Silicon-Micromachined Waveguide Calibration Shims for Terahertz Frequencies

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Silicon-Micromachined Waveguide Calibration Shims for Terahertz Frequencies

James Campion, Umer Shah and Joachim Oberhammer

Dept. of Micro and Nanosystems, KTH Royal Institute of Technology, Stockholm, Sweden jcampion@kth.se

Abstract — A new method of realising precision waveguide shims for use in THz Through-Reflect-Line (TRL) calibrations, based on silicon-micromachining, is introduced. The proposed calibration shims combine a thin λ/4 silicon layer, co-fabricated with a thicker layer which provides mechanical support. This design overcomes the limitations of CNC milling for the creation of calibration shims, facilitating use of standard TRL calibration at currently challenging frequencies. The novel shim fits inside the inner recess of a standard waveguide flange and is compatible with conventional flange alignment pins. Five micromachined shims were fabricated in a silicon-on-insulator process for operation in the WM-570 waveguide band (325 – 500 GHz).

The fabricated shims show excellent performance across the entire band, with return loss in excess of 25 dB, insertion loss below 0.2 dB and high uniformity between samples. Verification reveals that the micromachined shims have an electrical length within 2% of the expected value. Comparative measurements of a DUT calibrated with the proposed shim and a previously un-used conventional metallic shim show that the novel concept offers equivalent, if not better, performance. The mechanical design of the micromachined shim and the rigid nature of silicon ensure that it will not suffer from performance degradation with repeated use, as is problematic with thin metallic shims. This work enables the creation of low-cost, highly-repeatable, trace- able calibration shims with micrometer feature-sizes and high product uniformity, surpassing the limits of current techniques.

Keywords — micromachining, terahertz, submillimeter- wave , calibration, TRL, waveguide, THz

I. I NTRODUCTION

Waveguide calibration shims are used to realise the λ/4 Line standard required for the well-known Through- Reflect-Line (TRL) calibration technique [1]. Due to its self- calibration nature and the need for a single known stan- dard, TRL is the calibration method of choice for precision waveguide measurements above 100 GHz [2]. At frequencies below 300 GHz, calibration shims are typically manufactured via standard CNC milling, as the thickness of the shim is sufficient for it to be mechanically stable and the mechanical tolerances are small relative to the size of the waveguide. CNC milling is a high-cost, sequential process, rendering it unsuit- able for the production of low-cost calibration standards. It offers tolerances of the order of ± 5 µm for in plane features, while tolerances for out-of-plane features are often higher.

These tolerances affect both the cross-section and thickness of typical waveguide calibration shims, reducing their accu- racy and repeatability. Table 1 outlines the required physical thickness of a calibration shim for the various frequency bands between 220 – 2600 GHz, as per [3]. Components of

thickness less than 500 µm are challenging to fabricate via CNC milling and are greatly affected by the various tolerances of this process. Furthermore, shims of such thickness become too fragile for practical use, prohibiting use of the standard TRL method at frequencies above 300 GHz. This approach to the creation of calibration shims is no longer feasible at submillimeter-wave /THz frequencies.

For this reason, some manufacturers of waveguide calibration kits adopt the Line-Reflect-Line (LRL, [4]) method at these frequencies. Two precision waveguide lines, with a difference in thickness of a quarter-wavelength, are then required. LRL calibrations are unsuitable for precision waveguide measure- ments above 110 GHz, as all four S-parameters of the first Line standard must be known [5]. Other solutions include the use of a pair of 3λ/4 length standards, each designed for one half of the waveguide band of interest [3]. As with λ/4 shims, such structures quickly become too thin for practical use at frequencies above 1.1 THz.

Tracebility, the ability to trace a given measurement back to the International System of Units, is central to ensuring the reliability and quality assurance of any measurement. For waveguide S-parameter measurements, traceability is estab- lished through the dimensional accuracy and precision of all waveguide components and is directly limited by their me- chanical tolerances. Attempts to establish traceability above 100 GHz have been made in recent years [6], [5], based on the use of custom/high-precision waveguide flanges. Mechanical tolerances in waveguide components also create uncertainty in S-parameter measurements, due to misalignment between waveguides. This limits the maximum return loss that can be accurately measured [7] and contributes a shunt reactance to the measured S-parameters [2]. If precise coupling to the source and load ports is required, as in waveguide filters, these reactances can greatly detune measured performance [8]. A new approach to the fabrication of waveguide cali- bration shims is required to enable the use of standard TRL calibration and facilitate traceability at THz frequencies, while also overcoming the problems caused by misalignment of waveguide flanges.

Silicon-micromaching is an established technology for the creation of high-precision waveguide components, offering small feature sizes, low tolerances and high repeatability [9].

Prime silicon wafers can be manufactured with ± 0.5 µm

(3 σ) thickness uncertainty, far surpassing the limitations of

traditional CNC milling. Batch processing techniques allow

Table 1. TRL Line Standard Lengths for Submillimeter-wave Frequencies Waveguide Frequency Range λ/4 (µm)

WM-864 220 – 330 GHz 360

WM-570 330 – 500 GHz 239

WM-380 500 – 750 GHz 158.5

WM-250 250 – 1100 GHz 109

WM-164 1.1 – 1.7 THz 73

WM-106 1.7 – 2.6 THz 48

silicon-micromachined components to be manufactured in parallel, reducing fabrication costs and increasing product uniformity. These properties make silicon-micromachining highly suitable for the implementation of calibration stan- dards, which require high dimensional accuracy and unifor- mity.

Here, a new design of calibration shim is proposed, based on the use of a thin micromachined silicon layer (in which the waveguide line is defined) which is connected to a thicker silicon layer, providing mechanical support and rigidity. This novel calibration shim design is initially implemented for 325 – 500 GHz. The design allows for the use of extremely thin silicon layers without any of the classical mechanical concerns. The dimensional accuracy of micromachined struc- tures enables traceable measurements to previously unobtain- able frequencies, without the need for multiple waveguide lines or the use of LRL methods. Metallic shims for use at submillimeter-wave wave frequencies are commonly regarded as suitable for only a handful of calibrations, after which their performance degrades so significantly as to prevent further accurate measurements. The mechanical support added to the shim and the inherent stiffness of silicon ensure that this design will not degrade with use.

The design and fabrication of the calibration shims, based on silicon-on-insulator (SOI) wafers, is outlined in Section II.

Characterisation of the electrical properties of the shims is reported in Section III. Section IV summarizes the work and its impact.

II. C ALIBRATION S HIM D ESIGN & F ABRICATION

Creation of a silicon calibration shim can be performed using standard micromachining processes. Deep reactive ion etching (DRIE) allows for the creation of high aspect ratio etch profiles with smooth surfaces. This property is essential for the creation of low-loss waveguides of accurate geometry.

A simple waveguide λ/4 line can be created by etching a standard silicon wafer of a suitable thickness, in a manner similar to [10]. This approach is limited to relatively low frequencies (below 300 GHz); silicon layers with thicknesses below 300 µm are fragile and require careful handling, in much the same manner as thin metallic shims.

Here, a new design of micromachined calibration shim is presented, consisting of a thin λ/4 silicon layer which is mechanically supported by a thicker silicon layer, allowing for the use of conventional Line standards at frequencies unachievable with existing techniques. The features of the proposed design are shown in Fig. 1. The shim is designed

Elliptical Alignment Hole

Tight Circular Hole Oversized Circular Hole

Waveguide Opening

Oversized Circular Hole

(a)

λ/4 Layer

Inner Flange Boss

Recess

Supporting Frame

(b) Micromachined

Calibration Shim Alignment Pin

(c)

Fig. 1. CAD model of the micromachined shim, showing (a) the front side of the shim with all alignment features, (b) its back side, featuring the large circular recess for the flange inner boss and (c) the shim mounted on a standard waveguide flange.

to fit inside the inner recess of a standard waveguide flange (Fig. 1c). This decreases the overall size of the shim, greatly reducing the cost per shim. Furthermore, this reduces the area of the membrane formed by the thin quarter-wavelength layer, increasing its rigidity. The outline of the shim is shaped to match the curvature of a fingertip, allowing it to be easily held in one hand. The inner flange boss is accepted into the large circular recess on the backside of the shim (Fig. 1b). Elliptical holes [10] are utilised for both inner and outer alignment pins, ensuring accurate alignment between the two waveguide ports. The holes account for the worst case tolerances of both the inner and outer pins, allowing both sets of pins to be used.

Etching the structure shown in Fig. 1b in a standard silicon wafer will result in a curved surface with significant surface roughness, as shown in Fig. 2a. The curvature of this surface will create a small gap between the surface of the resulting shim and the waveguide flange to which it is connected. This gap, combined with the effect of the surface roughness, creates a poor ohmic contact between the two waveguides, preventing proper calibration and measurement.

To overcome this issue, we instead fabricate the shims from a

silicon-on-insulator (SOI) wafer, consisting of pair of silicon

layers (device/handle layer), separated by a thin buried oxide

(BOX) layer (Fig. 2b). In this approach, the thickness of the

device layer is chosen such that it corresponds to λ/4 at the

frequency of interest. The low thickness variation of prime

silicon wafers provides precise control of the thickness of

Si SiO

Au

Standard silicon micromachining

1.

2.

3.

DRIE

DRIE _Rough

surface

(a)

This work SOI based approach

DRIE DRIE

DL

HL BOX

(b)

Fig. 2. Fabrication process flow for calibration shims realised using (a) standard silicon micromachining and (b) the SOI based approach described here. (1.) Initial wafer layer stack. (2.) DRIE etching of the required features.

(3.) Metallisation of the final shim.

the shim. Due to availability, a thickness of (218 ± 1) µm is used; corresponding to an electrical length of 90 ^◦ at 433 GHz for a standard WM-570 waveguide. The other layer must be chosen such that it is thinner than the depth of the inner boss recess. A thickness greater than 300 µm provides excellent mechanical stability to the shim, preventing any bending or deformation during use. A 381 µm thick handle layer was used in this work. The BOX layer (1 µm) of the SOI wafer acts as an etch stop during DRIE, ensuring that the resulting etch profile is free from curvature. This layer is later removed, leaving a flat, optically polished surface on both sides of the shim. The resulting SOI based shim provides excellent ohmic contact between the shim and waveguide flange.

The shims are fabricated using a process similar to that described in [11]. Using oxide hard masks, the waveguide opening is patterned in the device layer (DL) of the SOI wafer while the circular recess to accept the inner flange boss is patterned its handle layer (HL), (Fig. 2b). Holes for both the inner and outer alignment pins are patterned in both layers.

Fabrication begins with etching of the DL using a modified Bosch process. The SOI’s BOX layer acts as an etch stop, ensuring the resulting etched surface is flat. Following etching of the handle layer structures, individual chips are released from the wafer. The oxide mask is removed via wet etching using hydroflouric acid. This process also underetches the BOX layer, as illustrated in Fig. 2b. Gold sputtering is used to metallise the shims on both sides; the final gold thickness is 1.25 µm.

III. RF C HARACTERISATION

The performance of the shims was evaluated by connect- ing them to standard WM-570 waveguide test ports, driven by a pair of Virginia Diodes Inc. (VDI) frequency extenders con- nected to a Rohde & Schwarz ZVA-24 VNA. To characterise the S-parameters of a single shim, standard TRL calibration was performed using a VDI metallic λ/4 shim, with an IF

Fig. 3. A fabricated micromachined calibration shim mounted on the test waveguide flange.

340 360 380 400 420 440 460 480 500 Frequency (GHz)

−60

−50

−40

−30

−20

−10 0

|S 11 |, |S 22 | (dB)

S

S

S

−1.0

−0.8

−0.6

−0.4

−0.2 0.0

|S 21 | (dB)

(a)

340 360 380 400 420 440 460 480 500 Frequency (GHz)

−120

−100

−80

−60

−40

θ( S 21 ) ( ◦ )

Shim 1, 452 GHz Shim 2, 442 GHz Shim 3, 426 GHz Shim 4, 419 GHz Shim 5, 408 GHz Theory, 433 GHz

(b)

Fig. 4. (a) Measured S-parameters of five fabricated shims. (b) Phase response of the micromachined shims. The theoretical response and a line representing a 90 ^◦ phase offset is shown for reference; the frequency at which the response of each shim crosses this line is also listed.

bandwidth of 100 Hz. Five samples from a single wafer were characterised sequentially, with calibration performed prior to measurement of the first sample. The measured S-parameters of the fabricated shims are shown in Fig. 4a.

Return loss of the calibrated shims is in excess of 25 dB across the band, while the insertion loss is below 0.2 dB.

The closeness of S ₁₁ /S ₂₂ indicates that the waveguide port

−60

−40

−20

|S 11 | (dB)

−60

−40

−20

|S 22 | (dB)

340 360 380 400 420 440 460 480 500 Frequency (GHz)

−1.00

−0.75

−0.50

|S 21 | (dB)

MEMS Shim VDI Shim

Fig. 5. Measured S-parameters of a 1 inch WM-570 waveguide piece, where calibration was performed using the proposed micromachined shim (MEMS Shim) and a standard metallic one (VDI Shim).

has the same cross-section on both sides; underetching during the DRIE process was minimised to ensure this was the case.

All five shims show very similar performance, due to the accuracy of the micromachining process and the excellent repeatability of the elliptical alignment hole method [10]. The electrical length of the shims was verified by examining their phase response, plotted in Fig. 4b. Shim 3 represents a λ/4 line at a frequency of 426 GHz, within 2% of the expected value given the physical thickness of the shim’s device layer.

Discrepancy in the electrical length of the shims may be due to the effects of thermal drift in the measurement setup. All measured electrical lengths are still within 5% of the expected value, however.

Proof-of-concept testing was performed by measuring the S-parameters of a separate waveguide line (VDI WM-570, 1 inch), where calibration was performed using both the conventional metallic shim and the proposed micromachined one. The metallic shim was previously un-used and free from deformation. A comparative plot of the measured S- parameters of this DUT is shown in Fig. 5. The return loss calibrated with the micromachined shim is particularly flat across the entire frequency range, due to the highly accurate alignment between the calibration shim and test waveguide flange. Overall, the proposed shim offers equivalent or better performance than a conventional metallic one. Due to its me- chanical rigidity, the silicon micromachined shim is expected to offer superior performance over time.

IV. C ONCLUSION

A new design of silicon micromachined calibration shim, which overcomes the limitations of conventional techniques for the fabrication of waveguide calibration shims, was pre- sented. The design incorporates a λ/4 silicon shim with a thick supporting layer which provides mechanical rigidity.

Silicon-on-insulator wafers are used to realise highly accu- rate, repeatable and compact calibration shims for operation between 325 – 500 GHz. Five samples of the proposed design were measured and found to have similarly excellent

performance (RL >25 dB, IL <0.2 dB). Comparison of TRL calibrations performed with the novel shim and a conventional one shows that it offers equivalent, if not better, performance.

The use of silicon, and the novel design of the shim, ensures that it offers far higher mechanical rigidity than conven- tional shims, preventing performance degradation and greatly enhancing repeatability of the shim. This factor, combined with the similarity between multiple shims (Fig. 4), opens the possibility of using silicon micromachined calibration shims for traceable S-parameter measurements at 300 GHz and above.

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