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Friday, February 9, 2018

Weekly Tip ISC: Micromachining of High Frequency Components at ISC/UPNA

Por Jorge Teniente Vallinas


The classical fabrication techniques (milling and drilling of both metallic and dielectric materials to generate the required three-dimensional geometries) used to manufacture high frequency components, start to fail at mm-wave, submm-wave and THz frequencies and new alternatives need to be found.

The machining tolerances of modern CNC-milling machines are around +/- 1 mm and this should be enough for those aforementioned high frequencies (at least up to 1 THz), but as the minimum structure size of milling techniques is simply limited by the smallest available milling cutter, which is typically between 150 and 200 mm, the machining of comparatively small structures as they are required for waveguides or antennas, becomes a sincere challenge and new techniques have to be employed for manufacturing such components.

In Fig. 1, a high frequency component at the limit of modern CNC-milling machines manufacture can be seen. The structures that can be manufactured with this technique are limited by such size of the smallest available milling cutter, so the positive structures machined must be thicker than 50 mm and the negative structures (trenches) thicker than 150 mm. This limits the maximum achievable frequency for the components that can be made by this technique to around 200 GHz.








Figure 1.- Waveguide component at 100 GHz manufactured in aluminum by a high precision CNC-milling machine whose cavity sections are milled with a 200 mm diameter milling cutter with fixed thicknesses tk = 100 mm and iris openings dk in the range from 0.7 to 1.2 mm while lk ranges from 1.6 to 1.8 mm.

In addition to conventional milling techniques, there are other manufacture techniques employed for high frequency components fabrication as electrical discharge machining (EDM), electroforming based on electro-chemical growth of large structures over a negative mold that is removed chemically, bulk silicon micromachining using dry and wet etching techniques, thick photoresist (SU-8) techniques and LIGA-processes based on X-ray or UV exposure. All these techniques require the availability of dedicated expensive equipment and among the techniques, only the last four ones allow the fabrication of large aspect ratios (above 10:1) and small negative and positive structures (below 1 mm).


2.Silicon Michromachining

At the Institute of Smart Cities of the Public University of Navarra (ISC/UPNA), we have a 40 m2 clean room facility (ISO 7) where we are able to process bulk silicon with wet and dry etching techniques as well as thick SU-8 photoresist techniques.

2.1 Wet etching of bulk silicon

Wet etching of silicon can be used to generate certain geometries by using the different etch-rates along various planes of the crystal. Etching is mostly done using strong alkaline substances (pH > 12) as KOH or TMAH solutions on {100} or more rarely {110} oriented silicon wafers, see Fig. 2.















Figure 2.- Wet etching of bulk silicon procedure.

a) Crystalline structure of bulk silicon and wet etching results depending on the etching plane.
b) Procedure for wet etching using SiO2 as hard mask.
c) Micromachined structures for high frequency components wet anisotropically etched in bulk silicon.

Since the bonding energy of Si atoms is different for each crystal plane, Si etching is highly anisotropic: while the {100} and {110} crystal planes are being etched, the stable {111} planes acts as an etch stop. {100} oriented wafers can only be used to machine trenches with slanted sidewalls (at 54.7º), so the etching process forms square based pyramids with {111} surfaces. {110} orientated wafers can only be etched in one direction and form perpendicular trenches with {111} side-walls which restricts the possible structures to one-dimensional comb-like geometries forming perpendicular trenches.

The maximum aspect ratio is mainly limited by the selectivity of the mask, which is usually made from SiO2.

2.2 Dry etching of bulk silicon

The poor geometrical flexibility which is the major disadvantage of the wet-etching techniques can be overcame using silicon dry-etching processes, such as the Bosch process. The dry etching processes allow to define arbitrary lateral shapes because they are not limited to the crystal planes of silicon. In any case, for both wet and dry etching, it is difficult to generate structures with multiple depths or structures which require extremely precise depth control, because the depth can only be controlled by the etching time. In fact, this aspect is object of research at the moment in our ISO7 clean room since we are capable of creating a mask with a circuit patterned in SiO2 (using an e-beam evaporator, see Fig. 3a) with different SiO2 mask thicknesses so as to delay the dry etching process and generate at least three different depths in the same micromachined circuit.

Dry etching is made through injection of several gases combinations in a vacuum chamber where a plasma has been generated, see Fig. 4. Dry etching is achieved by means of two etching methods that happen at the same time: a chemical etching due to ionized radicals in plasma (F-, Cl-, O=), this etching method is highly isotropic and highly selective and a mechanical etching (milling) due to accelerated ions in the                                                         self-bias voltage (SF5+, Ar+, …) which is highly anisotropic (directive) but with poor selectivity.     

Figure 3.- ISC/UPNA ISO7 clean room facilities for dry etching.

a) Angstrom Engineering e-beam evaporator for SiO2 and other materials deposition.

b) Oxford Instruments DRIE NGP80 ICP65 machine with Bosch silicon etch.

To achieve deep etches in silicon, the etch must have a relatively high etch rate, also the etch must have a high selectivity (ability to etch only the desired material, relative to the etching of mask and/or other substrate materials), the etch must remain anisotropic (verticality of the etch profile) throughout the etching process and the roughness of the final structured silicon (vertical etched walls and horizontal etched walls) must be as low as possible. To date, only two etching modalities have the potential to stand up to these rigorous requirements: a pulsed mode performed in the Bosch silicon etch process and mixed mode performed in the cryogenic silicon etch process. At the ISC/UPNA we have an Oxford Instruments DRIE NGP80 ICP65 machine with Bosch silicon etch process, see Fig. 3b.


Figure 4.- Dry etching of bulk silicon procedure

In the Bosch process (also known as Deep Reactive Ion Etching (DRIE)), the etching is accomplished by means of short alternating SF6 and C4F8 gases process steps where SF6 provides the etching (both mechanical and chemical) and C4F8 provides sidewall passivation (in order to protect vertical walls) by means of polymer deposition, see Fig 5. DRIE is a room temperature processing with no corrosive gases but requires high density plasma with high radical density, low ion energy as provided by ICP tools and ion bombardment to remove polymer from horizontal surfaces. The net result is deep anisotropic, the silicon is etched with a control of profile verticality in the range 89 to 95 degrees (deep trenches), high aspect ratio (> 20:1) with high etch rates (4 µm/min), very good selectivity over photoresists and SiO2 (25:1 over positive photoresist, 50:1 over negative photoresist and 75:1 over SiO2) but the sidewalls are not perfectly smooth, see Figs. 5 and 6.

Figure 5.- Deep Reactive Ion Etching (DRIE) procedure (Bosch process).

In the Bosch process, sidewalls are not perfectly smooth due to sequential etch (SF6) / deposition (C4F8) steps (the etch steps take sequential ‘bites’), these are called scallops, see Figs. 5 and 6. Scallops size and roughness can be controlled via reduction of etch and deposition time but is size is always smaller than 1 mm.

Figure 6.- Vertical sidewalls with the resultant roughness in the Bosch process.

As it has been explained in the previous paragraph, at the ISO7 clean room facility of the ISC/UPNA we are at the moment in a very interesting research for silicon micromachined fabrication of high frequency components as filters, diplexers, OMTs, mixers, multipliers, antennas, etc. for ultrafast communications, imaging and space applications (radiometers, …) in the THz range, see Fig. 7.

Figure 7.- Silicon micromachined compact low-pass filters with low insertion loss in WR 2.2 waveguide (330 GHz – 500 GHz).


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