玻璃湿法蚀刻

1. INTRODUCTION  

Interesting properties such as high chemical  resistance, high heat resistance, high electrical  isolation, biocompatibility, large optical transition  range and low optical absorption are desired from the  device material. In MEMS devices fabrication, after  silicon, glass is the second most widely used  material.

2. GLASS ETCHING TECHNIQUES  

There are three major groups of techniques used  for glass etching: mechanical, dry and wet.

Mechanical methods include traditional drilling  with diamond tipped drill bits, ultrasonic drilling  [10], electrochemical discharge [11] or powder  blasting [12, 13]. These methods are commonly use  for performing etching through the glass wafer.  However, smooth surfaces cannot be generated using  such methods. 

Dry etching techniques include plasma and laser  etching of glass. Ronggui and Righini in [14]  reported the first etching of glass in plasma with an  optimal etching rate of 10 nm/min. Better results are  reported using Deep RIE in ICP systems [15] using  SF6 as gas and electroplated Ni (20 µm-thick) as  masking layer. We also reported in [16] etching of  pillars in an ICP reactor using C4F8 chemistry and 40  µm-thick bulk silicon as masking layer. The main  advantage of this technique is the opportunity to  generate structures with high aspect ratio, or  microchannels with vertical walls. The disadvantages  of plasma etching consist of relatively low etching  rate (0.5-0.6 µm/min) which is halved by the cleaning  process of the equipment required usually after each  10 µm depth etch. The rough surface generated (high  energy sputter etching process) is another  disadvantage of the process.

3. INFLUENCE OF GLASS COMPOSITION

These insoluble products are deposited on the  generated surfaces and act as masking layers. As a  result, after etching, the surface becomes rough and,  in time, the etching rate decreases. For Hoya SD2 a  large amount of Al2O3– (about 20%) is presented [28]  while in the composition of soda lime, CaO (8.8%),  MgO (4%) can be found [29]. In the composition of  Corning 7740 the amount of insoluble products is  only 2% (Al2O3) [29]. For this reason wet deep  etching of glass is recommended to be performed on  glasses with low concentration of oxides that give  insoluble products in HF. This is why in our  experiments we focus more on Pyrex glass  (Corning7740).

4. ETCH RATES

The etch rate is a characteristic for each type of  glass, especially due to different oxides and different  composition used during fabrication. For the HFbased etching solution the etch rate is determined by  the concentration of HF enchants. To achieve a high  etch rate, a standard concentration of 49% should be  used. Figure 2 presents the influence of HF  concentration on the etch rate for Corning 7740  Pyrex glass. It should be noted that by increasing the  HF concentration from 40% to 49% a rapid increase  of etch rate of 50-60% can be achieved (4.4 µm/min  to 7.6 µm/min for non-annealed glass). At the same  time no major difference in the resistance of mask in  the etchant was observed.

Fig2

Other methods tested for increasing the etch rate,  were: warming the solution at 50OC and using  ultrasonic for agitation. The first method was tested  in a sealed Teflon container warmed in hot water at  the above-mentioned temperature. The increasing of  etch rate was significant (about twice). However, this  method is not recommended for safety reasons. By  applying ultrasonic agitation we also noticed  increases of the etch rate, but the resistance of the  masking layer in the etchant is drastically reduced.

The pinholes on the non-etched glass surface are  very often observed, mainly due to the tensile stress  and the defects produced during the Cr and Au  deposition. During cooling after the deposition  process, due to the high tensile stress, small cracks  are generated in the Cr/Au layer. The aggressive HF  etch liquid can easily penetrate through these cracks,  mainly due to the hydrophilic surface of Au and start  to generate the pinhole. It is known that the liquid is  pull in through the micro-channel with hydrophilic  surface and pull out if the micro-channel surface is  hydrophobic. Longer etching results in larger  pinholes whose sizes are proportional to the glass  etch depth because of its isotropic etch profile. Figure  7 illustrate the generation of the pinholes.