ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia

Characterization of Si3N4 Metal-Insulator-Metal (MIM)Capacitors for Monolithic Microwave Integrated Circuits

(MMIC) Applications

Lee Hock Guan, Mohd Nizam Osman, Asban Dolah, Ahmad Ismat Abdul Rahim, MohamedRazman Yahya and Abdul Fatah Awang Mat

Telekom Research and Development Sdn. Bhd.,Idea Tower I & II, UPM-MTDC, Technology Incubation Centre One,

Lebuh Silikon, 43400 Serdang, Selangor Darul Ehsan.

E-mail: leegtmrnd.com.my

Abstract The fabrication of the MIMcapacitors is illustrated in the paper. Theeffective capacitor areas are designed at 60 x60, 240 x 240 and 380 x 380 Pim2. From the s-parameter measurements, the fabricatedcapacitors showed capacitive capability up tofrequency of 40GHz. Strong parasitic effectwas observed at low frequency whileattenuation effect was observed at highfrequency.

I. INTRODUCTION

Capacitor is widely used in MMIC's technologydesign where the capacitance is included inMMIC circuits in any of four basicconfigurations i.e. an open-circuit transmissionline, coupled lines or interdigitated capacitors,Schottky diodes, and metal-insulator-metal(MIM) capacitors. Coupled lines and open-circuit transmission lines provide fairly lowcapacitance values. For these two capacitor types,the capacitance is dependent on the electricallength of the transmission lines. Therefore, thecapacitance is highly frequency dependent. Theadvantage of these capacitors is that they areeasily fabricated since they require only a singlemetal layer. The most popular type of capacitorfor MMICs design is MIM capacitor because ofthe high capacitance per unit area can beobtained. Therefore, smaller and less costlycircuits can be designed. A schematic of an MIMcapacitor is shown in Fig. 1. The structure iscomposed of two metal plates separated by a thinlayer of dielectric material. Typically, thedielectric material overlaps the first metal layerand the upper metal layer has a smaller area thanthe lower metal layer. This configuration helps tominimize fringing fields to ground and shortsbetween the upper and lower capacitor plates.

Usually the air bridge shown on Fig. 1 is notrequired in the design. It is often designed in tofurther minimize parasitic capacitance. Thetypically dielectric used is silicon nitride Si3N4,of thickness ranges from 0.1 to 0.4 mm. in whichit also has been used in the encapsulation ofMMIC fabrication process. SiO2 and Ta2O5 areother type dielectric films used too. Since thedielectric layer is substantially thinner than thesubstrate thickness, MIM capacitors exhibitsignificant fringing effects, which are a functionof the perimeter. Careful experimentation todetermine the magnitude of this effect forspecific process parameters, such as dielectrictype and thickness, is essential for any stableprocess. Test capacitors should also be includedon the wafer for in-process verification [1, 2].

PLAEDINTERCONNECTLINE\

TOP-LEVEL METAL ,,PLATED AIR BRIDGE

FIRST-LEVEL METAL DIELECTRIC

SEMI-INSULATING GaA

Figure 1: MIM capacitor using air bridge [3].

The model circuit of the MIM capacitor is shownin Fig. 2. The capacitor is represented by C-prime and Res components, whereas metal 2 andmetal 3 is represented by inductances andcapacitances of L M2, C M2, L M3 and C M3[3, 4, 5].

515

.1

ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia

L M: L M: 1Agilent 8722

10-06-2006 14:28:17

Figure 2: Equivalent circuitofMM c ito 0.6a i

_ _ 0.2t / < Xt 5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Figure 2: Equivalent circuit ofMIM capacitor 0.2-0.4-0.6-0.8-1-15 45-15°

II. EXPERIMENTAL PROCEDURE

The fabrication of the capacitor was done using -04

silicon as the substrate material. Three maskswere used, i.e. (1) bottom contacts, (2) Si3N4etching and (3) top contacts. Both masks for the (a)contacts consist of Radio Frequency (RF) pads S1 1for the RF characteristics measurement. Agilent 8722Aluminum was used as the metal contact for both 10-06-2006 14:25:56

top and bottom contacts, which was deposited 0 a 15using thermal evaporator. The thickness of the 0

2

Aluminum is estimated to be around 0.6ptm. 0< 3

Silicon Nitride is sandwiched between these twocontacts as the dielectric material. The deposition I\of the Silicon Nitride was done in spuftering 0

system. The thickness is about 960A. The020406081152fabrication steps are summarized in Fig. 3. Upon Xacompletion of the capacitors, the S-parameter -0 5measurement was carried out using HP 8722 3network analyzer sweeping from 100MHz to 40GHz to determine the RF characteristics of thecapacitors [6, 7].

(b)III. RESULTS AND DISCUSSION s 1I1

Agilent 872210-06-2006 14:14:34

Fig. 4 shows the measured S-parameters of thesputtered Si3N4 MIM capacitors with and three 06/1-5capacitor sizes which are 60 x 60, 140 x 140 and 2

380 x 380ptm. From the s-parameter0\4

3

measurement, the fabricated capacitors are 02 5

showing capacitive effect throughout thefrequency range from 1OOMHz to 40 GHz.Although the capacitors are showing capacitive 02X4V 081 X 0teffect throughout the swept frequency, furtheranalysis need to be carried out to determine its -0

suitability to operate at the designated frequency.From the analysis, the capacitors showed

2

reduction in the capacitance values as thefrequency increases as illustrated in Fig. 5.

(c)Figure 4 S-parameter (Si,) of MIM capacitors withvarious sizes. (a) 60 x 60, (b) 140 x 140 and (c) 380 x380pim2.

516

ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia

One attribute of the capacitance reduces as thefrequency increases is due to slow carriersrespond time at high frequency resulting nocapacitance obtained [8, 9] Thus, capacitancedropped as the frequency increases. In addition,the trend of the graphs also signifies that highattenuation of the capacitance at high frequencyas the capacitance is practically near to zero,which is the natural phenomenon. Hence, thecapacitors might not be suitable to be operatingat high frequency. In Figure 6, the capacitanceversus dimensions is illustrated for both low andhigh frequency. For low frequency (Figure 6 (a)),the capacitance showed reduction as thedimensions increases where else the capacitancesuppose to increase according to the capacitanceformula stated in Equation (1) [10].

c= oErAC=od

Capacitance(pF)

3000.00 X

2500.00

2000.00

1500.00

1 000f1

500.00

0I000.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00

380um 140um 60um Frequency (GHz)

Figure 5: Capacitance vs. frequency corresponds todimensionsCapacitance

(pF)0uuu.uu

(1)2500.00

where C is the capacitance; co is the permittivity

in vacuum; cr is dielectric constant of theinsulator; A is the area of the capacitor and d isthe thickness of the insulator.

Because of the usage of Aluminum as the metalcontacts, the electrodes are easily get oxidized,consequently high parasitic capacitance wasobtained at low frequency. As a results, thedominancy parasitic effect at low frequency onboth the contacts area was observed, which wasrepresented by L M2, C M2, L_M3 and C_M3shown in Fig. 2. It is likely that these parasiticeffect response better at low frequency as at highfrequency (Fig. 6 (b)), the capacitance showedincrement as the dimension increases. Thisfurther strengthens the fact that the parasiticeffect is dominant at low frequency because athigh frequency, slow parasitic carriers cannotresponse to the high frequency changes [8-10],therefore the capacitance obtained increases asbeen stated in Equation (1). On the other hand,the grounding of the devices during measurementwould also cause the parasitic effect to bedominant. Since the capacitors were fabricatedwithout via-hole grounding, therefore theparasitic effect is significant and quantifies [8-10].

2000.00

1500.00

1000.00

500.00

0.000

10GHz_

15GHz j A A

20G l=

100 200 300 400

Dimension (um)

Figure 6 (a): Capacitancecorresponds to low frequency

vs. dimensions

Capacitance

70.06

60.00 30G

50.00

40.0035G

30.00 40G

20.00

10.00u

0.00 "0 100 200 300 400

Dimension (um)

Figure 6 (b):Capacitance vs. dimensions correspondsto high frequency

517

00 I i

Iqa

ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia

IV. CONCLUSION

Results of preliminary experimental fabricationand characterization of Si3N4 MIM capacitor waspresented. Aluminum was used as the metalcontacts and sputtered silicon nitride Si3N4 as thedielectric materials. S-parameter measurementshows that the capacitor is capacitive throughoutthe swept frequency i.e. from 100MHz to 40GHz. However, due to strong parasitic effect atlow frequency and high attenuation at highfrequency, the fabricated capacitors are yetappropriate to be used in MMIC circuits. Furtherimprovement need to be carried out in thefabrication processes and materials used as themetal contacts in order to reduce the oxidation ofthe metal contacts which cause the parasiticcomponent in the measurement. Via-holegrounding should be incorporate in the nextfabrication to reduce parasitic and improve thegrounding during the measurement.

[6] C. H. Ng, K. W. Chew, and S. F. Chu,"Characterization and Comparison of PECVDSilicon Nitride and Silicon Oxynitride Dielectric forMIM Capacitors", IEEE Electron Device Letters,Vol.24, No.8, pg 506-508, (2003).

[7] Zhichun Wang,, Jan Ackaert, Cora Salm,, Fred G.Kuper, Marnix Tack, Eddy De Backer, PeterCoppens, Luc De Schepper, and Basil Vlachakis,"Plasma-Charging Damage of Floating MIMCapacitors", IEEE Transaction on Electron Devices,Vol.51, No.6, pg017-1023, (2004).

[8] Ph Lombard, J.D. Arnould, 0. Exshaw, H. Eusebe,Ph. Benech, A. Farcy and J. Torres, "MIMCapacitors Model Determinationl and. Analysis ofParalmeters Influence,IEEE ISE, June 20-23, 2005,Dubrovnik, Croatia (2005).

[9] Dieter K. Schoder, "Semiconductor Material andDevice Characterization", John Wiley & Sons, Inc,pg 420-427, 455-484 (1998).

[10] "Applications and Design of Thin Film Capacitors",MIC Corporation Report, (1995).

ACKNOWLEDGMENT

The author would like to thank TelekomMalaysia for sponsoring the work throughresearch project R05-0607-0 and Institute ofMicro Engineering and Nanoelectronics (IMEN)for the support in laboratory facilities. Specialthanks to Dr Lim Soo King from UniversitiTunku Abdul Rahman for his consultancy andadvice on this work.

REFERENCE

[1] Giancarlo Bartolucci, Franco Giannini, ErnestoLimiti, and Steven P. Marsh, "MIM CapacitorModeling: A Planar Approach", IEEE Transactionson Microwave Theory and Techniques, Vol. 43,No.4, pg 901-903 (1995).

[2] Jae-Hak Lee, Dae-Hyun Kim, Yong-Soon Park,Myoung-Kyu Sohn, and Kwang-Seok Seo, "DC andRF Characteristics of Advanced MIM Capacitors forMMIC's Using Ultra-Thin Remote-PECVD Si3N4Dielectric Layers", IEEE Microwave and GuidedWave Letters Vol.9, No.9 pg 345-347 (1999).

[3] Sammy Kayali, George Ponchak, Roland Shaw,"GaAs MMIC Reliability Assurance Guideline forSpace Applications", JPL Publication 96-25, pg.58(1996).

[4] I.D. Robertson, S. Lucyszyn,, "RFIC and MMICDesign and Technology", The Institution ofElectrical Engineers, London, pg 90-93 (2001).

[5] Jeffrey A. Babcock, Scott G. Balster, Angelo Pinto,Christoph Dirnecker, Philipp Steinmann, ReinerJumpertz, and Badih El-Kareh, "AnalogCharacteristics of Metal-Insulator-Metal CapacitorsUsing PECVD Nitride Dielectrics", IEEE ElectronDevice Letters, Vol.22, No.5, pg230-232, (2001).

518

ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia

(a)

(b)

(c)

(d)

(f)

(g)

(h)

[1 1zI

(i)

(e) ()Fig. 3 Fabrication processes ofMIM capacitor with cross sectional and top view.(a) deposition of metal; (b) & (c) photolithography to etch metal for bottom contact; (d) formation of the bottomcontact; (e) deposition of Si3N4 layers; (f) & (g) photolithography to etch Si3N4 layers on RF pads; (h)deposition of metal-top contact; (i) photolithography to etch metal for op contacts; (j) completed MIMcapacitors.

519