Etching

Etching is a technique to create structures in glass and silicon substrates by chemically removing (parts of) layers from the surface of the wafer. This procedure can be performed using wet or dry etchants, depending on the substrate material and the desired layout of the etched structure.

Dry etching

Deep Reactive Ion Etching (DRIE or plasma etching) is a dry etching technique to create deep, high density, and high aspect ratio structures in glass and silicon substrates. In DRIE etching layers of the substrate are (partially) removed as a result of a bombardment of the material with ions. Etching with steep sidewalls (anisotropic etching) as well as with rounded walls (isotropic etching) is possible, with depths varying between 1 µm cavities and complete wafer-through holes. Using the DRIE process, high aspect ratios can be realized.

DRIE of silicon

Silicon structures can be etched very deeply using the DRIE Bosch process. To achieve these high aspect ratios, the process alternates every few seconds between the etch gas SF6 and the passivation gas C4F8. The etched vertical sidewall is immediately passivated and protected in the following etching step. These sequences of etching and passivation are repeated until the chosen etching-depth has been reached or until a through-hole has been created.

For DRIE of silicon, Micronit can offer the following:

• Aspect ratio 1:50
• Through-wafer etching
• Multiple depths
• Variable steepness of the sidewalls
• DRIE on Silicon-on-insulator (SOI) wafers
• Stopping on silicon oxide or -nitride membranes
• Aligned front and backside etching

DRIE of glass

Micronit is one of the few companies able to use the DRIE method in glass substrates. With this unique technique, Micronit is able to create high aspect ratio structures in glass substrates. Not only the pure quartz glass can be dry-etched using this technology, but also less costly glass such as BF33® or D263®. Glass wafers of Borofloat (BF33®) can be anodically bonded to silicon. This way, complex and highly integrated technologies for multidisciplinary applications can be fabricated.

For DRIE of glass, Micronit can offer the following:

• Aspect ratio 1:3
• Depths up to 80 µm
• Multiple depths
• Variable steepness of the sidewalls
• Positive taper
• Aligned front and backside etching

Wet etching

Micronit applies wet chemical etching techniques to create structures in glass and silicon, using hydrofluoric acid (HF) as an etchant. HF is commonly used for cleaning metal and etching glass. The structures that are created with the wet etching technique, can be positioned with an accuracy within one µm. Masks are used to define the shape of the structure. Next to etching glass and silicon, Micronit offers the etching of various metals such as Au, Cr, Cu, Ni, Ti and the etching of non-conductors such as oxide and nitride.

Isotropic etching (glass)

In wet etching, hydrofluoric acid (HF) is the liquid etching agent. Wet etching is isotropic: glass dissolves in all directions equally fast. As a rule, in isotropic etching, a channel is always twice as wide as it is deep. Besides, the corners of the channel are rounded. The surface of the etched structures is, corresponding to the type of glass used, very smooth.

Anisotropic Etching (silicon)

Anisotropic etching using alkali hydroxides such as KOH, NaOH, or amine-containing solutions such as TMAH, etches silicon along the crystal plane. KOH is the most popular, having been in industrial use for decades. Depending on the temperature and concentration, various etch rates and surface roughnesses can be created. TMAH is used when the oxide is to be maintained. The masking is done using a nitride layer, which is lithographically deposited.

Powder blasting

Besides dry etching and wet etching, powder blasting is another method to create structures or through-holes in silicon and glass. In powder blasting, a particle jet is directed on a target for mechanical material removal. The material is ablated only there, where the glass or silicon is not protected by a resist layer. This mask is deposited at high precision to the existing structures on the wafer, to ensure a maximum fine-tuning of the position of the powder blasted structure. The removed material is sucked off during the process so that the glass or silicon surface remains undamaged. After the powder blasting process, the substrate can be bonded to another glass or silicon substrate.

Powder blasting is a flexible, cost-effective, and accurate technique to create fluidic channels and interconnections. It has an accuracy of within 2 µm and a feature size accuracy of around 25 µm. The layout of the structure can be very flexible: wells can be round, rectangular, or triangular. The sides of the wells will not be completely vertical but sloped at an angle of about 70°. Furthermore, the average roughness of the channels will be between 0,8 and 2,5 µm, depending on the chosen process.

For powder blasting, Micronit can offer the following:

• Depths up to 1500 µm
• Multiple depths
• Positive taper (angle 70°-75°)
• Roughness depending on the design, average 0,4 µm - 1,5 µm (Ra)
• Resolution limit photoresist 150 µm

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