Nanocone arrays are widely employed for applications such as antireflection structures and field emission devices. Silicon nanocones are typically obtained by an etching process, but the profile is hard to attain because anisotropic dry etching generally gives vertical or only slightly tapered sidewall profiles, and isotropic dry plasma etching gives curved sidewalls. In this work, we report the fabrication of cone structures by using masked etching followed by maskless etching techniques. The silicon structure is first etched using fluorine-based plasma under the protection of a hard metal mask, with a tapered or vertical sidewall profile. The mask is then removed, and maskless etching with an optimized nonswitching pseudo-Bosch recipe is applied to achieve the cone structure with a sharp apex. The gas flow ratio of C4F8 and SF6 is significantly increased from 38:22 (which creates a vertical profile) to 56:4, creating a taper angle of approximately 80°. After subsequent maskless etching, the sidewall taper angle is decreased to 74°, and the structure is sharpened to give a pointed apex. The effect of an oxygen cleaning step is also studied. With the introduction of periodic oxygen plasma cleaning steps, both the etch rate and surface smoothness are greatly improved. Lastly, it was found that the aspect ratio-dependent etching effect becomes prominent for dense patterns of cone arrays, with a greatly reduced etch depth at a 600 nm pitch array compared to a 1200 nm pitch array.

Si nanocone arrays are widely used in solar cells for antireflection and light trapping,1–3 optical sensors,4 and other optical devices due to their excellent ability to capture light.5 This cone structure normally holds the sharp tip which is always applied for atomic force microscopy (AFM)6 and electron field emitter arrays.7 

Nanocone structure fabrication is commonly based on nanosphere lithography, which is a low-cost self-assembly process.8,9 However, this process is uncontrollable with irregular sphere coverage, resulting in areas that are, respectively, bare or covered by multiple stacked sphere layers. Some researchers have used anisotropic KOH wet etching technique to obtain an ordered cone array,10 but with this approach, the slope of the sidewall is determined by the crystalline orientation and is, thus, not tunable. It has also been reported that cryogenic etching using SF6/O2 gas below −80 °C can fabricate a cone structure,11 with good profile control at such low temperatures. However, this process requires precise control of the temperature, setting a higher standard for etching equipment that is not cost-effective. In addition, the cryo-process is often subject to uncontrollable “grass” formation due to over-passivation.

The deep reactive ion etching (DRIE) technique using the Bosch process provides a promising approach to fabricate cone structures, with the ability to control the profile of anisotropic etching by means of adjusting the passivation and etching half-cycle durations. Increasing the passivation half-cycle relative to the etching half-cycle will lead to the positively tapered profile needed for cone fabrication, but at the cost of greatly reduced etch rate and the eventual uncontrollable formation of silicon grass at random locations. Moreover, the Bosch process gives a scalloped sidewall profile due to the switching half-cycles. Therefore, the pseudo-Bosch process also called the mixed process (with simultaneous C4F8 and SF6 in the chamber) is employed in this study to give a smooth sidewall.12,13 It has been reported that this recipe can give a perfectly vertical sidewall profile.14 In this process, the passivation and etching proceed simultaneously, and the profile is controlled by both lateral and vertical etching that depends on the competition between fluorine free radical etching and inhibitor formation or passivation (associated with SF6 and C4F8, respectively).15 A brief comparison of these three different silicon dry etching processes is shown in Table I. Thus, the taper angle of the profile is tunable with the gas flow ratio of the etching and passivation gases. With higher passivation gas flow, the profile should be more positively tapered.

TABLE I.

Comparison of three main silicon dry etching techniques.

TermsBosch processCryogenic etching processPseudo-Bosch process
Working process Pulsed mode Mixed mode Mixed mode 
Main gases SF6 and C4F8 SF6 and O2 SF6 plus C4F8 or O2 
Passivation layer Flurocarbon polymer SiOxFy Flurocarbon polymer 
Process temperature Room temperature Lower than −80 °C (Ref. 16Room temperature 
Etching rate 1–10 μm/min (Ref. 171–10 μm/min (Ref. 16100 nm–20 μm/min (Ref. 17
Etching selectivity (Si to photoresist) 10 to 150:1 (Ref. 17Normally 100:1 (Ref. 16Normally less than 10:1 (Ref. 17
Sidewall roughness Rough due to scalloping Smooth Smooth 
TermsBosch processCryogenic etching processPseudo-Bosch process
Working process Pulsed mode Mixed mode Mixed mode 
Main gases SF6 and C4F8 SF6 and O2 SF6 plus C4F8 or O2 
Passivation layer Flurocarbon polymer SiOxFy Flurocarbon polymer 
Process temperature Room temperature Lower than −80 °C (Ref. 16Room temperature 
Etching rate 1–10 μm/min (Ref. 171–10 μm/min (Ref. 16100 nm–20 μm/min (Ref. 17
Etching selectivity (Si to photoresist) 10 to 150:1 (Ref. 17Normally 100:1 (Ref. 16Normally less than 10:1 (Ref. 17
Sidewall roughness Rough due to scalloping Smooth Smooth 

However, besides the problems of very slow etch rate and random grass formation at the over-passivation region, it was found that the sharp apex formed only for small feature size; for large feature size, it is difficult to achieve a sharp point due to the insufficient lateral etch under the protection of mask during etching. Therefore, we studied the maskless etching process to refine the etch profile.18 By applying a maskless etching procedure on a pre-etched structure, the profile will be optimized to achieve a cone shape, with both a tapered smooth sidewall and a sharp apex due to the lateral etching and shrinkage without a hard mask. This two-step etching process, with masked etching followed by maskless etching, is more capable of attaining a desired size and shape for ordered nanocone arrays than single step etching with a hard mask.

For this experiment, a Cr mask is used for high etch rate selectivity (>50:1) to silicon in contrast to a photoresist or SiO2 mask (<10:1). In addition, Cr gives a relatively large mask undercut during the etching process, desirable for cone fabrication.19,20 The 4-in. Si wafer is first processed with RCA clean followed by de-ionized water rinse. A ZEP replacement resist (Allresist GmbH) is spin-coated on the wafer at 1000 rpm for 1 min, followed by 3 min baking on a hotplate at 180 °C. Electron beam lithography (JEOL JBX-6300FS EBL system operating at 100 kV and 20 nA beam current) is used to define the pattern. The dose for the pattern area is 250 μC/cm2. The sample is then developed in ZED-N50 for 1 min followed by 1 min isopropanol rinse. 30 nm Cr is then deposited by e-beam evaporation. A lift-off process transfers the pattern to Cr. For the lift-off process, the sample is immersed in Remover PG solvent (Kayaku advanced materials, Inc.) heated up to 80 °C with mild sonication. After a delay of 30 min, the sample is rinsed by isopropanol.

The ICP-RIE etching process is applied after the preparation of the sample, using the Oxford Instrument PlasmaLab 100 system. The sample is first etched to get a primary height, with vertical or tapered profiles resulting from different etching recipes. The Cr mask is then removed by wet etching, and further maskless etching is performed to finalize the structure. This process is illustrated in Fig. 1 and the taper angle is defined there. In the definition of the taper angle, it should be noted that the profile is vertical at 90° but becomes sharper as the taper angle decreases. For vertical profile etching, the classical nonswitching pseudo-Bosch process is applied with the recipe listed in Table II.21 For taper profile etching, we modified this recipe, increasing the gas ratio of C4F8/SF6 from 38:22 to 56:4 to promote passivation over-etching. It has been reported that by increasing the gas ratio of C4F8/SF6, the etch profile will be more positively tapered.22,23 The RF bias power can also control the taper angle,22,23 but RF power is a complex factor in the etching process. High RF bias power can promote ion bombardment to result in a more directional etch, but, it will also increase the sheath thickness of the plasma, increasing the chance of collision between ions and molecules, which may instead result in a less directional etch.24 Considering this aspect, we did not vary the RF power significantly and only modified the gas glow ratio significantly. Another concern in this process is the large amount of C4F8, as excessive fluorocarbon polymer will deposit and accumulate in the chamber. This will prevent or disturb further etching. A periodic oxygen plasma cleaning process is introduced to solve this problem.12 The detailed parameters of the etching profile recipe are listed in Table III. Since the gas ratio of C4F8 and SF6 is 56:4, this recipe is identified as the 56:4 recipe.

FIG. 1.

Illustration of the formation of silicon nanocones. (a) Starting from the Si substrate, patterned with Cr mask; (b) Si dry etch; (c) Cr mask removal; and (d) further Si dry etch to achieve taper profile.

FIG. 1.

Illustration of the formation of silicon nanocones. (a) Starting from the Si substrate, patterned with Cr mask; (b) Si dry etch; (c) Cr mask removal; and (d) further Si dry etch to achieve taper profile.

Close modal
TABLE II.

Parameters of the classical nonswitching pseudo-Bosch process that gives vertical profiles.

C4F8 (SCCM)SF6 (SCCM)RF power (W)ICP power (W)Temperature (°C)Pressure (mTorr)
38 22 20 1200 15 10 
C4F8 (SCCM)SF6 (SCCM)RF power (W)ICP power (W)Temperature (°C)Pressure (mTorr)
38 22 20 1200 15 10 
TABLE III.

Parameters of the “56:4” recipe that gives tapered profile.

ParametersEtching stepOxygen depassivation step
Time (s) 60 
O2 (SCCM) 20 
C4F8 (SCCM) 56 
SF6 (SCCM) 
RF (W) 15 15 
ICP (W) 2000 1000 
Tempertature (°C) 15 15 
Pressure (mTorr) 10 10 
ParametersEtching stepOxygen depassivation step
Time (s) 60 
O2 (SCCM) 20 
C4F8 (SCCM) 56 
SF6 (SCCM) 
RF (W) 15 15 
ICP (W) 2000 1000 
Tempertature (°C) 15 15 
Pressure (mTorr) 10 10 

As discussed, the sample is first etched with the optimized pseudo-Bosch process to get a tapered profile. This recipe is a switching process, with each cycle consisting of two steps of etching and oxygen depassivation. Figure 2(a) shows the result of the pattern after 12 cycles of the 56:4 recipe etching, which gives a 975 nm-height silicon structure with a sidewall taper angle of 80°.

FIG. 2.

SEM images of positively tapered Si nanostructures (images taken at 70° tilt). (a) After 12 cycles of the 56:4 recipe with a Cr mask. (b) After 4 min etching with 100 SCCM C4F8/4 SCCM SF6 gas flow (no periodic O2 plasma cleaning), followed by 5 min oxygen plasma cleaning with Cr mask retained. The undercut here is just sufficient to result in the Cr mask falling off, producing a pointed apex. The scale bar is 100 nm shown at the bottom of the images.

FIG. 2.

SEM images of positively tapered Si nanostructures (images taken at 70° tilt). (a) After 12 cycles of the 56:4 recipe with a Cr mask. (b) After 4 min etching with 100 SCCM C4F8/4 SCCM SF6 gas flow (no periodic O2 plasma cleaning), followed by 5 min oxygen plasma cleaning with Cr mask retained. The undercut here is just sufficient to result in the Cr mask falling off, producing a pointed apex. The scale bar is 100 nm shown at the bottom of the images.

Close modal

To obtain a larger taper profile, a higher flow of C4F8 gas is studied, and Fig. 2(b) gives the result of 4 min modified etching, identical to the etching used in Fig. 2(a) except for C4F8 gas flow, which is increased from 56 to 100 SCCM. However, the improvement is insignificant as the sidewall taper angle barely increases. There might exist a limit to the possible taper angle when using this nonswitching etching process. The C4F8 gas will produce an excess amount of CxFy species, which will form a fluorocarbon polymer and deposit on the sample surface and chamber walls. This excessive passivation layer requires more fluorine radicals to break through, otherwise, the etching process will be inhibited.25 When large amounts of C4F8 are applied, the SF6 gas will be diluted, also reducing the etch rate. Overall, even though greatly reducing the SF6 flow rate to increase the C4F8/SF6 ratio may give a larger sidewall slope angle, it will also greatly reduce the etch rate, cause grass formation, and contaminate the chamber with a harmful species. Therefore, for further etching processes, the gas flow ratio of the tapered profile etching recipe is fixed at 56:4.

Since there is a practical limit of the taper angle when using one-step masked etching, maskless etching is applied to further shape the etched structure. The pattern without the protection of a mask will be etched isotropically. However, since the evolution of undercut, the cross section of the origin pattern is trapezoidal. Thus, with maskless etching, a cone shape can be formed.

After 12 cycles of masked etching, the Cr mask is removed and further maskless etching is performed, which gives a tapered profile with a pointed apex. Figures 3(b)3(d) show the results after 5, 15, and 20 cycles of maskless etching using the 56:4 recipe, respectively. As further maskless etching is performed, the taper angle decreased from 78° [Fig. 3(b)] to 74° [Fig. 3(d)]. Interestingly, as maskless etching proceeded the top diameter of the structure decreased significantly, whereas the bottom diameter does not change significantly. One explanation is that the top part is subject to more direct ion bombardment. It can also be found that, without the protection of mask, the height of the profile decreased from 975 to 690 nm. This deduction in height is remarkable. It means that the final height of the profile after maskless etching is close to the original height achieved by masked etching. Thus, the height of the cone array can be controlled by the masked etching duration, and the diameter of the cone array can be determined by the original mask pattern design. Previous research related to wet etching by using the mixture of HF and HNO3 is also a promising way for final structure tuning, however, this method is only suitable for dense arrays.26 It can be concluded that the combination of masked tapered silicon etching and maskless etching is an effective way to achieve an ordered cone array with tunable size and shape.

FIG. 3.

SEM images of sample evolution after 12 cycles of the 56:4 recipe with a Cr mask [(a) taken at 70° tilt and (b)–(d) taken at 60° tilt]. (a) 12 cycles 56:4 recipe masked etching; (b) after Cr mask removal and additional five cycles of 56:4 etching, with tapered sidewalls; (c) sample continued with ten cycles of 56:4 maskless etching; and (d) final additional five cycles 56:4 maskless etching. The scale bar is 100 nm shown at the bottom of the images.

FIG. 3.

SEM images of sample evolution after 12 cycles of the 56:4 recipe with a Cr mask [(a) taken at 70° tilt and (b)–(d) taken at 60° tilt]. (a) 12 cycles 56:4 recipe masked etching; (b) after Cr mask removal and additional five cycles of 56:4 etching, with tapered sidewalls; (c) sample continued with ten cycles of 56:4 maskless etching; and (d) final additional five cycles 56:4 maskless etching. The scale bar is 100 nm shown at the bottom of the images.

Close modal

Despite achieving a relatively large taper angle with the 56:4 recipe, the etching rate of 50 nm/min is rather slow. With the periodic oxygen cleaning step, the etch rate can increase to 80 nm/min, still very low compared with the traditional Bosch process and the nonswitching pseudo-Bosch process. This motivated us to replace the 56:4 recipe for the masked etching step with the standard nonswitching pseudo-Bosch recipe, which gives a much faster etch rate with a vertical profile. We then carry out a maskless etch to convert the vertical pillar structures into cone structures.

Figure 4 shows the results of this two-step etching process. Figure 4(a) is the result after 1 min of classical nonswitching pseudo-Bosch etching with a Cr mask that, as expected, gives a vertical profile. The mask was then removed and maskless etching with the 56:4 recipe was performed. For this sample, only two cycles of maskless etching were performed. The final nanocone array has a sidewall taper angle of 74° with a sharp cone apex. It should be mentioned that for wider pillars, more cycles of maskless etching would be required to make a pointed apex.

FIG. 4.

SEM images of Si nanostructures (images taken at 70° tilt). (a) After 1 min classical nonswitching pseudo-Bosch etching, with vertical sidewall and (b) after Cr mask removal and additional two cycles of 56:4 etching, with tapered sidewalls. The scale bar is 100 nm shown at the bottom of the images.

FIG. 4.

SEM images of Si nanostructures (images taken at 70° tilt). (a) After 1 min classical nonswitching pseudo-Bosch etching, with vertical sidewall and (b) after Cr mask removal and additional two cycles of 56:4 etching, with tapered sidewalls. The scale bar is 100 nm shown at the bottom of the images.

Close modal

We have so far presented our efforts to achieve sharp nanocone array structures. Below, we examine two more issues in our etching process development: the effects of oxygen plasma cleaning and array pattern density, which are discussed in this section and in Sec. III E. To investigate the effect of the oxygen depassivation step in the 56:4 recipe, two groups of samples are etched and compared, with and without the oxygen depassivation step. The duration of the total etching time is 8 min for both. That is, as shown in Table III for the one with the oxygen depassivation step, 7 s of cleaning are added after 1 min of etching for eight etch cycles for the oxygen-cleaned sample.

As shown in Fig. 5(b), the surface of the sample etched without periodic oxygen depassivation is much rougher than the cleaned sample in Fig. 5(a). This is caused by fluorocarbon polymer deposition on the surface during the etching process. Moreover, the structure etched with periodic oxygen depassivation is nearly twice the height of its uncleaned counterpart, with fluorocarbon polymer accumulation slowing the etch process in the latter. Though the polymer formation on the sample surface should be uniform, the etching chemical reaction causes local heating to reduce polymer formation and promote the etch at the heated spots, generating more heat; this positive feedback loop gives instability in the local etch rate and, thus, generates a rough surface or even tall “grass” at random locations.

FIG. 5.

SEM images of Si pillars after 8 min etching (images taken at 70° tilt). (a) With oxygen depassivation step. (b) Without oxygen (continuous etching). The scale bar for (a) is 100 nm and for (b) is 1 μm shown at the bottom of the images.

FIG. 5.

SEM images of Si pillars after 8 min etching (images taken at 70° tilt). (a) With oxygen depassivation step. (b) Without oxygen (continuous etching). The scale bar for (a) is 100 nm and for (b) is 1 μm shown at the bottom of the images.

Close modal

However, based on our investigation, the oxygen cleaning step has adverse effects. The duration of the oxygen cleaning, half-cycle should be optimized. Prolonged oxygen cleaning will remove the fluorocarbon polymer and then oxidize the silicon to form a SiOx layer which is more difficult to etch than silicon. Inadequate oxygen cleaning cannot remove fluorocarbon polymer completely. Another concern is mask erosion by oxygen chemical etching and/or ion bombardment, which will lower the etching selectivity between the mask and silicon.12,27,28 For a metal mask, the reduced selectivity is generally still acceptable; however, for a photoresist mask, oxygen plasma will etch the photoresist rapidly to greatly reduce the selectivity. As such, mask erosion should be taken into consideration, especially for fabricating high aspect ratio features.

The pattern density (i.e., the array periodicity) also affects the etching profile. To investigate the effects of pattern density arrays of two different pitches, 1200 and 600 nm, were designed. Both samples are etched with 12 cycles of the 56:4 recipe with a Cr mask, followed by 5 cycles of the 56:4 recipe with the mask removed.

As shown in Fig. 6(a), for the dense pattern of 600 nm array pitch, the etch depth is not uniform, with cones inside the pattern area much shorter than those at the pattern’s edge. However, the pattern of the sparse array with 1200 nm periodicity shown in Fig. 6(b) was etched uniformly. The reason for this phenomenon is expected to be the aspect ratio-dependent etching (ARDE) effect. For the central pattern, there is a smaller opening between cones, and transport of the etchant and products into and out of the open area is more difficult. Ion bombardment of the bottom surface is more shadowed by the etch mask and the upper part of the structure (the ions have an arrival angle range, rather than all normal incidence),29 also reducing the etch rate. In this scenario, the ARDE effect is further magnified by the heavy passivation, for which an increase in fluorocarbon polymer inhibitor from even a slightly reduced removal process can greatly decrease the etch rate.

FIG. 6.

SEM images of (a) small pitch (600 nm) pattern (image taken at 60° tilt). (b) Large pitch (1200 nm) pattern after etching (image taken at 70° tilt). The scale bar for (a) is 1 μm and for (b) is 100 nm, shown at the bottom of the images.

FIG. 6.

SEM images of (a) small pitch (600 nm) pattern (image taken at 60° tilt). (b) Large pitch (1200 nm) pattern after etching (image taken at 70° tilt). The scale bar for (a) is 1 μm and for (b) is 100 nm, shown at the bottom of the images.

Close modal

We developed a two-step process to fabricate ordered nanocone array structures. The first step is to etch a silicon structure using a Cr mask, with tapered or vertical sidewall profiles. The Cr mask is then removed, and maskless etching is carried out to modify the profile to a sharp cone shape. The optimized silicon etching recipe that gives a tapered profile is based on the classical nonswitching pseudo-Bosch recipe, with an increased gas flow ratio of C4F8/SF6 from 38:22 (vertical profile) to 56:4 (80° taper angle). The subsequent maskless etching can greatly increase the taper angle and produce a pointed apex. Moreover, the introduction of a periodic oxygen cleaning step is effective in removing excess fluorocarbon polymer and improves the etch rate and surface smoothness. The size of the cone array is largely determined by the pattern design since the lateral etching of the cone bottom is insignificant compared to the lateral etching of the upper part. Lastly, when etching a dense array, the ARDE effect seriously influences the uniformity of the etch profile. Such nanocone arrays can be applied in many devices such as photodetectors and solar cells.

The University of Waterloo’s QNFCF facility was used for this work. This infrastructure would not be possible without the significant contributions of CFREF-TQT, CFI, Industry Canada, the Ontario Ministry of Research and Innovation, and Mike and Ophelia Lazaridis.

The authors have no conflicts to disclose.

Zheng Yan: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Huseyin Ekinci: Formal analysis (equal); Investigation (equal); Validation (equal); Writing – review & editing (equal). Aixi Pan: Conceptualization (equal); Investigation (equal); Writing – review & editing (equal). Bo Cui: Funding acquisition (equal); Project administration (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

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