The Etch-Stop Process

It is often required to produce not holes or trenches but channels or sack holes respectively depressions, i.e. to etch a desired geometric structure only to a definite depth. This is particularly the case for microfluidic devices containing channels and cavities or for structures used in the bio or medicine technique. 

The surface roughness at the bottom of a structure is significantly higher than at the side walls, which is caused by the etching attack on the crystals and the reduced chemical attack on the residual glass phase which results in the formation of a porous layer of the residual glassy phase. An example is shown in Fig. 9.11 (right). 

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- 

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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. 

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 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. 

Another foil type offered by laminate supphers, reverse-treated foil (RTF), offers an advantage for producing fine lines. The RTF copper has adhesion promoter applied to both sides and is classified by 4562 as code R (reverse-treated bond enhancement [cathode side] stain-proofing on both sides).This approach provides advantages to imaging fine lines. When the copper tooth is reversed, the fabricator can improve fine definition by allowing the etch chemistry process to stop at the surface of the laminate. 

In the upper part of Figure 1.18, the first oxide layer in the Poly MUMPS process has been patterned with the ANCHOR 1 mask level, with the etch stopping on the nitride layer on the left and on the PolyO later on the right. A polysilicon layer, Polyl, is then deposited and... 

Thus the rate of etching decreases continuously with increasing depth of the structures to be created. In this case the etch process does not stop abruptly at a given depth, and distinguishes consequently from the etch stop during the anisotropic etching of silicon at the (lll)-layer. 

Electrochemical etching (i.e. etching using an electrochemical potential for the control and stopping of the etching 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... 

The epitaxial emitter structure was fabricated, as shown in Figure 6.26 [23]. In this case, only 1,000-A-thick, p-type base layer doped at 2 x 10 cm is grown. This is followed by an epitaxial growth of 3,000-A-thick n emitter layer. The emitter layer is etched using RIE to stop at the base layer. The rest of the process details are similar to those described in Section 6.4. The most difficult step in this process is the etching of the emitter layer and stopping at the base layer. The uniformity of the RIE is critical at this step. 

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]. 

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