A 60GHz radio chipset fully-integrated in a low-cost packaging technology

Abstract

We present a cost-effective plastic packaging technology for a fully-integrated 60GHz radio, used for communication in the 60GHz ISM band. The chipset supports 1-3 Gbps directional links using a ASK or PSK modulation, or it can be used in 500Mbps-1Gbps omni-directional links using an OFDM modulation. The antenna is integrated inside of the package and does not require any high-frequency external connection. The fabrication process of a direct-chip-attach (DCA) and surface mountable land-grid-array (LGA) package technology is presented. Both packages are robust against variations of the electrical properties of standard plastic mold materials

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A 60GHz Radio Chipset Fully-Integrated in a Low-Cost Packaging Technology

U. Pfeiffer, J. Grzyb, D. Liu, B. Gaucher, T. Beukema, B. Floyd, and S. Reynolds
IBM T.J. Watson Research Center
1101 Kitchawan Road, Yorktown Heights, NY 10598
ullrich@us.ibm.com

Abstract
We present a cost-effective plastic packaging technology
for a fully-integrated 60GHz radio, used for communication
in the 60GHz ISM band. The chipset supports 1-3 Gbps
directional links using a ASK or PSK modulation, or it can be
used in 500Mbps-1Gbps omni-directional links using an
OFDM modulation. The antenna is integrated inside of the
package and does not require any high-frequency external
connection. The fabrication process of a direct-chip-attach
(DCA) and surface mountable land-grid-array (LGA) package
technology is presented. Both packages are robust against
variations of the electrical properties of standard plastic mold
materials.
I. Introduction
The use of conventional low-cost chip-scale packaging
(CSP) technologies, like plastic ball grid arrays (PBGA) or
direct chip attach (DCA) technologies has not yet been
reported at mmWave frequencies. In parts this is because
higher frequencies often demand low-loss materials with well
characterized dielectric properties. Furthermore, high-
precision machining is often needed, paired with high-
resolution photo-lithography process technologies and
accurate alignment techniques. Furthermore, complex multi-
chip modules (MCMs) or monolithic microwave integrated
circuits (MMICs) technologies
integration capabilities are often employed to combine circuit
chips within a single package. With recent progress in the
semiconductor technologies, however, the integration level of
mmWave circuits has been dramatically improved to the point
that highly-integrated single-chip I/Q radios are possible [1-
5]. Single-chip radio packages with integrated antennas, for
example, do not have to support conducted I/O frequencies
higher than the base-band I/Q signal bandwidth which is
typically <1GHz. At 60GHz, the antenna dimensions are on
the order of millimeters, which does not add significantly to
the overall package size. The small form-factor of the
integrated antenna package also enables application to phased
array systems to enable digital beam forming or spatial power
combining. To realize the benefits of the integrated antenna, a
novel strategy for the antenna integration is required, which
allows its performance to be as independent as possible from
unknown mold material parameters.
In this paper we present a low-cost packaging fabrication
technique for a receiver (Rx) and transmitter (Tx) chipset
capable of gigabit per second wireless communications in the
60GHz ISM band. Direct chips attach (DCA) and surface
mountable land-grid array (LGA) package technologies are
presented (see Fig. 1). First, the packaging concept and
antenna design is outlined including a description of the
package fabrication method. Next, the electromagnetic
modeling of the antenna is presented, showing the robustness
of the antenna efficiency and return loss against variations in
with passive device
the electrical properties of mold materials. Anechoic chamber
measurements show the performance of the packaged
transmitters after molding. Finally, system link tests using a
IEEE 802.11a-based OFDM-QPSK modulation show data
rates of 630Mbps with up to a 10m coverage range.


II. 60GHz Chipset
The 60GHz chipset is fabricated using a silicon-
germanium process technology with fT and fmax of 207GHz
and 260GHz respectively [7]. A superheterodyne system
architecture was used including a phase-locked loop (PLL), a
voltage-controlled oscillator (VCO), and on-chip image
rejection and intermediate frequency (IF) filters. For
operation in the 59-64GHz industrial-scientific-medical (ISM)
band, the frequency plan results in an IF frequency of 8.4-
9.1GHz, a VCO frequency of 16.8-18.3GHz, and an image
frequency of 42-46GHz. The maximum bandwidth of I/Q
base-band signals is 700MHz. Measured result of such
circuits have previously been published in [2]. The die sizes
are 3.4x1.7mm2 for the receiver and 4x1.6mm2 for the
transmitter.
III. 60GHz Chip-Scale Package With Integrated Antenna
The integrated super-heterodyne circuit architecture has
two advantages with respect to packaging. First, the
frequency conversion to base-band allows the frequency of a
package lead to be substantially lower than the RF or IF
signals. Once the antenna is integrated on the chip or within
the package, only I/Q base-band signals, supply, control, and
bias signals need to be connected from the outside. Second,
the high-integration level does not require any I/Os for
connection to external passive or active IF/RF devices,
providing an inherent advantage compared to lower
integration levels [7]. However, the 60GHz RF signal needs
to be connected to the antenna with low losses and good
impedance match. Unlike alternative approaches, where the
antenna is located directly on the chip, we have connected the
antenna to the silicon chip via a flip-chip ball interconnect.
Figure 1: LGA packaging option left (a) and DCA
option right (b). Each package uses a standard low-cost
mold material. The LGA package can either include a
single chip (Tx or Rx) or both as shown.
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Due to the increase in die area, on-chip antennas are not
considered a cost-effective solution.
Low-cost single-chip plastic packaging technologies are
attractive for cost-effective system designs; however,
mmWave antenna designs are susceptible to assembly and
material parameter tolerances. Hence, a critical factor for the
antenna design is a well-controlled electromagnetic (EM)
environment that can decouple the antenna from its
surrounding environment. The antenna and its reference
ground, therefore, have been fabricated separately from the
package in such a way that lossy mold materials of the
package have only a small effect on the antenna radiation
pattern and impedance bandwidth. The antenna is supported
by a cavity structure which achieves high radiation efficiency
and a large bandwidth, even though standard plastic mold
materials are being used. Fig. 1 shows the DCA and LGA
packaging options that currently exist. Each package uses the
same standard low-cost mold material. The LGA package can
either include a single chip (Tx or Rx) or both as shown in the
figure. The size of the LGA package is 13x13mm2, no more
than 2.8 times the original die plus antenna size. The size of
the DCA package is 7x11 mm2. Optionally, the LGA package
can be equipped with only a single transmitter or receiver
chip, with a size of 6x13 mm2. Note, the cavity-backed
antenna is self-contained and achieves the same performance
in either of the packages.


Figure 2: Fabrication process of the LGA package.
Fig. 2 shows a sketch of the fabrication steps involved for
the LGA package. The antenna radiating element is a half-
wavelength folded dipole. It is fabricated as a separate
component, made out of a fused silica substrate which is
attached to a kovar metal frame prior mounting. First, the
silicon chip is attached to the LGA die-paddle using a
thermosetting adhesive. It is wire-bonded using a standard
gold ball-bonding technique. Next, the antenna is connected
to the chip using a thermal compression flip-chip mounting
technology. Flip-chip interconnects have shown excellent
performance at mmWave frequencies and have been
characterized and modeled up to 110GHz [8]. The flipped
antenna with the covar frame forms an air-filled cavity
together with the package ground directly below the radiating
dipole. The fused silica acts as a superstrate with the radiation
going through it. The cavity provides both the mechanical
support for the antenna and a well-controlled EM
environment, which makes the antenna design less sensitive
to the surrounding package-level dielectric and metal layers.
This build-up leads to the shortest possible interconnect to the
antenna without the need for vias in the antenna substrate.
The antenna feed-line passes through a dedicated opening in
the frame which was encapsulated prior to the mounting of
the antenna. In this configuration, the folded-dipole antenna
achieves 7-dBi gain and ±30o 3-dB beam-width. The
interconnect shows a very large impedance bandwidth of over
30% defined by a 10-dB return loss and a radiation efficiency
above 90% over the entire ISM band.
IV. Electromagnetic Modeling
The metal frame considerably influences the antenna input
match and its radiation pattern. It defines a cavity depth of
500µm, being a trade-off between the available chip
thicknesses and the required antenna bandwidth. The internal
cavity size is 3x4.2mm2 and its metal walls are 400µm thick.
The 1.46mm long dipole is fed by a quarter-wavelength
transformer line followed by a uniform 100Ω CPS line in
series with a tapered CPS line. The lateral dimensions (strip
and slot width) of the tapered line were adjusted so that it
accommodates the influence of the mold material on the feed
structure. The dielectric constant and loss tangent of the mold
were 4.4 and 0.03 at 60GHz, respectively [12]. Fig. 3 shows
the 3D physical model used for full-wave electromagnetic
(EM) simulations of the antenna performed with HFSS. Note
that only some length of the feed line between the chip and
antenna located outside of the frame is covered by the
encapsulant. The antenna radiates through the fused silica
substrate only and the mold material flowing around the
antenna frame has only minor influence on the antenna
performance.
The shape and size of the cavity was optimized in view of
the antenna input impedance bandwidth. Another important
parameter is the strip width of the dipole and the spacing
between its front and rear part. Two additional metal bars
were placed on the top of the fused silica substrate to
minimize the radiation in the antenna plane.
The antenna with its surrounding cavity was measured
stand-alone in an anechoic chamber using a probe-tip based
measurement setup [10]. The measured return loss is better
than -15dB from 56-65GHz. Very good agreement between
the measurement and simulation was found. The simulated
antenna efficiency is above 90% (see Fig. 4). The lossy
encapsulant reduces the antenna efficiency by only 5%.

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Figure 3: Geometry used for full-wave electromagnetic
simulations performed with HFSS. The orientation of the
figure is such that the radiation is going downwards.

We have shown in [13] that the similar antenna, however
designed not to be encapsulated, is able to preserve most of its
input impedance bandwidth even for ±20% and ±33%
changes in the cavity depth and cavity area, respectively. For
the antenna presented in this paper we also investigated the
influence of the dielectric constant and loss tangent
uncertainties of the encapsulant material on the antenna
performance. Fig. 4 shows the HFSS simulated antenna
efficiency for the loss tangent ranging from 0 to 0.1.
Dielectric permittivity of 4.4 was assumed in the simulations.

55 57 59
Frequency [GHz]
61 636565
75
80
85
90
95
100
Efficiency [%]
tanδ = 0.03
tanδ = 0
tanδ = 0.02
tanδ = 0.04
tanδ = 0.06
tanδ = 0.1

Figure 4: HFSS simulated antenna efficiency vs. loss
tangent of the mold material. Dielectric permittivity of 4.4 was
assumed in simulations.
It can be seen that antenna encapsulated using a very lossy
material with a lost tangent of 0.1 still shows radiation
efficiency higher that 80%. The influence of the dielectric
permittivity uncertainty of the encapsulant being used on the
antenna return loss is shown in Fig. 5. A high dielectric
constant variation between 3 and 5 practically does not impact
the antenna input impedance bandwidth. The corresponding
antenna gain changes by less than 0.2 dB across the
considered bandwidth.

555759
Frequency [GHz]
6163 6565
−45
−40
−35
−30
−25
−20
−15
−10
S11 [dB]
εr = 3
εr = 3.3
εr = 3.6
εr = 4
εr = 4.4
εr = 4.6
εr = 5

Figure 5: HFSS simulated antenna return loss vs.
dielectric permittivity of the mold material. A loss tangent of
0.03 was assumed in simulations.

V. Packaged Chipset Performance
The packaged chipset performance was measured at
various steps during the packaging process. The transmitter
chip was mounted on a board and measured in an anechoic
chamber. This allowed the Tx radiation pattern and large-
signal gain compression to be measured simultaneously.
These measurements require an accurate gain calibration, I/Q
input power calibration, and interconnect cable loss
calibration. All measurements were done with the DCA
package using a 100-MHz CW I/Q input tone. The
measurement of the overall gain in the main radiation
direction includes the chip's conversion gain, the antenna
gain, interconnect and packaging related losses. Measurement
accuracy is within 1dB through careful calibration. The
vertical radiation pattern after molding with a standard low-
cost material is shown in Fig. 6. The data was taken along the
theta=0-120o direction for phi=0o at 61GHz. The cross-
polarization is 20-dB down (not shown). The overall gain in
the main radiation direction is around 41dB with a 3-dB
beam-width of ±30o. The pattern is smooth with little package
and board-level interference. The molded package shows a
slightly higher disturbance at ±30 due to refraction at the
package boundary.
Receiver and transmitter boards have been used in system
link tests to validate the link budget and to show achievable
data rates and the maximum coverage range. A 700Ms/s ARB
was used to generate Tx baseband IQ modulation and a
700Ms/s 8bit PCI ADC including a software demodulator was
used to demodulate the Rx IQ output. An IEEE 802.11a-
based OFDM-QPSK modulation was used at 630Mbps for
system performance testing. At 630Mbps, the link ran with a
<10% frame error rate (FER) up to a maximum separation of
10m. This result agrees with the expected coverage range
based on system link budget computations and validates the
inferred package interconnect loss and antenna gain. Further
system performance evaluation will include high-gain (20-
dBi) antennas with ASK/PSK modulation for directional 1-
3Gbps applications. High-gain antennas can be used to extend
the coverage range of 60GHz directional links into the km
range. Future work will investigate other packaging options to
support a waveguide interface to external high-gain antennas.

Mold
Cavity
Superstrate
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Figure 6: Radiation pattern along theta after molding of
the chip and antenna at 61GHz.
Summary
A packaging fabrication technique with integrated
antennas for highly-integrated mmWave chipsets was
presented. A 60GHz radio was packaged in a direct chip
attach or land-grid array that provides a cost-effective
packaging solution for gigabit per second wireless
communication in the 60-GHz ISM band. The transmitter
chip achieves 3-dB beam-width of ±30o with a 41dB total
conversion gain in the main radiation direction. The chip and
its antenna have been encapsulated using standard mold
materials with an interconnect loss that is less than 2dB across
the ISM band. The cavity-backed antenna decouples the
design from the physical properties of the package such as
dimension and mold material. The antenna performance
shows an extremely low sensitivity to the cavity
manufacturing tolerances and uncertainties of the mold
material dielectric properties, being a key factor enabling the
use of the presented low-cost packaging technology at
mmWave frequencies. Envisioned applications of the
packaged chipset include 1-3 or more Gbps directional links
using ASK or PSK modulation and 500Mbps to 1Gbps omni-
directional links using OFDM modulation. The results
demonstrate that mmWave chipsets can be packaged today,
with a level of integration that is unique for millimeter-wave
frequencies systems.
Acknowledgments
The authors would like to thank the IBM SiGe technology
group (IBM Burlington, Essex Jct., VT 05452, USA) for chip
fabrication and NASA (NAS3-03070) and DARPA (N66001-
02-C-8014 and N66001-05-C-8013) for partial funding. The
authors thank T. Zwick for early antenna developments, D.
Beisser for layout support, R. John for encapsulation and
wire-bonding, M. Soyuer and M. Oprysko for their support
(all with the IBM T.J. Watson Research Center, Yorktown
Heights, NY 10598, USA).

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13462006 Electronic Components and Technology Conference