Ultrafast fabrication of high-aspect-ratio macropores in P-type silicon: toward the mass production of microdevices

ABSTRACT Etching rate is a major concern for the effective mass production of high-aspect-ratio microstructures, especially in p-type silicon. In this work, controlled electrochemical growth of high-aspect-ratio (from 15 to 110) macropores in p-type silicon at ultrafast etching rate (from 16 to 30 µm min−1) has been studied. Based on current-burst-model, pore formation was systematically investigated from the nucleation phase to stable pore growth. Good macropores with depth up to 180 µm and aspect ratio beyond 110 was achieved in just 11 min. This sets a new record on state-of-the-art p-type silicon microfabrication and can promote the development of microdevices. GRAPHICAL ABSTRACT Impact Statement High-aspect-ratio macropores at ultrahigh etching rate were achieved in p-type silicon, which sets a novel record among p-type silicon microfabrication and can promote the effective mass production of microdevices.


Introduction
Porous silicon, produced by electrochemical etching in hydrofluoric acid (HF) containing solution, has been a subject of great attention due to its specific physical and chemical properties. Depending on the substrate type and the etching parameters, numerous different pore morphologies can be obtained, resulting in a wide range of applications over the past few decades. Macroporous silicon, in particular with high aspect ratio (AR), has been used in many technical fields, such as microelectromechanical systems (MEMS) devices [1,2], biosensors [3,4], fuel cells [5,6], microelectronics [7] and photonic crystals [8,9]. All these applications require the development of high-quality pore structures with fast etching speed, which is one of the most important factors in effective mass production.
With mass fabrication in mind, there has been significant interest in the fast pore etching on n-type macroporous silicon in the past few years. The conventional technique [10] of macropore formation on n-type silicon was based on photo-assisted electrochemical etching, where the etching rate was around 1 μm min −1 . By modulating the HF-containing electrolytes with organic solvents [11,12] or strong oxidizers [13,14], perfect macropores can be produced more rapidly on all kinds of n-type silicon substrates (low-doped [13], moderately-doped [11] and highly-doped [14]) in the dark (without backside illumination). Furthermore, under illumination, fast macropore formation is realized in aqueous HF electrolytes on low-doped n-type silicon with optimal bias and HF concentration ([HF]) [15]. Recently, Barillaro [16] reported on the controlled electrochemical etching of high-aspect-ratio (from 5 to 100) macropores in n-type silicon at the highest etching rates (from 3 to 10 μm min −1 ). On the other hand, in p-type silicon, it's much more complicated to achieve macropores with high AR because the hole concentration is much higher than in n-type silicon and hence cannot be controlled in the pore walls [17]. A few studies have been done on high-aspect-ratio macropore formation on p-type silicon [17][18][19][20][21]. However, the etching rates in above works were no more than 2 μm min −1 . So far few reports have been concentrated on fast macropore etching in p-type silicon.
The purpose of this paper is to obtain fast growing macropores with good quality (straight and smooth pore walls) and high AR on p-type silicon substrates. We demonstrate that macropores with extremely large AR (over 110) and high depth (up to 180 μm) can be achieved in p-type silicon at the highest etching rates (from 16 to 30 μm min −1 ) using a set of optimized parameters, the electrolyte composition, [HF] and current density. The etching rates from the nucleation to stable pore growth and from the smaller to higher depths were investigated. The current-burst-model (CBM) [22] was employed to justify the observed results.

Experimental
In our experiments, p-type wafers (20-40 cm, 500 μmthick, CZ-grown, (1 0 0)-oriented and polished) were used. An 800 nm-thick aluminum film was sputtered on the backside of samples to establish a good ohmic contact. All samples were prepared in the dark on an electrochemical etching platform in a standard decontamination chamber at room temperature (18 ± 1°C), with the etching area of 1 cm 2 . The electrolytes used for anodization were based on a mixture of 48 wt.% aqueous HF and two different organic solvents: dimethylformamide (DMF) and dimethylsulfoxide (DMSO). Mainly four types of electrolytes were prepared in our experiments:

Results and discussion
First, a set of anodizing experiments were performed at different current densities (30, 150, 300 mA cm −2 ), for a given electrolyte (HF: DMF = 3:7), in order to investigate the dependence of pore morphology and etching rate on the current density. The etching rate was evaluated (unless otherwise stated) as the ratio between the etching depth and etching time. The cross-section and top-view SEM micrographs of Figure 1 show how the pore sizes and morphologies are affected by the current densities. The micrographs in the middle column show the high magnification views of the regions defined by the black squares in the left column.
At 30 mA cm −2 , heavily branched mesopores were obtained (Figure 1(a)). The average diameters and the top diameters of the main pores were about 2.1 (Figure 1(b)) and 1.4 μm (Figure 1(c)), respectively. The depth of mesopores was about 18 μm, corresponding to the average etching rate of 1.2 μm min −1 . While current density increased to 150 mA cm −2 , straight pores with average diameters of 1.8 μm were obtained (Figure 1(d,e)) with the pore openings decreased to 1.2 μm (Figure 1(f)). The pore walls were generally straight, cylindrical and relatively smooth with fewer branches. The pore depth was about 42 μm after anodization for 5 min, with the average etching rate increased up to 8.4 μm min −1 . By increasing current densities to 300 mA cm −2 , nice macropores with depth of 20 μm were obtained in 1.5 min (Figure 1(g)), and the average etching rate was increased to 13.3 μm min −1 . As can be seen from the enlarged micrograph (Figure 1(h)) and the corresponding topview image (Figure 1(i)), the macropores were cylindrical and generally uniform with rather straight and smooth pore walls. The average sizes of stable macropores were about 1.4 μm, with the top diameters around 0.8 μm.
Thus the pore morphology and etching rate are very sensitive to the current density. From Figure 1(a-g), the current densities increase from 30 to 300 mA cm −2 , and the etching rates increase from 1.2 to 13.3 μm min −1 . However, the pore diameter and opening decrease with increasing the current densities, from 2.1 to 1.4 μm, and 1.5 to 0.8 μm, respectively. In addition, the pore diameters are significantly larger in the middle than at the opening, comparing the cross-section and top-view images of the pores. This observation is attributed to the transition, commonly seen in the electrochemical etching of silicon [23], from the initial to the stable-growth state.
With the aim to gain more insight into the dependence of the macropore etching rates on the applied current densities, two sets of experiments were then carried out at different current densities (10-300 mA cm −2 ) for different HF: DMF ([HF] = 30 vol%, 43 vol%, 57 vol%) electrolytes and different electrolyte compositions (DMF and DMSO). Figure 2(a) shows the experimental data on the etching rate versus current density as a function of [HF] in electrolytes. It is apparent that the macropore growth rate consistently increases with the current density for any [HF]. The curves are in good agreement with the results in Figure 1 and Ref. [20]. Furthermore, the increase of [HF] leads to an enhanced pore etching rate for any current density. By changing the [HF] from 30 to 57 vol%, the average etching rate increase by around 60%. Figure 2(b) shows the dependence of the average pore etching rate with current density for two different electrolyte compositions with 43 vol % [HF]. One consistent trend is that the etching rate in electrolyte using DMF as organic solvent is larger than that in electrolyte containing DMSO, varying the current densities from 10 to 300 mA cm −2 . Moreover, in agreement with the trend in Figure 2(a), macropore growth rate increases consistently with the current density for any electrolyte composition.
Based on these experimental results, we can infer that at least three prime factors pertain to fast pore etching on p-type silicon, namely [HF], electrolyte composition and current density. According to the CBM [22], two basic dissolution reactions (direct dissolution via HF acid and indirect dissolution by oxidation) take place at the pore tips simultaneously in a current burst during anodization. High [HF] will accelerate the direct removal of silicon and promote the dissolution of the oxides, thus shortening both the time of direct and indirect dissolution in every single current burst, compared with a lower [HF]. Therefore, for a given current density (300 mA cm −2 ), as shown in Figure 2(a), the average etching rate over 5 min increases from 13.3 to 23 μm min −1 , while the [HF] increases from 30 to 57 vol%.
Secondly, the etching rate is dependent on the electrolyte composition. It has been shown that organic solvents, act as mildly oxidizing reactants, can suppress the oxidation process compared to the pure aqueous electrolyte [24]. This allows for shorter indirect-dissolution time and better macropore growth. Besides, as stressed in CBM, electrolyte compositions determine the velocity of the H-termination, which is regarded as the basic passivation mechanism of the pore walls [25]. The better the passivation of pore walls, the higher is the etching rate at the pore tips. Therefore, the availability of H, i.e. the H donor ability, plays an important role in fast pore etching. In general, the H donor property of DMF is better than that of DMSO [24], resulting in higher dissolution rate of pore tips in DMF-contained electrolyte. Thus, a clear trend can be seen in Figure 2(b), for a given [HF],  the etching rate in electrolyte using DMF is much higher than that in electrolyte containing DMSO.
The last crucial factor for fast pore etching lies in the applied current density. A larger current density means more available transferred charges, thus enhancing the electronic-hole-intensive reaction [26]. In addition, raising current density leads to a shorter time consumption for H-termination process, and thus optimize the pore nucleation phase [22]. SEM pictures in Figure 3 show the nucleation process of p-type macropore formation for a given 57 vol% [HF] under different current densities (30 and 300 mA cm −2 ). At 300 mA cm −2 , a layer of mesopores were obtained after 5 s anodization (Figure 3(a)). The depth of mesopores was about 400 nm, corresponding to an average etching rate of 4.8 μm min −1 . According to the CBM [27], it was in the nucleation phase and the mesoporous layer was the nucleation layer. While the etching time increased to 7 s, the mesoporous nucleation layer grew to a depth of 1 μm and some wavy macropores formed (Figure 3(b)), showing the nucleation was completed and pore formation was transformed to the reorganization process. The pore depth was about 4.2 μm, with the average etching rate increased to 36 μm min −1 . By increasing the etching time to 35 s, straight macropores with depth of 22 μm were obtained (Figure 3(c)), and the average etching rate was increased to 37 μm min −1 , demonstrating the stable pore growth.
For comparison, pore formation at lower current density (30 mA cm −2 ) with the same electrolyte was investigated. A layer of mesopores with thickness of 260 nm (Figure 3(d)) was formed during 40 s anodization, and the reorganization phase appeared when the etching time prolonged to 60 s (Figure 3(e)). Therefore, for a larger applied current density, the time of nucleation phase was optimized from dozen of seconds to no more than 7 s. Figure 3(f) shows the pore growth curves in nucleation phase with two different current densities (30 and 300 mA cm −2 ) in 1 min. From these curves, it is obvious that increasing the current density dramatically shortens the pore nucleation time and thus leads to a great enhancement of the etching rate.
Following these experimental results and analysis, we tried to optimize the etching conditions for stable p-type macropore formation with high AR and high etching rate. Macropore etching in p-type silicon with a number of first optimized parameters (HF: DMF = 4:3, 300 mA cm −2 ) was investigated as a function of the etching time (from 0 to 11 min). The macropore growth curve (pore depth versus etching time) was recorded in Figure 4(a). The pore depth consistently increases with etching time, and macropores with depth of 180 μm were achieved by increasing the etching time to 11 min. This result demonstrates significant enhancement of macropore etching rate with respect to the literatures [17][18][19][20], in which it takes about 2 h to produce p-type macropores with the same depth. Figure 4(b) shows the average etching rate as a function of the etching depth. The etching rate was relatively small (4.8 μm min −1 ) in the nucleation phase, followed by largely varying etching rates in the reorganization phase and relatively stable etching rates (above 15 μm min −1 ) in the stable growth phase. This indicates that the optimized process can dramatically enhance the etching rate at any depth. The average etching rate was reduced from 30 μm min −1 at the depth of 30 μm to 16 μm min −1 at the depth of 180 μm, probably due to the reactant diffusion limit over deep depth. Figure 4(c,d) show the SEM micrographs of the obtained macropore structure after electrochemical etching of 11 min. The pore walls were very straight and smooth (Figure 4(d)) with the depth up to 180 μm and diameters of 1.6 μm, corresponding to the AR above 110 and the average etching rate of 16 μm min −1 . Therefore, ultrafast macropore fabrication can be obtained with ultrahigh AR at higher depth without loss of pore quality.

Conclusions
In summary, we demonstrate that p-type macropores with ultrahigh AR can be fabricated at ultrafast etching rate. Under the conditions of high current density, high [HF] and DMF-containing electrolyte, the time of nucleation phase was greatly shortened, and the etching rate was dramatically enhanced (16-30 μm min −1 ) both at smaller and higher depths (up to 180 μm) without any significant loss of good 'quality', which was at least ten times larger than that with state-of-the-art microfabrication processes. The experiment results verify the judgment inferred from the CBM and three prime factors are responsible for the great rate enhancement.

Disclosure statement
No potential conflict of interest was reported by the authors.