Silicon-based waveguide technology for millimeter-wave antennas

This paper describes a method for the preparation of complex structured waveguides that can be used for millimeter-wave antennas based on a bulk silicon MEMS process. In this paper, a millimeter-wave band array of ridge waveguide slit antennas with multilayer step-matched feeds is realized by this method. A bulk silicon MEMS process is used to realize the complex structure and the high precision and flatness requirements on the millimeter scale, which cannot be achieved by machining. And the step height difference of more than 100um is realized by combining multiple lithography and silicon oxide mask. The step difference is larger than that achievable by dry etching using the etching selection ratio of silicon oxide and silicon. The waveguide fabricated by this method can work well in the millimeter wave band and is used for antenna feeding, power distribution, etc. It is characterized by compact structure, high precision, and high performance.


MEMS Processes for Silicon-based Waveguide with Complex Structures
Firstly, photolithography is used to make a mask for cavity patterning.The photoresist mask thickness is selected according to the cavity etching depth.If the cavity depth is relatively deep, it is necessary to grow dielectric film first on the surface of the wafer for patterning as an etch mask by chemical vapor deposition [1] .
The next step is to perform a dry etching process to etch out the waveguide cavity, the structure inside the cavity, and the through-holes for the feed ports by using process gases.When etching multi-layer steps in the cavity, it will be abnormal if the mask pattern is produced for the steps with larger areas after the etching of the steps with smaller areas.The photoresist on the sidewalls of the steps is not easily exposed due to the UV light from the photolithography machine coming down from directly overhead [2] .As a result the photoresist on the sidewalls of the steps is very likely to remain after development and cannot be removed [3] .When the step patterns with larger areas are dry etched, the sidewalls are obscured by residual photoresist and will form spikes.As shown in figure 3 below.Figure 3. Etched spikes on the sidewall of the step So when multi-layer steps within a cavity need to be etched, steps with large areas are etched first, steps with small areas of are etched afterwards.Through-holes are etched from the other side of the wafer, and the initially grown silicon oxide can be used as a mask.Through-holes are etched from the other side of the wafer, and the initially grown silicon oxide can be used as a mask.
In the structure shown in Fig. 1, the notch on the ridge is a planar structure, which can be patterned by photolithography.And the structure can achieve high precision as the etching error is within 2 um.
Both the 191/250um step height difference at the feedthrough and the through-hole are longitudinally three-dimensional.To avoid the etching spikes mentioned above, the 191um deep cavity is etched first.Due to the relatively large etching depth, the photoresist is not sufficient to protect the non-etched areas.A double mask process such as a silicon oxide dielectric film coupled with photoresist is applied.Depth error is within 5um after the etching process is completed.The area with a depth of 250um is then etched 59um down from the front side.The depth is shallow and spraying photoresist as a mask is sufficient.The feed port vias are etched out from the backside using silicon oxide as a mask.The structure of Figure 2 is realized in the same way.
When the multilayer wafers are all etched, the dielectric film, which is used as a mask layer, is removed and the dielectric layer is re-grown to ensure that the isolation is effective.
The wafer surface is then sputtered with metal, bias sputtering is used to ensure that the sidewalls of deep cavities, steps, and through holes are covered with metal.
The structure described in this paper uses bias sputtered TiW/Au to satisfy metal adhesion requirements and intermetallic stress matching.
Next, both sides of the wafer are plated separately using a single-sided plating process.Compared to direct double-sided plating, the uniformity of the metal is better in this way and the through-holes are less likely to bulge and undulate.
Finally, multiple layers of wafers are bonded together to form a complete waveguide cavity structure by wafer gold-gold bonding, which produces a continuous intermetallic solid-state fusion process at high temperature and high pressure.The following figure 4

SIMULATION AND DISCUSSION
The physical dimensions of this antenna are basically consistent with the design requirements with high accuracy.After the physical measurements, the positive and negative tolerance of the external dimensions of the antenna structure is within 0.005mm.And the bottom waveguide port dimensions have a positive or negative tolerance of 0.005mm or less.The etching error in the planar dimensions of the cavities on each wafer layer is less than 5um.The etching depth error for cavities on each wafer layer is less than 10% of the etching depth.The bonding alignment error between wafer layers is less than 20um.The following figure shows the actual radiation performance test results of this antenna.The antenna works well in the preset frequency bands with high gain and low side lobe.

CONCLUSION
This paper describes a process for preparing silicon-based waveguides with complex structures using the MEMS process.A 220 GHz ridge waveguide slit antenna array is successfully realized.This antenna is processed by the MEMS process with high precision.The radiation performance test results of the antenna are also consistent with the simulation results.This is a successful attempt at MEMS process application.It proves the practicality of the process method introduced in this paper for the preparation of complex structured waveguides.And it is promising in the field of millimeter-wave antennas.

Figure 1 .
Figure 1.The inner ridge and feed ports of the waveguide.

Figure 2 .
Figure 2. Matching steps and feed ports in the waveguide and figure 5 show the internal physical diagram of the waveguide and the test results of this antenna.

Figure 4 .
Figure 4. Internal structure of the waveguide antenna: the radiating part

Figure 6 .
Figure 6.The radiation performance of the antenna