(a) Schematic of the reactive ion etching machine used in this process. All different parts are controlled using computer programming. (b) Process steps of deep silicon vertical etching (from A to D). The etching requires a passivation subcycle plus an etching subcycle to achieve a deep vertical etching.
SEM images of samples processed by silicon vertical etching with various heights and sizes. Top-left image belongs to parallel trenches with different sizes, whereas the top-right image shows a high aspect ratio etching of a 150 nm width rod with the height of about . The smiley face structure has height and aspect ratio of 30:1. The middle right image represents a cross section of a vertically etched hole in silicon with a height of . An array of vertical rods with 300 nm width and a height of .
(Color online) Observation of RIE lag for small openings. For a small opening of , there is a slight drop in the etch rate. The red line compares the bottom of a large opening with smaller features. As seen the depth of the narrower opening is slightly less than the larger windows. The bottom image shows the slight reduction in the size of opening due to this effect.
(Color online) (a) Etch rate vs passivation time where the duration of the etch subcycle has been fixed at 15 s. If the passivation time exceeds 2 min, etch rate drops considerably to less than . (b) Etch rate vs rf power during the etching subsequence. As seen by raising the plasma power the total and sequence etch rates greatly increase.
(Color online) (a) AFM analysis of a silicon sample after vertical etching has been performed. The surface roughness with an average of 46 nm is observed from this image. (b) Surface improvement after the sample has been oxidized where the average roughness drops to 23 nm.
(Color online) (a) Severe underetch results in a crownlike structure. The gearlike chromium mask is remained on top of the structure. (b) The same structure with lower amount of underetch suitable for three-dimensional fabrication. The control over underetching is an important factor for the formation of highly featured three-dimensional structures. (c) The same structure is etched with some underetching at the first steps followed by high aspect ratio vertical etching. The arrow shows where the underetch has recovered.
Some silicon 3D structures obtained from controlled underetch during vertical (anisotropic) etching process. (a) Image shows an unusual sample where the initial underetching has been recovered and the remaining of the process has been vertical. Lowering the plasma power and duration of passivation process for the first 15 etching sequences has led to the underetching, whereas for the remaining 15 sequences, the recipe is the same as the one used for the results in Fig. 2. b)–(d) are opposite in the sense that initial etching sequences (40) are fully vertical and the last 10 sequences are with lower hydrogen flow. Cuplike structures with little holes inside are seen as dark spots in (b).
(Color online) Process steps of a desired 3D shape silicon etching, (a) starting with mask patterning, followed by (b) etching with desired underetching and (c) recovery, and (d) oxide passivation and (e) removal from the bottom. (f) Final structure depends on the number of times these steps are repeated.
(Color online) Collection of micrometer and submicrometer wavy 3D structures with underetching and recovery of the order of few micrometers. The arrayal structure shows a clean surface where the roughness is quite low.
(Color online) Process steps for fabricating 3D nanowall structures. (a) After fabricating the 3D structure, (b) a wet thermal oxidation of about 20 min is performed at , and by (c) removing the top chromium mask, the Si surface at the top of the structure becomes available. By restarting the etching process, the Si layer is removed and the film remains, leading to (d) 3D nanowall structures.
(Color online) Formation of 3D structures with nanowall features. Both rows correspond to various steps during which the three-dimensional structure becomes hollow from top and the edges are nanometric . The arrow shows a point where the thickness of the remaining layer has been successfully measured to read 120 nm.
Etching parameters selected for observing the effect of sweeping the passivation time.
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