Study of the cross-sectional profile in selective formation of porous silicon

Abstract

Porous Si is the semi-insulating state of Si, with low thermal expansion mismatch with bulk Si. As a result, it is an excellent material for crosstalk isolation in mixed-signal integrated circuits. We study the formation of isolated porous Si regions in p−-type and p+-type Si substrates with emphasis on the cross-sectional profile of the porous regions. Our study reveals that in addition to the primary undercut due to the isotropic nature of the anodization process, there exists a secondary undercut that is similar in shape to the bird’s beak commonly observed at the edge of field oxides in conventional Si complementary-metal-oxide-semiconductor process. The shape and the extent of the secondary undercut are dependent on the type of mask materials used during selective formation of porous Si as well as the substrate resistivity. The combined experimental and simulation studies pointed to two likely origins of secondary undercuts: the weak adhesion of some of the mask materials and current crowding in bulk Si substrates near the edge of the mask openings. Secondary undercuts result in the erosion of the precious Si chip surface area when a porous Si trench is used for rf crosstalk isolation, and should be minimized.

Porous silicon 2D photonic crystals

“…The pore directions in the open areas and under the mask are aligned vertically and parallel to the surface, respectively (see Figure 2 a). It can be explained by the hole current direction in the silicon substrate [16]. Alternating layers obtained from nearby open spaces, even after contacting each other, don't produce continuity between adjoining structures (see Figure 2 b).…”

“…The second is due to the weak adhesion between the mask material and silicon and also the interaction between the mask material and electrolyte. The shape and spread of the undercut is dependent on the type of mask materials used for selective formation of porous silicon as well as the substrate resistivity [16]. In this work, all experiments were performed on heavily doped p + silicon wafers and only mask materials were changed.…”

Porous Silicon Characterization and Application: General View

“…This gas is not selective to silicon and could deteriorate the substrate during the patterning or the porous layer during its removal. In addition, silicon carbide is expensive and has a high intrinsic stress and poor adhesion (Kim et al 2003). Silicon nitride tends to show better results but is not completely resistant against fluoric solution.…”

Optimized plasma-polymerized fluoropolymer mask for local porous silicon formation

“…In the literature, different kinds of HF resistant materials have been tested and proposed as a masking layer for deep PS formation. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Among them, silicon nitride (Si x N y ), 15,16 bilayer of polycrystalline silicon/silicon dioxide, 18,22 or tri-layer stack of poly-Si/Si x N y /SiO 2 12 are the most widely used masking materials due to the maturities of deposition techniques and the compatibility with current silicon processing. However, the common drawback of all these masking materials is the complicated post-etching mask removal procedures when bare silicon surface is ultimately required.…”