Etch-stop process

No matter what etchant is employed, in order to achieve high predictability and reproducibility in the micromachining process, the etching step must be combined with some technique for stopping it automatically. Even the most casual examination of the present micromachining literature reveals that virtually all reported micromachined structures rely on at least one etch-stop technique. In its most comprehensive definition, an etch-stop process is any method that allows for selective removal of a specific material to produce a predefined relief. 

The etch-stop process can be used to produce narrow geometrical features up to a depth of a half of the sample thickness. However, the process is not suitable to produce foils with a thickness of less than 200 pm. The major advantages of the etch-stop process are the low technical effort, short process-... 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

Sensors for measurements of physical parameters such as pressure, rotation or acceleration are commonly based on elongation or vibration of membranes, cantilevers or other proof masses. The electrochemical processes used to achieve these micromechanical structures are commonly etch-stop techniques, as discussed in Section 4.5, or sacrificial layer techniques, discussed in Section 10.7. 

In the manufacture of micromechanical devices electrochemistry is commonly used to realize etch stop structures or to form porous layers. The first of these is discussed in Section 4.5. In the latter case, the use of PS as a preserved layer or as a sacrificial layer can be distinguished. In the first case PS is an integral part of the ready device, as discussed in Sections 10.4 to 10.6, while in the latter case the PS serves as a sacrificial layer and is removed during the manufacturing process. 

The membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

The steam reformer is a serpentine channel with a channel width of 1000 fim and depth of 230 fim (Figure 15). Four reformers were fabricated per single 100 mm silicon wafer polished on both sides. In the procedure employed to fabricate the reactors, plasma enhanced chemical vapor deposition (PECVD) was used to deposit silicon nitride, an etch stop for a silicon wet etch later in the process, on both sides of the wafer. Next, the desired pattern was transferred to the back of the wafer using photolithography, and the silicon nitride was plasma etched. Potassium hydroxide was then used to etch the exposed silicon to the desired depth. Copper, approximately 33 nm thick, which was used as the reforming catalyst, was then deposited by sputter deposition. The reactor inlet was made by etching a 1 mm hole into the end... 

An important component in the process flow is a through-wafer via with a nitride liner (and metal fill) that can act as an etch stop for the grinding and polishing wafer-thinning step. This capability allows uniform thinned layers and could provide good wafer-scale planarity for subsequent processing (although characterization of wafer-level planarization has not been reported to date) [41]. Available product information describes the performance and specifications of 3D components [40,42]. 

FIGURE 15.11 Process flow for thinning to a hurried oxide etch stop, rebonding, and repeating to allow electrical testing of the original structures (from Ref. 49). 

Anisotropic etching of silicon is routinely used in the fabrication of three-dimensional structures [1,2]. These micro fabrication techniques take advantage of orientation-dependent etch rates where the planes of lowest etch rate, usually the (111) planes, act as etch stops for the dissolution process [3, 4]. In electrolytes such as KOH, the etch rates of the (100) and the (110) planes may be more than two orders of magnitude faster than those of the (111) planes. In buffered NH4F solutions, etch rate enhancements as high as 15 have been reported for the (100) plane in comparison with the (111) surface [5]. 

The electron deficiency model implies that etch stop should not occur on n-Si where there are abundant electrons in the conduction band. Unable to explain the etch rate reduction on phosphorus- or germanium-doped n-Si, Seidel et al suggested that the etch rate reduction observed on phosphorus-doped n-Si is caused by a different mechanism. Also, the electron deficiency model assumes that electrochemical reactions are responsible for the etching process in alkaline solutions. However, experimental results indicate that the etching process is mainly of chemical nature as described in detail in Chapter 5. 

C. A. Desmond, C. E. Hunt, and S. N. Farrens, The effects of process-induced defects on the chemical selectivity of highly doped boron etch stops in silicon, J. Electrochem. Soc. 141, 178, 1994. 

The effect of potential is shown in Fig. 24 etch stops at passivation potential and decreases with cathodic polarization for both n-Si and p-Si [138, 139]. The etch rate dependence on potential is rather different from that in KOH in which p-Si etch rate varies only slightly with cathodic polarization (Fig. 21). Also, etch rates at OCP as shown in Fig. 24 are much smaller than the peak values indicating that electrochemical processes play a more important role than that in KOH. Etch rate in EDP is independent of illumination [46]. 

The shortened electron life model could not explain the etch stop on -Si, in which there are abundant electrons in the conduction band. Also, this model assumes electrochemical reactions responsible for the etching process in alkaline solutions, in which they are in fact largely in chemical nature. 

A planar substrate, such as silicon wafer, could be micromachined by a sequence of deposition and etching processes. This results in three-dimensional microstructures which can be implemented in cavities, grooves, holes, diaphragms, cantilever beams etc. The process referred to as silicon micromachining often employs anisotropic etchants such as potassium hydroxide and ethylene diamine pyrocatechol. The crystallographic orientation is important as the above-mentioned etchants show an etch-rate anisotropy. The ratio for the (100)-, (110)- and (111)- planes is typically 100 16 1. The technique of electrochemical etch stop could be applied for control of the microstructural dimensions. An alternative... 

The etching profile can be fully modeled by combining the etch stop model with the output of a process simulator, such as ATHENA /SSUPREM3 , which defines the location of the pn junctions and the electrical conductivity of the specific layers. 

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