MEMS Variable Capacitor Versus MOS Variable ...

ESSCIRC 2002

MEMS Variable Capacitor Versus MOS Variable Capacitor For a 5GHz Voltage Controlled Oscillator Manuel Innocent , Piet Wambacq,
St´ephane Donnay,

Harrie A. C. Tilmans, Hugo De Man , and Willy Sansen IMEC, Kapeldreef 75, Leuven, Belgium. Also Ph.D. student at Katholieke Univ. Leuven.
Also prof. at Katholieke Univ. Leuven.

ESAT-MICAS, Katholieke Univ. Leuven. Kardinaal Mercierlaan 94 Leuven, Belgium. [email protected] Abstract MEMS variable capacitors (varicaps) are an alternative to MOS and PN junction varicaps as a tuning element in voltage controlled oscillators. There is however no comparative data available on oscillators with a MEMS and a MOS varicap. Therefore we did a design experiment with two 5GHz oscillators in 0.35 CMOS. The worst case phase noise over the tuning range improves by 5dB when changing a MOS to a MEMS varicap while keeping power consumption and tuning range almost constant. There are however some requirements that need to be fulfilled before MEMS varicaps can be used in mainstream applications: the required tuning voltage should be obtainable in standard CMOS and accidental pull-in of the capacitor has to be avoided.

the version with MEMS varicap uses an inductor in the MEMS technology. The MCM-D [4] technology and the MEMS [5] technology are very similar. They are both a stack of metal and dielectric layers on a glass substrate. Both allow high-Q (Q  50) spiral inductors and large decoupling capacitors. The MEMS process (figure 1) features a sacrificial polymer layer which is etched away. This layer allows structures to move freely. These free structures can be used to make electrostatically tuned varicaps and switches. Bridge metal Sacrificial layer CPW metal Dielectric

1. Introduction MEMS components have recently been introduced in wireless RF systems as an attempt to improve the performance or add new functionality [1]. Examples are MEMS antenna switches, tunable filters based on MEMS variable capacitors (varicaps) [2] and voltage-controlled oscillators with a MEMS varicap [3]. There is however no quantitative data available on the change in oscillator phase noise or tuning range when using a MEMS varicap instead of a more conventional varicap such as a junction diode or a MOS varicap. Therefore we did a design experiment in which we designed two voltage-controlled oscillators (VCO). One VCO uses a MOS varicap and the other uses a MEMS varicap. The rest of the oscillator is as identical as possible to allow a fair comparison.

2. MOS, MCM, and MEMS technology The VCO circuits consist of two parts. The active part (in 0.35 CMOS) is flip-chip mounted on the passive part which is realized in two different technologies. The version with MOS varicap uses an MCM inductor while

Contact metal Substrate       "!#$&%'!)(+*,.-/,10123%+4 5%'6'87:93   ? % @"% ! ? 23 1$A  @ B.%
$C 593 D%34 E"F % @/@ %5B
G H ? 323 >I 7J93 1;+4 > + ? % @ K"!
%3 ? % @ % 63%3F  L M !3 >N;BH%POM 2F"@ %F  J593%
 ' 23 #4  JH% !Q 3@ R%
S M$4  #93%FJ6 @ @ L593 ;+4 > + Q>
T!'

The behavior of a MEMS varicap can be modeled by a mass-spring-damper model (figure 2). A voltage over the varicap results in an electrostatic force between the top and the bottom plate. This force pulls the top plate down to the position where it is in equilibrium with the elastic force in the top plate. When the voltage is higher than a critical voltage, called the pull-in voltage ( U8VW ), there is no equilibrium anymore and the top plate collapses on the bottom plate. This is unwanted for a varicap, but it is the normal operation of a capacitive switch. Mechanical inertia makes the MEMS varicap behave as a lowpass filter on the tuning signal. This is an important difference when compared to an MOS varicap which does not show this lowpass behavior at all.

487

$ $&% c

V

1.4

k m

V=Vbias+vin

Figures 3 and 4 show the tuning curves of the two types of varicaps used in the VCOs. Figure 4 shows no measurements beyond 40V because this is the maximum voltage for the used equipment. $ 2.8 $&%

1 0

10

20

30

0.5 ')(+*,1-

50 'O(*,- 60

(V)

    QP"SRL  ? % @  5 > % 63%3F % !3 )$G593 P,P-/,H0 23%+4 5%'6 %3  $R !3SS ! $ 593 G !+ !F J2F"@ %3 + +L7J93  5G%+ ? %F5  ? !
 6' !
R I593 .@  !3  #93 H93 5 C SS % @ ? '> 3@"E'R >N F "9 93 5 6' !
R:;BDU TF%'6"@ %
S !FW V > 2 $/59+ 23%+4 5%'6 = 7J93 ? 5%35  1 O  X Y[Z % !3> 593 93 5 C SS % @ ? % T ? ? 5 !+ !F .%F  P  593  !+ !F P%
S     4

Design of two oscillators that differ only in the LC tank

This section presents the design of the two oscillators, one with a MOS varicap and one with a MEMS varicap. These oscillators are designed for a superheterodyne wireless LAN receiver in the 5.15-5.35GHz band with an IF of 880MHz. They are optimized for maximum tuning range to overcome process variations and still cover the band between 4.22 and 4.52GHz. Figure 5 shows the schematic of the two oscillators. The only difference between the two is in the LC tank. The MOS varicaps are placed in series with metal/metal capacitors and the bias is applied through large resistors between the two capacitors. The bulk of the MOS varicap is connected to the supply. The MEMS varicaps are connected directly to the active part of the oscillator and bias is applied on the top plate which is an AC ground node. The MOS varicap-based oscillator shows a simulated tuning range of 800MHz and a phase noise bet-

488

 DFEHG/I G IKJ

1.1

MCM

-3 -2.5 -2 -1.5 -1 -0.5

3.

B"C

1.2

 "   ,1%FR* 56= !F *A> % ? 6+  ? '> @$M% ! @ 55+A%
S % @ @ B % 3%F > ,P-/,H0 23%+4 % 6
  D93 > 56 @ %3 ? !
    593 N%+C 5% 3$Q593 6 @ %F +  %'!3> %+C 93 ? %3R  593 N> % ? 6+ !F %'!3> 93 R  !3 5C 56 5S 23 3@ B
  !"  M93 ? 93% !+ % @  5!3%'!3 593 `\H0 TJ 59 ,P-/,H0P23%+4 % 61^4 9
;_57J93 .> R% !3 1;+G  TG  !H593 D5%'63%3F R'G6 @ %
 5 %3/$R !3  "!M$593 J!F' ? % @   5>:2F@ %F  G23  93 ,.-/,10Q23%+4 % 6 ^="[6 _% !3>