A 410-GHz CMOS push-push oscillator with an on

A 410-GHz CMOS push-push oscillator with an on
A 410GHz CMOS Push-Push Oscillator with
Chip Patch Antenna
Eunyoung Seokl, Changhua Caol2, Dongha Shim', Daniel J. Arenas',
David B. Tanner', Chih-Ming Hung3 Kenneth K. 01
'Univeristy of Florida, Gainesville, FL, NXP Semiconductors, Austin, TX
'Texas Instruments, Dallas, TX
The uses of terahertz systems (300GHz to 3THz) in radars, remote
sensing, advanced imaging, and bio-agent and chemical detection
[1, 2] have been extensively studied. A compact and low-cost signal
source is a key circuit block of terahertz systems. Traditionally, the
circuits have been built using highly optimized III-V technologies
[3, 4]. With the advances of CMOS, it has become realistic to consider terahertz circuits in CMOS. This paper reports a signal
source operating near 410GHz that is fabricated using low-leakage
transistors in a 6M 45nm digital CMOS technology.
The maximum oscillation frequency is limited by the unity maximum available power gain frequency, fmax of a transistor. To obtain
even higher frequencies, the push-push oscillator architecture [5,
6] using the second harmonic has been used to generate a 192GHz
signal in a 0.13,m CMOS technology [6]. Figure 26.1.1 shows a
schematic of the push-push oscillator with a patch antenna.
Presently, high frequency probes for measurements above 325GHz
are not available. The on-chip patch antenna is used to extract the
signal. Additionally, the parasitics of a package and interconnections between on-chip components and an external antenna
impede signal transmission at terahertz frequencies. This causes
signal attenuation and unwanted coupling. The use of on-chip
antennas reduces loss and size as well as cost of systems.
Cross coupled transistors M, and M2 form the oscillator core.
Inductors L1 and L2, and the capacitances of the cross-coupled pair
determine the oscillation frequency. At the virtual ground nodes
the out-of-phase fundamental signals are attenuated, and the 2"'harmonic is extracted and radiated through the on-chip patch
antenna. In the push-push oscillator, the cross-coupled transistor
pairs generate differential signals and eliminate the concern for
even-mode oscillation. Quarter-wavelength transmission lines provide a high impedance at the 2nd-harmonic frequency. Two oscillators are implemented. The first oscillator includes buffers to measure the fundamental signal, while the second version does not
include the buffers, which increases the output frequency. The fundamental oscillation frequency of the latter is ~16GHz higher than
that of the oscillator with buffers. The buffers are tapered in order
to lower the capacitive load in the oscillator core (Fig. 26.1.1). At
200GHz, since the low-leakage transistors do not have much gain,
the buffer is expected to attenuate the signal. The primary role is
to isolate the oscillator core from the 50Q load. The total bias current for buffers is lOmA.
For layout symmetry, the core is biased using two PMOS transistors through two quarter-wavelength transmission lines [6]. To
have a higher operating frequency, varactors are not included. The
transmission lines are formed using a grounded coplanar waveguide (GCPW) structure [7]. Compared to the conventional CPW,
the insertion loss is lower because the ground plane isolates the
line from the lossy silicon substrate. The lines are formed using
the top metal layer with thickness of -1.2,am, and the ground
plane is formed using metal 1 and 2 layers. The characteristic
impedance is 50Q and the length is 90gm. The separation between
the signal and ground lines and between the signal and ground
plane are -4gm. The transmission lines and patch antennas are
simulated using Ansoft HEFSS.
The on-chip patch antenna consists of a patch and a ground plane
as shown in Fig. 26.1.2 [8]. The ground plane once again shields
the antenna from the lossy silicon substrate and makes the antenna characteristics to be less dependent on nearby metal structures.
The ground plane also reduces the signal coupling from and to
nearby circuits and metal lines. The patch length is about half
wavelength. The patch size is 200N200km' which is not excessively large. The aluminum pad layer is used to form the patch. The
ground plane is formed by shunting metal 1 to 5 layers to lower the
loss. The thickness of each metal layer is -0.2tm. The separation
between the patch and ground plane is -4tm. The simulated directivity of the patch antenna is 5 and efficiency is 22%. The input
resistance of the on-chip patch antenna is 50Q at 390GHz.
For fundamental output measurements, the buffer output node of
the oscillator is connected to a GGB WR-5 (140 to 220GHz) waveguide probe. The fundamental output spectrum is measured using
an OML M05HWD (140 to 220GHz) harmonic mixer and an
Agilent E4448A spectrum analyzer. The conversion loss of the harmonic mixer is 60dB. Figure 26.1.3 shows the measured spectrum of 189GHz fundamental signal. The output power after deembedding the -60dB harmonic mixer loss is about -27dBm.
Detection of 400GHz signals is challenging. The output power of
push-push oscillator decreases with frequency due to the decrease
of transistor gain and increase of interconnection loss. The harmonics mixer loss also increases with frequency. More importantly, high-frequency probes as mentioned are not available at these
frequencies. Because of these issues, optical techniques are utilized. The 410GHz 2nl -harmonic is measured using a Bruker 113V
Fourier-transform infrared spectroscopy (FTIR) system. The FTIR
system shown in Fig. 26.1.4 consists of a source, an interferometer,
and a silicon bolometer. The oscillator is placed on the source position. A 5mil Mylar beam splitter is used to increase the sensitivity
of interferometer at 400GHz. The scanning speed of interferometer has been changed to distinguish the noise of interferometer.
Figure 26.1.5 shows the measured spectrum. The fundamental signal at 20OGHz is also seen. For the absolute power measurement,
the circuit is placed directly at the input of bolometer. Figure
26.1.6 shows the measured radiated power and operating frequency versus bias conditions. The supply voltage is 1.5V. As the current is increased, the oscillation frequency decreases, and the radiated power increases. The optical responsivity of silicon bolometer
(HD-3, 1378) from Infrared Laboratories is 1.1x10VW. At limA,
the output frequency is ~410GHz and output power is ~2OnW or
-47dBm. This low power is mostly due to the low gain of transistors and losses of thin metal and dielectric layers in the CMOS
process, as well as the mismatch between the tuned frequencies of
patch antenna and oscillator. The efficiency of the patch is in addition limited by the thin dielectric layer between the patch and its
ground plane. In principle, the losses of metal lines and antenna
efficiency can be drastically improved without significantly
increasing the cost by using a back-end process better suited for
terahertz applications. When the bias currenit is varied from 10 to
limA, the frequency can be tuned by -2GHz and the output power
changes by ~3dB. Figure 26.1.7 shows the die micrographs of the
two circuits. The chip area of circuits with buffers and without
buffers is 35OxS40Om2 and 390x640gml, respectively. Despite the
low power, this circuit suggests the possibility of CMOS THz sys-
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vol. 53 no. 11, pp. 3557 -3561 NoV. 2005.
Oscillator Core
Figure 26.1.1: Schematic of push-push oscillator with an on-chip patch antenna.
Figure 26.1.2: Patch antenna structure and HFSS simulated directivity.
-20 I
-30 F
Frequency [GHz]
Figure 26.11.3: Power spectrum of the fundamental output measured using a
Figure 26. 1.4: FTIR measurement setup
harmonic mixer. The -60dB loss of the harmonic mixer is de-embedded.
_A I
1O ESa-
- 0.05
Frequency [GHz]
Bias Current [mA]
Figure 26.1.5: Spectrum of the push-push oscillator measured using a FTIR Figure 26.1.6: Output powers versus bias current measured using a bolometer.
Continued on Page 629
Figure 26.1.7: Die micrographs.
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