EP1052726B1

EP1052726B1 - Manufacturing method for a slot coupled micromachined waveguide antenna

Info

Publication number: EP1052726B1
Application number: EP20000870097
Authority: EP
Inventors: Hocine Ziad; Ezzeldin Soliman; Guy Vandenbosch
Original assignee: Katholieke Universiteit Leuven; Interuniversitair Microelektronica Centrum vzw IMEC
Current assignee: Interuniversitair Microelektronica Centrum vzw IMEC
Priority date: 1999-05-05
Filing date: 2000-05-05
Publication date: 2008-02-20
Anticipated expiration: 2020-05-05
Also published as: EP1052726A1

Description

The present invention relates to a method for manufacturing slot-coupled micromachined waveguide antenna.

State of the art

Microwave antennas are usually to emit radiation in a medium such as air, ..., with a sufficient precision of directivity and are to be sufficiently small. State of the art millimetre wave antennas are, thanks to the antennas scaling laws, inherently small enough for being arrangeable in highly directive arrays. The larger the number of radiating elements in the array, the more directive the antenna can be. If the directivity of one element is already high, the number of elements required for reaching a target directivity will be smaller, and therefore the antenna itself can be smaller.
The state of the art directive antennas in the millimetre wave range are planar antennas, with patch antennas being the most widely used.
These planar antennas are very attractive for use within compact communication systems, (telecommunication, WLAN) owing to their simple integration with driving electronics and microwave circuits. However, they suffer from two serious disadvantages which are the limited bandwidth and the substrate losses.
Millimetre wave antennas furthermore find applications within the automotive market as a FLAR (Forward Looking Automobile Radar) or as automobile sensors.
Thus the millimetre wave portion of the spectrum is used for at least two important applications requiring small and directive antennas:

Wireless data transmission (38 GHz).
Automobile anticollision radar's (77 GHz).

Automotive applications are considered one of the two most important applications [H.H. Meinel, "Commercial Applications of Millimetre Waves. History, Present Status, and Future Trends", IEEE Transactions on Microwave Theory and Techniques. Vol.43. Nr 7, July 1995, pp 1639-1653.] in the millimetre wave communication range, the second being the short haul transmission links for PCN installations.
A FLAR is a radar used for measuring the relative velocity between two vehicles in a lane, and the distance between these vehicles, in order to issue warnings to the vehicle drivers. The FLAR consists of different parts:

A set of antennas (Tx and Rx) for emitting a millimetre wave signal, and receiving the corresponding signal echoed by any obstacle on the lane, both signals propagating in the air.
MMICs (Monolithic Microwave Integrated Circuit) called "transceiver", which comprises mixers, low noise amplifiers, power amplifiers, and an oscillator. The transceiver insures the generation of a millimetre wave oscillation, the mixing thereof with a signal of lower frequency (IF), and the amplification before emission. The transceiver also insures the recovery of the millimetre wave echoed signal and its downconverting to IF frequency range.
Analogue and digital processing units.

The FLARs are to be designed for operation at different frequencies, depending on the geographic area:

in Europe, at 76,5 GHz
in Japan, at 60 GHz
in the United States, there are several frequency bands allocated by the US Federal Communications Commission (F.C.C) for traffic radar's, including the 10.5 GHz and 24.1 GHz frequencies and the 33.4 to 36 GHz range. Radar's operating at 94 GHz are also under development.

Radar systems for the automobile have been studied for more than 20 years by major car companies in collaboration with RF companies and chips manufacturers.
Basically, there are two dominating factors that drive the technology for millimetre wave automotive radar's: cost and hardware size. Low cost is the key factor for consumers to accept the radar as a safety and affordable component in their vehicle. The size constraint is essential for easy integration of the radar on the vehicle without major impact on the vehicle design and performance.
An antenna for an Automobile anticollision radar (77 GHz) system needs to fulfil the requirements of scanning the road ahead. Thus, radar techniques are used requiring antennas which are:

Directive: there should be no confusion between in line and adjacent lanes.
Compact: The antenna size should not be detrimental to the car aesthetics. Furthermore, an easy and straightforward link between stearing electronics and the antenna should be feasible.

Most of the radar developments so far are built around GaAs (or other III-V) MMIC's, but also other developments have been going on. Daimler Benz is active in developing SIMMWIC and has reported mid 1995 the successful fabrication of

schottky diodes for mixers
PJN diodes for switches
low noise oscillators using SiGe HBT's
IMPATT diodes for mmwave power generation on high resistivity Si (> 10k.cm). [J F. Luy et al, "Si/SiGe MMIC's". IEEE Transactions on Microwave Theory and Techniques. Vol.43. N04, April 1995, pp 706-719 and A. Stiller et al, A Monolithic Integrated Millimetre Wave Transmitter for Automotive Applications. IEEE Transactions on Microwave Theory and Techniques. Vol.43. Nr 7, July 1995, pp 1654-1657.]

Another approach followed by Hughes Research Labs is the flip-chip mounting of GaAs MMIC chips on low cost duroid substrates. Successful realisation of a mixer for the 77 GHz by flip-chip mounting GaAs schottky diodes was reported [R.S. Virk et al, "A Low Cost W-Band MIC Mixer Using Flip-Chip Technology", IEEE Microwave and Guided Wave Letters. Vol. 7, Nr 9, September 1997, pp 294-296.]
Therefore, the cost of devices fabricated in a III-V process technology remains higher than silicon-based devices.
Furthermore, it is known that the radar size can be reduced by the following measures

By higher degrees of integration i.e. regrouping the different functional blocks of the communication system. This is not straightforward in practise: trade-offs are imposed on the performance of the different elements brought together;
By stacking several chips; ([M. Stotz et al, "Planar Millimetre Wave Antennas Using SiNx Membranes on GaAs", IEEE Transactions on Microwave Theory and Techniques, Vol.44. Nr 9, September 1996. Pp 1593-1595.].
By suitable antenna design. The radar size however cannot be reduced to less than the area occupied by the antenna. In practise, the antenna size is fixed by the radar specification of directivity.

Thus, there is a need for the development of a millimetre wave communication device that includes an antenna

which can be fabricated at low cost but at the same time with sufficient precision;
which has a sufficient directivity;
that is compact and can be integrated with other electronic components of the device;
that has a sufficiently large band width and is sufficiently efficient.

Several documents, among them WO96/27913 , US-5,724,049 and EP-0939451 (not yet published at the date of the priority of the present application), are describing microstrip-to-waveguide antenna's. The main problem of using such technology is that the thickness of the dielectric and conductive layer is rather important. Another drawback of this microstrip technology is that an additional impedance matching between the slot and the external space is required. For instance, in documents US-5,724,049 and EP-0939451 , it is described that a dedicated dielectric layer is added in order to obtain the desired impedance, which will avoid undesired reflection of signals. Furthermore, the directivity of said devices will not be high enough to be used in several applications, and more particularly for automobile anti-collision radars.

Aims of the invention

An aim of the present invention is to provide a method for manufacturing a slot-coupled micromachined waveguide antenna for millimetre wave communication device applications. These antennas are millimetre wave antennas emitting radiation, having high directivity and high efficiency. The antennas can be used for instance for telecommunications and for automotive radars.

Summary of the invention

The present invention relates to a method according to claim 1.
This method can further comprise the step of filling said cavity with a polymer, preferably BCB, the substrate being an MCM-D wafer.
In an embodiment of the method of the invention, the substrate can comprise Si and the first insulating layer can comprise a Si-oxide layer and a polymer layer, said polymer preferably comprising BCB.

Short description of the drawings

Figures 1a to 1d are representing two distinct embodiments of the cavity of the antenna. Parameter values are given in Table 1 and Table 2.
Figure 2a to 2f illustrate a method of fabricating the device according to the present invention.
Figure 3 shows a radar device.

Detailed description of several embodiments of the invention

The present invention is described in details based on a device having the following features:

Advanced micromachining (deep dry etch), microelectronics (SiGe/Si HBTs) and assembly techniques (flip-chip) are combined with CMOS processing, to make a self packaged, low cost, small size transmitter/receiver device. The high precision of Silicon manufacturing techniques allows for making a device with high precision.
The transmitter/receiver antenna can be designed for minimised size and maximal directivity.
The device can be used in the automotive market (FLAR), but it can easily be adapted for other automobile sensors (Doppler radar) and to other areas of applications (namely millimetre wave communications, such as in WLANs).

It is a MCM-D wafer, on which several circuits for system integration with the antenna are assembled.
In this embodiment of the present invention, it is preferred:

To use highly resistive Si as the substrate of the antenna. The RF functions can be integrated in the Si substrate or can be defined in other components, for instance in CMOS technology, CMOS-SiGe, bipolar technology or III-V technology, that are mounted on the first metallic layer. This mounting step can be achieved for instance by flip-chip mounting, ball-grid array technology and other techniques known in the art. It is nowadays practical to define the high frequency (>5 GHz) RF function in another substrate material than Si.
To adopt a directive antenna element design for the antenna, and to machine this element (alone, or arranged in arrays) into the bulk of the resistive silicon backside.

Because the active element chips in a millimetre wave transceiver device are made in a variety of technologies, an attractive direction for its fabrication is the bonding of these multiple chips on one platform, which might support passives as well. Such a platform is available in the multilayer thin film technology as used in the MCM-D, technology which is well known in the art.
Preferred embodiments of the present invention which are millimetre wave antenna micromachined on an MCM-D silicon platform are described hereunder. It is designed for radiating above 20 GHz. The radiating aperture of this antenna is a micromachined waveguide. The aperture is etched in the bulk of the silicon substrate. The cross section of the micromachined waveguide can take a rectangular (Figs. 1a and 1b) or a circular (Figs. 1c and 1d) cross section or a cross section of any geometry. The cavity may be filled with a low loss dielectric, material such as a polymer material (BCB), to shrink the antenna dimensions.
The proposed antenna's are fed by a coplanar waveguide (CPW) realised on the side of the substrate of the antenna facing the MCM-D substrate but separated therefrom by a dielectric material. The electromagnetic coupling from the feeding CPW to the antenna is achieved through a slot etched in the metal base of the aperture waveguide. The coupling slot has the same shape as the waveguide cross section.
Figs. 1a and 1b show a rectangular opening (slot) (37a) in the base of a micromachined rectangular waveguide (33a) while Figs. 1c et 1b show a circular opening (slot) (37b) in the base of a micromachined circular waveguide (33b).
The feeding CPW (23) are also shown on these Figures. Several radiating ends of the feeding CPW are used, such as open, short, capacitive, and inductive ends.
The frequency of the signal radiated by the antenna is the frequency of the signal propagating in the CPW waveguide, and fed to the antenna by electromagnetic coupling through the slot. In other words, the radiated wavelength is basically not set by the waveguide dimensions. The waveguide dimensions actually define the cut-off frequency of the respective modes which could be excited by a fed signal of given frequency.
A cylindrical aperture antenna is described on Fig. 1c. The walls of the cylinder are sputter coated with TiW/Au. An opening is made in this metal in the bottom of the cylinder, in order to feed (excite) the antenna located on chip's backside through a microstrip tine leaving the oscillator, and located on the frontside of the substrate.
In order to cancel the high order resonating modes of the antenna, the hole is either

filled with a low loss dielectric
or half wavelength deep.

In order to make the antenna a resonator, the following is done:

The cylinder is etched in Si, using a deep dry etch recipe from STS.
A photolithographic resist layer Shiplev PEPR 2400 [S. Linder et al. "Photolithography in anisotropically etched grooves", Proc. IEEE MEMS Worskhop San Diego. CA, pp 38-43.] based on combined planar and non planar technology (the term "non planar technology" refers in this case to the metallic waveguide) will be electroplated on MMIC chip's backside. An opening in resist is performed, and the slot etched (the front side being resist protected).
BCB can be dispensed in the cavity (or if not possible, the depth of this cavity will equal half wavelength).

A rectangular aperture antenna can be manufactured similarly.
The directivity of the integrated antenna was evaluated analytically. An empty waveguide was assumed for these first calculations (no BCB filling).
From results summarised in Tables 1 and 2 it can be shown that:

the circular aperture antenna compares in size with the micropatch, but is more directive (7.4dB against 5.4dB for the micropatch) and obviously more robust. The BCB filling is expected to further improve the directivity of the circular aperture antenna.
the rectangular aperture antenna is even more directive (9.8 dB against 5.4 dB for the micropatch), it is observed however that the rectangular aperture antenna features a larger size than the circular one (there is room in a rectangular aperture for two rectangular micropatch. And a two elements array factor should therefore be considered).

1. Production of an antenna according to the invention

As represented in Figs. 2, the process of fabrication of an antenna according to the present invention starts with a double side polished low resistivity silicon wafer (antenna substrate) (31), wherein the following steps are performed:

For DRIE dry etching through the silicon wafer (31), a thick oxide layer (22) is CVD deposited on the wafer backside, and further patterned into an oxide hard mask (29) (see Fig. 2a).
The MCM-D circuit (100) can be made on the front side of a silicon wafer using either spin-on dielectric (thick BCB or polyimide), and metal deposition/patterning as described in J F. Luy et al, "Si/SiGe MMIC's". IEEE Transactions on Microwave Theory and Techniques. vol.43. N04, April 1995, pp 706-719.
The hard mask is aligned with the MCM-D patterns on the wafer front side, in order to align the tip (23) of the coupling CPW (25), the etched slot_(37), and the waveguide. Double side alignment is performed on an Electronic Vision dedicated equipment (see Fig. 2b).
At this point, a resist protective layer (not represented) is coated on the MCM-D circuit (100).
A cavity (27) with desired cross section is etched vertically through the bulk of the silicon wafer (see Fig. 2c), using an STS recipe applied on wafer backside. The etch stops as soon as the interface between the silicon and the dielectric layer is reached, thanks to etch selectivity.
The residual masking oxide is removed using dry etching. The backside is then metallised with a blanket layer (33) of sputtered metal (see Fig. 2d).
Copper is then sputtered on the wafer backside. Copper is used as a seed layer for coating electrodeposable resist of type PEPR 2400 by Shipley.
Next, resist is exposed through a photolithographic mask (35), then developed. The unprotected copper is etched away. This completes the opening of the feed slot in the bottom of the metallised cavity (27). Resist is then stripped.
A layer of electrophoretic PEPR 2400 photolithographic resist from Shipley is electroplated and baked. The resist is further exposed (see Fig. 2d), and developed, this leaves an opening (37) in the resist in the bottom of the metallised cavity (27). The exposed metal is wet etched (see Fig. 2e).
The PEPR 2400 resist and the protective layer are stripped in a compatible solvent (e.g., acetone or hot resist stripper).
Finally (optional), BCB (41) can be dispensed in the cavity (27) (see Fig. 2f).

2. Manufacturing of a low cost, robust and small size radar

One can envisage to process two silicon chips separately, one highly resistive supporting all the RF functions plus the antenna (as e.g. in example 1), the second supporting all the CMOS and IF functions. These can to flipchip assemble the 2 chips, connecting the RF and IF circuits where necessary on a MCM-D substrate.
The result will be a low cost fully integrated, compact self packaged and robust radar as shown on Fig. 3.

Claims

Method for manufacturing a slot-coupled micromachined waveguide antenna for emitting and/or receiving a signal in the millimetre wave range,
characterised in that the method comprises the following steps:
a) depositing a first insulating layer (21,26) on a first side of a planar substrate (31) having two opposing sides and a thickness, said first insulating layer having an etch-stop functionality;

b) depositing a first metallic layer (25) on said first insulating layer (21),

c) etching a cavity (27) with predetermined dimensions in said substrate (31) at said second side thereof, wherein etching stops as soon as the interface between the substrate (31) and the insulating layer (21,26) is reached, so that the depth of the cavity (27) corresponds to the thickness of the substrate (31),

d) depositing a second metallic layer (33) that covers said second side of said substrate (31) including the cavity (27), and

e) removing a part from said second metallic layer located at the bottom of said cavity in order to create an opening (37) in said second metallic layer (33) located at the bottom of said cavity, the first metallic layer (25) overlying said opening (37), thereby defining a slot coupling.
The method as in claim 1, further comprising the additional step of filling said cavity with a polymer material (41), preferably a dielectric comprising BCB (benzocyclobutene).
The method as in claim 1 or 2, characterised in that the substrate comprises Si and the first insulating layer comprises a Si oxide layer (26) and a polymer layer (21), said polymer layer preferably comprising BCB ((benzocyclobutene).