2004 RF AND MICROWAVE CONFERENCE, OCTOBER 5 - 6,SUBANG, SELANGOR, MALAYSIA

Performance Comparison of MIM Capacitors and Metal Finger Capacitors: for Analog and RF ApplicationsQueennie S . I. Lim', Albert V. Kordesch2, and Richard A. Keating2'Department of Electrical, Electronics and System Universiti Kebangsaan Malaysia 43690 Bangi, Selangor, Malaysia 'Silterra(M) Sdn. Bhd.

09000 Kulim, Kedah, Malaysia

Abstract

- In this work we compare silicon nitride MIM capacitors to metal finger capacitors and clarify the performance and cost trade-offs. Our measurements show that MIM capacitors are superior to metal finger capacitors in terms of matching and process tolerance, but equivalent metal finger capacitors have higher Q, lower voltage coefficient and lower temperature coefficient.1.

Introduction

Today's mixed-signal CMOS and RF CMOS processes offer a wide variety of capacitors to designers: Metal-oxide-semiconductor (MOS) capacitors, Poly-insulator- poly (PIP) capacitors, metal-insulator-metal (MIM) capacitors, junction capacitors, gate capacitors, metal plate capacitors, and metal finger capacitors. When designing an analog CMOS or RF CMOS circuit, a designer has to choose between these available capacitors based on their desired properties such as linearity, quality factor, breakdown voltage, area, sensitivity to process variation, temperature coefficient, matching, cost and performance. The desirable characteristics of an integrated capacitor are low cost, small process variation, low temperature coefficient, low voltage coefficient, low leakage, low noise, low hysteresis, low parasitic ground capacitance, high capacitance density, good matching, and of course high reliability and high yield. For RF circuits the additional requirements are high self-resonance frequency, high quality factor, and constant capacitance over the usage frequency range. High breakdown voltage in capacitors is also desired in Wdesigns [I]. Precision low frequency analog circuits such as sample and hold, switched capacitor filters, amplifiers, A/D and DIA converters have stringent requirements for capacitors. Even though MOS capacitors and junction capacitors have high capacitance density, they are difficult to use for these circuits because they are non-linear capacitors, that is its capacitance changes with applied voltage. Furthermore, these capacitors are

very dependent on temperature [3]. In addition, they have a very poor quality factor due to high series resistance. Designers can choose the parallel plme metal-to-metal capacitors to achieve better linearity and quality factor. However, parallel plate capacitors suffer from a low capacitance density, as a result of large vertical spacing between the metals. In order to increae the capacitance density of parallel phte capacitors, interdigitated or metal finger capacitors were introduced. PIP capacitors have been used for analog applications in the past but in sub-micron CMOS, these capacitors have been replaced by MIM capacitors. The MIM capacitors have much better voltage linearity, lower series resistance, much lower parasitic capacitance, excellent matching and the fabrication process has less hea< which is important for sub-miaon transistors. In this work we will compare the MIM capacitors to the metal finger capacitors and clarify their tradeoffs.2.

T h e Comparisons

To probe further into the tolerance and variation properties of the capacitors, parametric 11% data of the MIM cap capacitances are taken from I65 sites on different wafers while for finger capacitors, measurements are taken across 30 sites of an 8-inch wafer with each site containing six different sizes of finger capacitors. MIM capacitors have less proccss variation compared to finger capacitors, as seen in Figure 1. Hence, manufacturing tolerance of the MIM capacitors can be much tighter. The MIM capacitance is determined by the thickness and the dielectric constant, k of the silicon nitride insulator between Ihe two plates. Most of the capacitance in the finger capacitors comes from the sidewall (fringing) capacitance between the metal lines, so the metal line space is the dominant factor. Silicon nitride deposition shows a higher accuracy and repeatability than Ihe lithography and etching process for finger capacitors. Capacitances in MIM capacitors demonstrated a

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standard deviation of 0.105% whereas the finger capacitors display a sigma of 0.652%. Evidently, the percentage of line width variation for the finger capacitors is much larger than the layer thickness variation of the MIM cap; so metal finger capacitors have a much larger tolerance window.Std dev = 0.652%

because the capacitance density (Plum') is lower, resulting in greater chip area and the net cost may actually be higher. Capacitance density for Silterra's MIM capacitors is I.0ff/um2, while the capacitance density for finger capacitors with five layer metals is 0.8ff/um2.E

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Figure 1: Process variation for (a) finger capacitors and (b) MIM capacitors respeclively Due to smaller process variation and manufacturability tolerances, MIM capacitors also demonstrate superior matching properties compared to the finger capacitors. So if matching is the major concem, MIM capacitors are the better choice. Figure 2 shows the measured mismatch of the MIM capacitors and the finger capacitors by obtaining the standard deviations of the difference of the normalized capacitance for each capacitor pair. The process control and uniformity for silicon nitide deposition is better than the minimum feature of lithography and etch, so we would expect the finger capacitors to have the worse mismatch. From the result, the capacitance matching properties of the MIM capacitors are three times better than the finger capacitors. MIM capacitors have better tolerance and matching properties, they require one extra mask layer for fabrication. Metal finger capacitors don't require any e x t a layers or extra process steps, so the process manufacturing cost for finger capacitors are lower. However there is a cost trade-off in finger capacitors

Figure 2: Capacitor matching results for (a) finger capacitors and (b) MIM capacitors Finger capacitors portray a much lower voltage dependency and temperature dependency than the MIM capacitors, as shown in Figure 3 and Figure 4. Temperature coefficient for the MIM capacitors is around 35ppmPC whereas for finger capacitors, temperature coefficient is at 15.32ppmPC. MIM capacitors attain higher voltage coefficient with its first order voltco as 14.22 p m N and its second order E voltco as 1.577ppmN. In tum, finger capacitors present better voltage linearity with first order voltco of O.5ppmN and second order voltco of 0.09 ppm/V'. Figure 3 and 4 show the voltco and tempco linearity for the respective capacitors. Figure 5 shows that finger cap leakage is lower than the leakage in MIM capacitors because silicon dioxide in finger capacitors is a better insulator than the silicon nitride in MIM capacitors. The dielectric constant for silicon nitride and silicon dioxide is around 6-8 and 4.5 respectively. In addition, finger cap demonstrates a much lower leakage than MIM at high voltages. From Figure 5 , it is clearly seen

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the voltage of 40V that leakage for finger capacitors ,ound 0.02 fNum') is significantly lower than the kage for MIM cap, which is approximately 28 Jum'

capacitance area for finger capacitors is much higher than of the MIM capacitors. It is believed that smaller capacitors receive an automatic shrink in parasitic capacitance 111. Higher parasitic capacitances also often come together with noise pickup from the substrate or from the nearby structures [4].

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Figure 3: Increasing trend of (a) finger capacitor capacitance and (b) MIM capacitor capacitance measured at various temperatures Figures 6 and 7 compares the quality factor (Q) and the capacitance versus frequency for MIM and finger capacitors. The finger capacitors typically have Q around 300 or more, up to IOGhz. Compare this with the Q of the MIM cap in Figure 6. Q is less than 40 at 2 GHz and drops to zero at the self-resonant frequency, just above 8 GHz. Finger capacitors have shorter metal lengths, this reduces the series resistance that will eventually lead to a higher Q. The selfresonance frequency is the point where C vs. F crosses zero in Figure 7. Self-resonant frequency for finger capacitors is significantly high. The inherent effects of smaller physical dimensions can explain the behavior of a higher Q. Series resistance will be reduced proportionally with the decrease of dimensions. This will in tum increase the self-resonant frequency of the finger capacitors [I]. Generally, finger capacitors devices are more stable across various frequencies when compared to its countetpart The parasitic capacitance of the lower plate to ground is much less for a MIM cap (typically 6.4%) compared to a Finger cap (typically 16.2%). Effective

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7 (b)Figure 7: Capacitance versus frequency for (a) finger capacitors and (b) MlM capacitors respectively Finally, Table I summarizes the properties of both the MIM capacitors and the finger capacitors. Table 1 MIM HIGH HIGH MEDIUM LOW LOW LOW MEDIUM LOW HIGH LOW HIGH HIGH HIGH LOW Metal Finger LOW MEDIUM HIGH MEDIUM LOW LOW LOW MEDIUM LOW HIGH MEDIUM HIGH LOW HIGH

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cost Area Capacitance density Process variation Temperature Coefficient Voltage Coefticient Leakage Parasitic Capacitance Matching Quality factor Frequency dependence Yield Parasitic Inductance Self resonance frequency

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3. ConclusionMIM capacitors are superior to metal finger capacitors in terms of matching and process tolerance, but metal finger capacitors have higher Q, better voltage coefficient and temperature coefficient, and are cheaper to manufacture. The standard deviation of capacitance for MIM is less than 0.11% (one sigma) compared to 0.6% (one sigma) for metal finger capacitors. Mismatch for capacitor pairs is 3 times worse for metal finger capacitors Metal finger capacitors typically have Q around 300 or higher, up to IOGHz while for similar MIM capaciton Q is less than

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40 at 2 GHz and drops to zero at the self-resonant frequency.

AcknowledgmentThe authors would like to thank Sia Choon Beng, the Device Modeling Group and the Integration Group from Silterra Malaysia Sdn. Bhd. for their contributions and help.

References[ I ] Aparicio R. and Hajimiri A., Capacity Limits andMatching Properties of Integrated Capacitors,

IEEE Jo-umal-of Solid-SroteQrcuit, voi. 37, pp. 384-393, March 2002. [Z] Chunqi 6..Anh D. M , Zheng Z.and Bo) land I.,A Scalable

KF Mudrl of the Mrul-Oxide-Mrlal (MOM) Capacitor, in Technicd Proceedings U/ rlie 2001 lnremnnonal Con/erence on Modeling andSinrulmion o/MIcros)srems, 2001,482-485

[3] Samarati H., Hajimiri A. and Shahani A. R., Fractal Capacitors, IEEE Journol o Soli& f SfoleCircuif, vol. 33, pp. 20352041, December 1998. [4] Rajagopalan I. and Basit H., Optimization c.f Metal-Metal Comb-Capacitors for RF Application, Publication in OEA /nrernoriona(. Inc, 2003. [ 5 ] Remmel T., Zurcher P., Kim M., Miller M., Raymond M., Straub S., Tarabbia M., Steimle 5.. and Chu P., Integration of Thin-Film Capacitors and Resistors into ULSI Technology for Wireless Application, Passive Component Induslry, pp. 24-30, IanuarylFebruary 2001. (61 Keating R. A., Silterra CMOS18 Mixed-Signal and RF Device Characterization Report, Rev. 3.0, pp. 20-24, June 2003. [7] TestChip Technologies, Inc., CMOS18 Analog Circuits, Rev. 1.O, August 2004. [8] Advanced RFlC (S) Pte. Ltd., RF Device Characterization and Modeling of MlM Capacitors for Silterra Malaysia Sdn. Bhd. 0.18um Mixed-Signal R F CMOS Process, Rev. 0.11. January 2004.

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