100 kHz.

Analysis of reliability characteristics of high capacitance density MIM capacitors with SiO 2–HfO 2–SiO 2 dielectrics

In this paper, reliability as well as electrical properties of high capacitance density metal–insulator–metal (MIM) capacitor with hafnium-based dielectric is analyzed in depth. The fabricated MIM capacitor exhibits not only high capacitance density but also low voltage coefficient of capacitance (VCC) and low temperature coefficient of capacitance (TCC). It also has a low leakage current level of about ∼1 nA/cm 2 at room temperature and 1 V. However, it is shown that voltage linearity has a different dependence on the polarity of applied bias as temperature increases maybe due to the bulk traps between the metal electrode and high- k dielectric interface. In addition, the effect of charge trapping and de-trapping on the voltage linearity is analyzed under constant voltage stress.

Physical and Electrical Characterization of Metal–Insulator–Metal Capacitors With $\hbox{Sm}_{2}\hbox{O}_{3}$ and $\hbox{Sm}_{2}\hbox{O}_{3}/\hbox{SiO}_{2}$ Laminated Dielectrics for Analog Circuit Applications

We report the first demonstration of metal-insulator-metal (MIM) capacitors with Sm2O3/SiO2 laminated dielectrics featuring low quadratic voltage coefficient of capacitance (VCC) and high capacitance density for precision analog circuit applications. In comparison with a HfO2 MIM dielectric, the Sm2O3 MIM dielectric is found to show a smaller quadratic VCC and a similar dielectric constant. We also investigated the cancellation of the positive quadratic VCC of Sm2O3 through its combination with a SiO2 layer having a negative quadratic VCC. Thus, MIM capacitors with a Sm2O3/SiO2 laminated dielectric were fabricated with various Sm2O3 and SiO2 thickness combinations. Capacitors with the Sm2O3/SiO2 laminated dielectric exhibit tunable quadratic VCC and high capacitance density. Very low quadratic VCC at various capacitance densities were achieved. The leakage current mechanism is related to Poole-Frenkel emission at a high positive bias. A smaller quadratic VCC is obtained at higher frequencies. We also conducted an extensive physical characterization of Sm2O3 using transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.

Performance Improvement of $\hbox{Sm}_{2}\hbox{O}_{3}$ MIM Capacitors by Using Plasma Treatment After Dielectric Formation

In this letter, we investigate the dependence of the performance of metal-insulator-metal (MIM) capacitors with Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dielectric on plasma treatment (PT) performed before Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> deposition, after Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> deposition, or both before and after Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> deposition. By performing PT in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient (PTN) after Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dielectric formation, the effective quadratic voltage coefficient of capacitance (VCC) can be reduced from 498 to 234 ppm/V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the effective linear VCC can be reduced from 742.3 to 172 ppm/V for MIM capacitor with Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> dielectric having a capacitance density of ~ 7.5 fF/mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The leakage current density at +3.3 V can be reduced from 3.44 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> to 1.60 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> by performing PTN in both before and after Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> deposition. PTN after dielectric formation is an effective way to improve the performance of high-kappa dielectric MIM capacitors for RF and analog/mixed signal IC applications.

Metal-insulator-metal capacitor with high capacitance density and low leakage current using ZrTiO4 film

Thin amorphous and orthorhombic ZrTiO4 film with a high-work-function Ni top electrode has been explored in metal-insulator-metal (MIM) capacitors for analog circuit applications. It has been found that even though the permittivity can be as high as 78.9 for orthorhombic ZrTiO4, the extraordinarily high quadratic voltage coefficient of capacitance (VCC), and leakage current make it ineligible for reliable MIM capacitors. On the other hand, amorphous ZrTiO4 demonstrates a high capacitance density of 29.12 fF/μm2 and a low VCC of 2341 ppm/V2. Because of the amorphous phase and the conduction mechanism of Schottky emission, a low leakage current of 1.3×10−7 A/cm2 at −2 V and a good thermal leakage up to 125 °C has also been obtained. Besides these promising characteristics, amorphous ZrTiO4 holds a great potential for high-performance MIM capacitors not only in its process simplicity but also in the full compatibility with incumbent backend integrated circuit technology.

Bulk and Interface effects on voltage linearity of ZrO2–SiO2 multilayered metal-insulator-metal capacitors for analog mixed-signal applications

Quadratic voltage coefficient of capacitance (VCC) for ZrO2–SiO2 multilayered dielectric metal-insulator-metal capacitors depends strongly on the stacking sequence of the layered dielectrics. The quadratic VCC of an optimized SiO2/ZrO2/SiO2 stack and ZrO2/SiO2/ZrO2 stack were +42 and −1094 ppm/V2, respectively, despite the same total SiO2 and ZrO2 dielectric thickness in the stack. The observed difference in quadratic VCC depending on dielectric stacking sequence is explained by taking into account both the interface and bulk dielectric responses to the applied voltage.

Effective Modulation of Quadratic Voltage Coefficient of Capacitance in MIM Capacitors Using $\hbox{Sm}_{2}\hbox{O}_{3}/\hbox{SiO}_{2}$ Dielectric Stack

We report the first demonstration of metal-insulator-metal (MIM) capacitors with Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> stacked dielectrics for precision analog circuit applications. By using the ldquocanceling effectrdquo of the positive quadratic voltage coefficient of capacitance (VCC) of Sm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and the negative quadratic VCC of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , MIM capacitors with capacitance density exceeding 7.3 fF/mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , quadratic VCC of around -50 ppm/V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and leakage current density of 1 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at +3.3 V are successfully demonstrated. The obtained capacitance density and quadratic VCC satisfy the technical requirements specified in the International Technology Roadmap for Semiconductors through the year 2013 for MIM capacitors to be used in precision analog circuit applications.

High performance TaYO x-based MIM capacitors

TaYO x -based metal–insulator–metal (MIM) capacitors with excellent electrical properties have been fabricated. Ultra-thin TaYO x films in the thickness range of 15–30 nm (EOT ∼ 2.4–4.7 nm) were deposited on Au/SiO 2 (100 nm)/Si (100) structures by rf-magnetron co-sputtering of Ta 2O 5 and Y 2O 3 targets. TaYO x layers were characterized by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) to examine the composition and crystallinity. An atomic percentage of Ta:Y = 58.32:41.67 was confirmed from the EDX analysis while XRD revealed an amorphous phase (up to 500 °C) during rapid thermal annealing. Besides, a high capacitance density of ∼3.7–5.4 fF/μm 2 at 10 kHz ( ε r ∼ 21), a low value of VCC (voltage coefficients of capacitance, α and β) have been achieved. Also, a highly stable temperature coefficient of capacitance, TCC has been obtained. Capacitance degradation phenomena in TaYO x -based MIM capacitors under constant current stressing (CCS at 20 nA) have been studied. It is observed that degradation depends strongly on the dielectric thickness and a dielectric breakdown voltage of 3–5 MV/cm was found for TaYO x films. The maximum energy storage density was estimated to be ∼5.69 J/cm 3. Post deposition annealing (PDA) in O 2 ambient at 400 °C has been performed and further improvement in device reliability and electrical performances has been achieved.

Influence of Ta and TaN bottom electrodes on electrical performances of MIM capacitors with atomic-layer-deposited HfO2 dielectric

Based on atomic-layer-deposited high permittivity HfO2 films on both Ta and TaN substrates, we compare the influence of different bottom electrodes on electrical performance of metal-insulator-metal （MIM） capacitors. The experimental results indicate that the TaN bottom electrode can give higher capacitance density （7.47?fF/μm2） and smaller voltage coefficients of capacitance （356ppm/V2 and 493ppm/V）, which are attributed to the high quality interface between TaN bottom electrode and HfO2 dielectric films. Moreover, a low leakage current of ～5×10-8A/cm2 at 3V is achieved for both types of capacitors, and TaN bottom electrode-based MIM capacitors exhibit higher breakdown field. Finally, the conduction mechanism of the TaN-based capacitor is studied, showing a Schottky emission at room temperature.

Improvement of Electrical Properties of $\hbox{Ba}_{{\bf 0.7}}\hbox{Sr}_{\bf 0.3}\hbox{TiO}_{\bf 3}$ Capacitors With an Inserted Nano-Cr Interlayer

The metal-insulator-metal (MIM) capacitors were prepared with Ba0.7Sr0.3TiO3/Cr/Ba0.7Sr0.3TiO3 (BST/Cr/BST) dielectric and Pt electrode. The multilayer BST/Cr/BST was sputtered onto Pt/Ti/SiO2/Si substrate. The presence of nano-Cr interlayer affects the electrical properties of the capacitors. The temperature coefficient of capacitance (TCC) of capacitors with 2 nm Cr is about 69% of that of capacitors without Cr. In a previous work, the formation of the TiO2 secondary phase was found after the BST/Cr/BST dielectrics were annealed at 1023 K in O2 atmosphere for 1 h. It is suggested that the nano-Cr interlayer as a catalyst leads to the TiO2 formation during the annealing in O2 atmosphere. The negative value of TCC of BST can be compensated by the positive TCC of TiO2, and the temperature stability in the dielectric constant can be realized for capacitors with nano-Cr interlayer. The voltage stability of BST is also improved with the insertion of nano-Cr interlayer, and the quadratic coefficient in voltage coefficient of capacitance (VCC) of Pt/BST/Cr(2 nm)/BST/Pt is about 30% of that of the BST capacitor without Cr. The effects of Cr thickness on TCC, VCC, dissipation factor, and leakage current density of Pt/BST/Cr/BST/Pt parallel plate capacitors are investigated.

Electrical Properties of the Amorphous BaTi4O9 Thin Films for Metal-Insulator-Metal Capacitors

Amorphous BaTi4O9 (BT4) film was deposited on Pt/Si substrate by RF magnetron sputter and their dielectric properties and electrical properties are investigated. A cross sectional SEM image and AFM image of the surface of the amorphous BT4 film deposited at room temperature showed the film was grown well on the substrate. The amorphous BT4 film had a large dielectric constant of 32, which is similar to that of the crystalline BT4 film. The leakage current density of the BT4 film was low and a Poole-Frenkel emission was suggested as the leakage current mechanism. A positive quadratic voltage coefficient of capacitance (VCC) was obtained for the BT4 film with a thickness of<70 nm and it could be due to the free carrier relaxation. However, a negative quadratic VCC was obtained for the films with a thickness ≥96nm, possibly due to the dipolar relaxation. The 55 nm-thick BT4 film had a high capacitance density of 5.1fF/μm2 with a low leakage current density of 11.6nA/cm2 at 2 V. Its quadratic and linear VCCs were 244ppm/V2 and -52 ppm/V, respectively, with a low temperature coefficient of capacitance of 961ppm/˚C at 100 kHz. These results confirmed the potential suitability of the amorphous BT4 film for use as a high performance metal-insulator-metal (MIM) capacitor.

Improved Stress Reliability of Analog TiHfO Metal–Insulator–Metal Capacitors Using High-Work-Function Electrode

We have studied the reliability of high-κ (κ∼49) TixHf1-xO (x ∼0.67) metal–insulator–metal (MIM) capacitors after constant voltage stress induction. The use of a high-work-function Ni top electrode improves not only the leakage current, and temperature- and voltage-coefficients of capacitance, but also the long-term capacitance variation after stress induction.

Electrical Characterization of Metal–Insulator–MetalCapacitors with Atomic-Layer-Deposited HfO2 Dielectrics forRadio Frequency Integrated Circuit Application

Metal–insulator–metal (MIM) capacitors with atomic-layer-depositedHfO2 dielectric and TaN electrodes are investigated for rfintegrated circuit applications. For 12 nm HfO2, thefabricated capacitor exhibits a high capacitancedensity of 15.5 fF/μm2at 100 kHz, a small leakage current density of6.4×10−9 A/cm2 at 1.8 V and 125°C, a breakdown electricfield of 2.6 MV/cm as well as voltage coefficients of capacitance(VCCs) of 2110 ppm/V2 and -824 ppm/V at 100 kHz. Further, itis deduced that the conduction mechanism in the high field rangeis dominated by the Poole–Frenkel emission, and the conductionmechanism in the low field range is possibly related totrap-assisted tunnelling. Finally, comparison of various HfO2MIM capacitors is present, suggesting that the present MIMcapacitor is a promising candidate for future rf integratedcircuit application.

High density Al2O3/TaN-based metal–insulator–metal capacitors in application to radio frequency integrated circuits

Metal–insulator–metal (MIM) capacitors with atomic-layer-deposited Al2O3 dielectric and reactively sputtered TaN electrodes in application to radio frequency integrated circuits have been characterized electrically. The capacitors exhibit a high density of about 6.05 fF/μm2, a small leakage current of 4.8×10−8 A/cm2 at 3V, a high breakdown electric field of 8.61MV/cm as well as acceptable voltage coefficients of capacitance (VCCs) of 795 ppm/V2 and 268ppm/V at 1 MHz. The observed properties should be attributed to high-quality Al2O3 film and chemically stable TaN electrodes. Further, a logarithmically linear relationship between quadratic VCC and frequency is observed due to the change of relaxation time with carrier mobility in the dielectric. The conduction mechanism in the high field ranges is dominated by the Poole–Frenkel emission, and the leakage current in the low field ranges is likely to be associated with trap-assisted tunnelling. Meanwhile, the Al2O3 dielectric presents charge trapping under low voltage stresses, and defect generation under high voltage stresses, and it has a hard-breakdown performance.

Electrical properties of the Sm2Ti2O7 thin films for metal-insulator-metal capacitor applications

A homogeneous crystalline Sm2Ti2O7 (ST) phase was formed in films grown at temperatures ranging between 100 and 200 °C and subsequently annealed at 900 °C. The ST film had a large dielectric constant of 58, which is similar to that of ST ceramics. The leakage current density of the ST film was low and the Poole-Frenkel emission was suggested as being the leakage current mechanism. The ST film had a negative quadratic voltage coefficient of capacitance (VCC), possibly due to the dipolar relaxation. The 100-nm-thick ST film had a high capacitance density of 5.2 fF∕μm2 with a low leakage current density of 1.34 nA∕cm2 at 2 V. Its quadratic and linear VCCs were −99.5 ppm∕V2 and 11 ppm/V, respectively, with a low temperature coefficient of capacitance of 135 ppm∕°C at 100 kHz. These results confirmed the potential for the ST film to be used as a high performance metal-insulator-metal capacitor.

Investigation on the Electrical Properties of the Ba2Ti9O20 Thin Films for Metal-Insulator-Metal Capacitor Application

Homogeneous crystalline phase was formed for the films grown at 700 and annealed at . However, second phase was also developed for the films grown at temperatures lower than and annealed at . A high capacitance density of along with a leakage current density of at were obtained for a thick crystalline BT film. This film had small quadratic and linear voltage coefficients of capacitance (VCC) of and , respectively, and a small temperature coefficient of capacitance (TCC) of at . A thick amorphous BT film grown at showed a high capacitance density of with a very low leakage current density of at . This amorphous film also showed small quadratic and linear VCCs of and , respectively, with a low TCC of at . These results demonstrate that BT films, particularly amorphous BT films, are good candidate materials for metal-insulator-metal capacitors.

$hboxBaSm_2hboxTi_4hboxO_12$Thin Film for High-Performance Metal–Insulator–Metal Capacitors

A high capacitance density of 4.84 fF/mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a low leakage current density of 4.28 fA/pFmiddotV were obtained for a 138-nm-thick crystalline BaSm <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (BST) film. The 100-nm-thick amorphous BST film exhibited a high capacitance density of 3.91 fF/mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a low leakage current of 1.24 fA/pFmiddotV. The crystalline BST film had quadratic and linear voltage coefficient of capacitance (VCC) of -295 ppm/V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> and 684 ppm/V, respectively, and a temperature coefficient of capacitance (TCC) of -136 ppm/degC at 100 kHz. The amorphous BST film also showed quadratic and linear VCCs of 48.6 ppm/V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and -738 ppm/V, respectively, with a low TCC of 169 ppm/degC at 100 kHz

New BST–silica suspension coating material for dielectric thin films fabricated at low temperatures

A new Ba0·7Sr0·3TiO3 (BST)–silica suspension has been developed as a coating material for dielectric thin films. Using a spin on glass (SOG) wet process, a thin layer consisting of nanoparticles of BST and silica was formed on an Al/SiO2/Si wafer. The BST–silica suspension was made by mixing a BST nanoparticle dispersion with methyl siloxane oligomer. A silicon wafer was coated with the BST–silica suspension using a spin coater and heat treated. The sample was then coated with methyl siloxane oligomer and heat treated again, and finally, a top electrode was applied. The Al/SiO2/BST–SiO2/Al/SiO2/Si stack thus prepared is a metal insulator metal (MIM) capacitor. The electrical properties of the MIM capacitor were evaluated and its capacitance, dissipation factor and voltage coefficient of capacitance were determined to be 1054 pF mm−2, <0·1 and <100 ppm V−1 respectively.