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Patterning of pyramidal recesses in (1 0 0)InP substrate
Microelectronic Engineering 88 (2011) 36–40
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Patterning of pyramidal recesses in (1 0 0)InP substrate P. Eliáš a,⇑, I. Kosticˇ b, J. Šolty´s a a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic Institute of Informatics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 07 Bratislava, Slovak Republic
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 30 June 2010 Accepted 18 August 2010 Available online 22 August 2010 Keywords: Inverted pyramids InP Wet-etching
a b s t r a c t Symmetrical pyramidal recesses were etched into (1 0 0)InP substrate in 3HCl:1H3PO4 at (16 ± 0.05) °C via 20 lm 20 lm square windows opened in InGaAs. The windows had sides aligned in h0 0 1i and cor 1. The recesses had each of the four sides at the square edges comners tapered along [0 1 1] and ½0 1 posed of a large ordinary facet (called pyramidal) and a small re-entrant facet. The pairs were 1 0g (pyramidal pairs), i.e. the ordinary pyramidal facets were identified to be initially close to {1 1 0}/f1 0Þ, and ð1 0 1Þ. However, they deviated towards planes with higher initially close to (1 1 0), (1 0 1), ð1 1 Miller indices with etching duration. The recesses were also confined to etch-stop {0 1 1} and fast-etch 1 taper edges. The ing {1 1 1}B facets at the [0 1 1] taper edges and etch-stop {2 1 1}A ones at the ½0 1 recesses evolved into sharp inverted pyramids with sub-100 nm extremities at the bottom if etched at least 30 min. The sharpening is possible thanks to the elimination of the etch-stop {2 1 1}A facets via a self-limited etching of the pyramidal pairs. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
The artificial structuring of semiconductor substrates, realized by localized dry- and wet-etching, lies at the core of any device technology. In III–Vs, such as InP and GaAs, mesas, ridges and grooves are the traditional patterns used for the definition and isolation of devices and in patterned epitaxy. The patterns are confined to sides whose profiles can range from rounded, containing ensembles of crystallographic facets, to straight ones, defined by a single facet [1–3]. The geometry of the latter is carefully defined to lead during epitaxy to the self-organized growth of quantum wells [4], wires [5,6], and dots [7]. This paper reports on pyramidal recesses exposed in (1 0 0)InP. The InP crystal contains twelve {1 1 0} crystallographic planes that have identical atomic arrangements. Four of them, (1 1 0), 0Þ, and ð1 0 1Þ, intersect (1 0 0) at 45° and theoretically (1 0 1), ð1 1 define inverted and upright pyramids based in (1 0 0). It was demonstrated previously that mesas confined to pyramidal facets 0Þ and (1 0 1)/ð1 0 1Þ pairs can be exposed in related to (1 1 0)/ð1 1 HCl via linear etching mask patterns aligned close to h0 0 1i [8– 10]. Below is a demonstration showing that flat-bottom and sharp inverted pyramids confined to such facets can be exposed in (1 0 0)InP by etching in HCl via square windows opened in InGaAs etching mask.
The recesses were etched into standard (1 0 0) semi-insulating InP:Fe substrate via a lattice-matched InGaAs layer, prepared by organometallic vapour phase epitaxy. The etching mask material exhibits superior masking properties because InGaAs (1) forms excellent interface with InP and (2) is inert to non-oxidizing HCl solutions, used to etch InP [11]. The patterning of InP substrates through such mask is well defined and highly repeatable [12,13]. The InGaAs etching mask was 120 nm thick, which allowed the observer to evaluate the amount of the InGaAs mask undercutting. We assume that as the mask was being undercut, edges of the released (free-standing) sections of the mask may have been subjected to some stress. However, we expect it had a negligible influence on the pyramidal recess formation. Windows in the InGaAs layer were opened with complete selectivity over InP in 1H3PO4 (85% w/w):1H2O2 (30% w/w):8H2O at 25 °C through openings in resist, defined by contact mode optical lithography. The windows were nominally 20 lm 20 lm squares with their sides aligned along h0 0 1i (Fig. 1(a)). The squares had 1. their corners tapered along [0 1 1] and ½0 1 Prior to the patterning, samples of the processed substrate were cleaned in acetone, isopropylalcohol and in oxygen plasma, then stripped off of surface oxides in 1HF(40% w/w):4H2O for 1 min at room temperature, and dried up in nitrogen flow. They were glued to GaAs holder chips (GaAs is not etched in HCl). The samples were etched in 3HCl:1H3PO4 at (16 ± 0.05) °C during 1, 2, 3, 5, 10, 15, 20, 30, 40, 60, and 80 min in light without agitation. (It was not crucial to keep the etching bath temperature fluctuation so small for this
⇑ Corresponding author. Tel.: +421 2 5922 2695; fax: +421 2 5477 5816. E-mail address: [email protected] (P. Eliáš). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.08.016
P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 1. (a) InGaAs etching mask: 20 lm 20 lm square windows with the sides of 1; (b) length s in h0 0 1i and corners of length t (3 lm) tapered in [0 1 1] and ½0 1 plane-view SEM micrograph of a recess etched for 1 min (InGaAs was etched off). {1 1 0} pyramidal facets and re-entrant facets f1 (appear bright) were exposed at the square sides. Re-entrant facets f2 (appear bright) linked with bottom ordinary {1 1 1}B-related ones were revealed at the [0 1 1] taper edges and ordinary {2 1 1}A 1 taper edges. related facets at the ½0 1
experiment. It will be more important for the patterning of the recesses with smaller size and pitch than those presented here. Such experiments are being in progress at our laboratory.) The solution was prepared by mixing electronic grade aqueousbased HCl (37% w/w) and H3PO4 (85% w/w) by volume ratio. To stop the etching process, the samples were dipped in H3PO4. They were rinsed with water and blown dry in nitrogen. The pyramidal recesses were studied with optical, scanning electron and atomic force microscopes (SEM and AFM, respectively). The SEM micrographs were taken with an S-800 HITACHI ultra-high vacuum (10–7 Pa) computer-controlled scanning electron microscope with a cold field emission electron source (single crystal tungsten, tip accelerating voltage between 1 and 30 keV, a 2 nm secondary electron image resolution). The AFM micrographs were obtained in non-contact mode with a Topometrix Explorer atomic force microscope from Veeco Instruments (maximum scanning field of 100 lm by 100 lm, maximum lift and resolution in the z axis of 10 lm and 0.2 nm, respectively). 3. Results Fig. 1(b) exemplifies a flat-bottom recess revealed via a 20 lm 20 lm square during 1 min. Its inverted pyramidal shape is evident: the sides along the square edges are confined to ordin 0Þ, and ð1 0 1Þ ary pyramidal facets close to (1 1 0), (1 0 1), ð1 1 ({1 1 0}) and smaller re-entrant facets f1. Other facets showed at the taper edges: top re-entrant facets f2 linked with bottom ordinary {1 1 1}B-related ones ({1 1 1}B) along [0 1 1]; and ordinary 1. The f2 and {2 1 1}A {2 1 1}A-related ones ({2 1 1}A) along ½0 1 facets did not undercut the mask. Fig. 2 captures the recess shape evolution during the first 3 min. It was the most dynamic period thanks to the pair of {1 1 1}B facets and (1 0 0)-related surface that disappeared after 2 and 3 min,
Fig. 2. Recesses in (a), (b), (c) etched for 1, 2, and 3 min, respectively; (b) {1 1 1}B facets disappear in 2 min and f3 facets emerge (black arrows); (c) (1 0 0)-related bottom surface etches off in 3 min and the {2 1 1}A facets intersect.
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respectively. With the fast-etching {1 1 1}B facets eradicated, the 0Þ and (1 1 0)/ð1 0 1Þ pairs of pyramidal facets be(1 0 1)/ð1 1 came closer to one another. However, they remained separated by four emerging f3 facets (Fig. 2(b), black arrows), which slightly enlarged with etching duration (Fig. 3(b), black arrows). As the (1 0 0)-related bottom surface etched off, the {2 1 1}A facets intersected (Fig. 2(c), black arrow). All the bottom intersections 1. Fig. 3 shows a recess cleaved projected onto (1 0 0) along ½0 1 1 view of the cleaved in the main orthogonal directions: (a) a ½0 1 recess showing the re-entrant f1 and f2, and emergent f3 facets (along with sections of the {1 1 0} facets); (b) a [0 1 1] view of the cleaved recess showing that f1 and f2 are re-entrant. The recess shape evolved slowly further on. The {2 1 1}A facets narrowed, started to become eliminated at the bottom, and let the neighbouring pyramidal facets intersect after 10, 20, and 40 min, respectively (Fig. 4). As a result, a sub-100 nm extremity formed at the bottom of the recesses (Fig. 5). Fig. 6 shows that the pyramidal facets were initially close to {1 1 0}, but they deviated towards planes with higher Miller indices with etching duration. Although the facets remained at 45° to (1 0 0) (see Figs. 8 and 9), their edges in (1 0 0) turned away from 1. It led to the undercutting of the InGaAs mask, h0 0 1i to ½0 1 which decelerated with etching duration. Fig. 7 shows a SEM micrograph of a recess with the InGaAs mask on. The black arrows mark the position at which the undercut data was taken for the graph in Fig. 6. The position lies at spot where the pyramidal facet intersected f1 in the (1 0 0) surface. Figs. 8(a) and 9(a) show AFM micrographs of a 6.20 lm deep flat-bottom recess and a 11.85 lm deep sharp-bottom one etched during 2 and 30 min, respectively. Figs. 8(b) and 9(b) depict sets of perpendicular cross-sections of the recesses along the top square edges at steps of 875 and 963.5 nm, respectively. The line 80 cross-section (Fig. 8(b)) shows a profile composed of lines related to (1 0 1)|1?2 (between points 1 and 2), (2 1 1)A|2?3, (1 0 0)|3?4, 4?5, and f1|5?6. Facets f1 appear steep (artifact), as the tip ð1 0 1Þ| cannot trace re-entrant surfaces. They remain present in the side up to a line between lines 95 and 100 (distance between lines 80 and 100 is 3.5 lm). The situation repeats with an inverse symmetry at line 140 diagonally close to the other corner. Fig. 8(c) shows the tilt of the ð1 0 1Þ-related pyramidal facet to (1 0 0) vs. distance along the top edge. It remains 45.35° between lines 80 and 140. Fig. 8(d) shows the pyramidal facet width from top to bottom. The facet occupied an area close to 49 lm2. The line 90 cross-section in Fig. 9(b) has two additional features vs. the one in Fig. 8(b): (1) a slanted 3 ? 4 section cut via the adjacent pyramidal facet, and (2) V-shaped groove between two f3 facets along section 4 ? 5. The 3 ? 4 line is not parallel to the x axis because the pyramidal facet turned away from (1 1 0). Fig. 9(b) shows a line 190 profile composed almost only of the pyramidal facet sections.
1 view of f1, f2, f3, and {1 1 0}; Fig. 3. Recess cleavages observed by SEM: (a) a ½0 1 (b) a [0 1 1] view showing that f1 and f2 are re-entrant. The recess was etched for 20 min (marker represents 5 lm).
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P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 4. Recesses observed by SEM in (a), (b), (c) etched for 10, 20, and 40 min, respectively; the {2 1 1}A were gradually eliminated (black arrows in (c)) and a sub-100 nm extremity was formed at the bottom.
Fig. 5. Sub-100 nm extremity, observed by SEM, at the bottom of a recess etched for 40 min.
Fig. 6. Full circles: the InGaAs mask undercut rate of the square sides vs. etching duration. Empty circles: pyramidal facet edges as seen in the top (1 0 0) surface 1 with etching duration. deviate from h0 0 1i to ½0 1
Fig. 9(c) shows the tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge. Fig. 9(d) depicts the width of the pyramidal facet from top to bottom. The facet occupied an area close to 77 lm2. 4. Discussion Unlike the ideal four-sided pyramidal recess confined only to four {1 1 0} crystallographic planes, the real recesses had each of the four sides composed of an ordinary {1 1 0} pyramidal facet and a re-entrant f1 facet. This can be explained considering observations published before. It was observed the facet revelation at
Fig. 7. Recess, observed by SEM, etched for 5 min with the InGaAs mask left on (marker represents 12 lm). The black arrows mark the spot at which the InGaAs mask undercut data was taken. The spot lies at the intersection between the (1 0 0), f1 and pyramidal facet.
etching mask patterns aligned in exactly h0 0 1i was ambiguous. 1, ordinary If the patterns were oriented slightly off h0 0 1i to ½0 1 {1 1 0} facets were exposed [8,9]. We showed ordinary {1 1 0} facets tilted at between 45° and 45.5° to (1 0 0) were revealed at 1 and re-entrant facets patterns aligned within h0.5°, 5°i off to ½0 1 0 0Þ were revealed at those tilted at between 55° and 45° to ð1 aligned within h0.5°, 5°i off to [0 1 1]. At 5° off h0 0 1i each way, the ordinary facet closed 45° to (1 0 0) and the re-entrant one 0 0Þ [14]. One can thus assume that facets f1 are similar 45° to ð1 in atomic configuration to the {1 1 0} pyramidal facets. This leads to a conclusion that the sides of the pyramidal recesses are initially 1 0Þ, (1 0 1)/ð1 0 1Þ, ð1 1 0Þ/ confined to pairs of (1 1 0)/ð1 ð1 1 0Þ, and ð1 0 1Þ/ð1 0 1Þ (all four pairs can be called 1 0g). {1 1 0}/f1 The pyramidal facets, being very close to {1 1 0} at early stages of etching, deviated to planes with higher Miller indices with etching duration. The longer a recess was etched, the more its pyramidal facets turned away from {1 1 0} and the slower they did so. Evidence of it is the deceleration of the undercutting rate of the InGaAs mask (Fig. 6). We showed previously that ordinary facets 1 revealed at linear patterns aligned beyond 5° off h0 0 1i to ½0 1 were etched at decreasing rates eventually dropping to zero [10]. The more the pyramidal facet pairs are turned away from {1 1 0}/ 1 0g, the slower they etch, which leads to an assumption that f1 the recess etching is self-limiting, i.e. proceeding at ever decreasing rates given sufficient etching duration. In practice, however, the recesses etched longer than 80 min had a worsened surface morphology, which suggests there is an etching mechanism through which HCl attacks the revealed InP facets if given excessive time. Apart from the pyramidal facets and f1 facets, the recesses were also confined to facets exposed along the taper edges: Re-entrant etch-stop f2 and fast-etching {1 1 1}B along [0 1 1] and etch-stop 1. Their exposure agrees with observations {2 1 1}A along ½0 1
P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
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Fig. 8. (a) AFM micrograph of a 6.20 lm deep flat-bottom recess etched for 2 min; (b) perpendicular cross-sections of the recess along the top square edge; (c) tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge. It is 45.35° for cross-sections between lines 80 and 140; (d) pyramidal facet occupied an area close to 49 lm2.
published in the literature: (1) facets f2 and {2 1 1}A were etchstop if revealed via InGaAs [12,15]; (2) facets f2 were re-entrant 1 and ½0 1 1 to ½1 0 0 [16]; and (3) fast-etching at 2–5° off ½0 1 {1 1 1}B facets etched almost as fast as (1 0 0) [12,15]. Considering the above results from the literature, f2 can be assumed to be 1Þ and ð0 1 1Þ crystallographic planes. We believe that close to ð0 1 it was useful to study the facetting at the blunt corners because sharp ones are hard to achieve for very small squares due to proximity effects in lithography. The evolution of the recesses with etching duration can be summarized as follows. Stage 1: the recess is at first confined to (1 0 0), 1ÞB, f2 ð0 1 1Þ and ð0 1 1Þ, {1 1 0}/f1 1 0g ð1 1 1ÞB and ð1 1 1ÞA. pairs, and (2 1 1)A and ð2 1 Stage 2: the fast-etching {1 1 1}B and bottom (1 0 0) surface become eliminated within 3 min. This ends the rapid period of the recess development and leads to the emergence of four f3 facets, which intersect along 1 and keep the ordinary {1 1 0} pyramidal facets apart. The ½0 1 1 0g pairs, recess is then confined to the slow-etching {1 1 0}/f1 slow-etching f3 facets, etch-stop f2 ð0 1 1Þ and ð0 1 1Þ, and the 1 0g pairs and f3 are etch-stop {2 1 1}A facets. The {1 1 0}/f1 etched further at diminishing rates. This is evident at the decelerated undercutting of the etching mask (Fig. 6). Interestingly, the etch-stop {2 1 1}A facets do not prevail in the recess shape but become partially eliminated. This leads to a pointed extremity at the bottom of the recess within 30 min. The etch-stop {2 1 1}A facets thus become eradicated whether between (1 1 0) facets at concave corners or between (1 1 0) facets at convex corners [14]. We believe that it is possible thanks to the 1 0g pairs having the freedom to be gradually etched. {1 1 0}/f1 The recess shape evolution can be expressed using the following inequalities on the etching rates: r ð100Þ ; r f111gB rf110g=f110g ; rf3 > r f211gA 0; rf2 0, with r f110g=f110g ! 0 for 3HCl:1H3PO4 at (16 ±
0.05) °C. Etching duration necessary to have such a recess pointed at the bottom depends on square size: the smaller the window, the shorter the etching time. Also, the smaller the window, the closer the pyramidal facets are to the ideal {1 1 0} sides. This experiment has provided an insight into the formation of pyramidal recesses in HCl for a limited set of parameters. To understand it in greater detail, one needs to explore the influences of HCl concentration, etching solution temperature, light, and type of substrate doping and conductivity. Some predictions can be made considering the literature. One may expect that the pyramidal recess formation in 3HCl:1H3PO4 will probably not depend on substrate conductivity or light. This assumption is based on the claim that InP is dissolved in undissociated HCl molecules through a purely chemical reaction of exchange type [17], which does not require the participation of free charge carriers at the etching front [18]. Huo et al. reported that the formation of V-grooves confined to (1 1 1)B-related facets in 5HCl:1H3PO4 was identical for Fe-, Znand Cd-doped InP layers on n-type InP(0 0 1):S substrate [19]. Neves and De Paoli observed that InP(0 0 1):Sn and InP(0 0 1):S substrates were etched in water and ethanolic HCl solutions (>5 M) at identical rates [20]. However, the [1 1 0]-oriented grooves etched into the substrates had side profiles that exhibited irregularities. The profiles were composed of variable length sections related to (1 1 1)B and (1 1 0). The density of such irregularities depended on the type of doping. It was correlated with the density of crystal defects associated with the type of dopant [20]. One may thus expect that the pyramidal recess shape can slightly vary with variables, such as the concentration of undissociated HCl and etching solution temperature. Also, it may be more profoundly changed or distorted due to different etching close to crystal defects. Although the present experiment did not show such irregularities in the pyramidal shape of the recesses, they may occur if
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P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 9. (a) AFM micrograph of a 11.85 lm deep sharp-bottom recess etched for 30 min; (b) perpendicular cross-sections of the recess along the top square edge; (c) tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge; (d) pyramidal facet occupied an area close to 77 lm2.
different HCl concentrations and etching solution temperatures are used. 5. Conclusion Symmetrical pyramidal recesses were etched into (1 0 0)InP substrate in 3HCl:1H3PO4 at (16 ± 0.05) °C via 20 lm 20 lm square windows opened in InGaAs. The windows had sides aligned 1. The recesses in h0 0 1i and corners tapered along [0 1 1] and ½0 1 had each of the four sides at the square edges composed of a large ordinary facet (called pyramidal) and a small re-entrant facet. The 1 0g (pyramipairs were identified to be initially close to {1 1 0}/f1 dal pairs), i.e. the ordinary pyramidal facets were initially close to 0Þ, and ð1 0 1Þ. However, they deviated towards (1 1 0), (1 0 1), ð1 1 planes with higher Miller indices with etching duration. The recesses were also confined to etch-stop {0 1 1} and fast-etching {1 1 1}B facets at the [0 1 1] taper edges and etch-stop 1 taper edges. The recesses evolved into {2 1 1}A ones at the ½0 1 sharp inverted pyramids with sub-100 nm extremities at the bottom if etched at least 30 min. The sharpening is possible thanks to the elimination of the etch-stop {2 1 1}A facets via a self-limited etching of the pyramidal pairs. Acknowledgements This research was supported by VEGA Agency Grants 2/0081/09 and 2/0214/09.
References [1] S. Adachi, H. Kawaguchi, J. Electrochem. Soc. 128 (1981) 1342–1349. [2] D.T.C. Huo, M.F. Yan, J.D. Wynn, D.P. Wilt, J. Electrochem. Soc. 136 (1989) 1828–1830. [3] P.H.L. Notten, J. Electrochem. Soc. 138 (1991) 243–249. [4] R. Bhat, E. Kapon, J. Werner, D.M. Hwang, N.G. Stoffel, M.A. Koza, Appl. Phys. Lett. 56 (1990) 863–865. [5] H. Fujikura, T. Iwa-Ana, H. Hasegawa, Jpn. J. Appl. Phys. 33 (1994) 919–924. [6] J. Wang, B.J. Robinson, D.A. Thompson, J.G. Simmons, Appl. Phys. Lett. 67 (1995) 2358–2360. [7] A. Hartmann, L. Loubies, F. Reinhardt, E. Kapon, Appl. Phys. Lett. 71 (1997) 1314–1316. [8] K. Hjort, J. Micromech. Microeng. 6 (1996) 370–375. [9] M. Tamura, K.-C. Shin, N. Serizawa, S. Arai, Jpn. J. Appl. Phys. 35 (1996) 2383– 2384. [10] P. Eliáš, J. Šolty´s, I. Kosticˇ, Mater. Sci. Eng. B 138 (2007) 172–179. [11] S.B. Phatak, G. Kelner, J. Electrochem. Soc. 126 (1979) 287–292. [12] R. Klockenbrink, E. Peiner, H.-H. Wehmann, A. Schlachetzki, J. Electrochem. Soc. 141 (1994) 1594–1599. [13] P. Eliáš, I. Kosticˇ, J. Šolty´s, S. Hasenöhrl, J. Micromech. Microeng. 14 (2004) 1205–1214. [14] P. Eliáš, J. Martaus, J. Šolty´s, I. Kosticˇ, J. Micromech. Microeng. 15 (2005) 1007– 1014. [15] J. Wang, D.A. Thompson, J.G. Simmons, J. Electrochem. Soc. 145 (1998) 2931– 2937. [16] L.A. Coldren, K. Furuya, B.I. Miller, J. Electrochem. Soc. 130 (1983) 1918–1926. [17] P.H.L. Notten, J. Electrochem. Soc. 131 (1984) 2641–2644. [18] P.H.L. Notten, J.E.A.M. van den Meerakker, J.J. Kelly, Etching of III–V Semiconductors: An Electrochemical Approach, Elsevier Advanced Technology, Oxford, 1991. p. 226. [19] D.T.C. Huo, M.F. Yan, J.D. Wynn, D.P. Wilt, J. Electrochem. Soc. 136 (1989) 772– 775. [20] S. Neves, M.A. De Paoli, J. Electrochem. Soc. 140 (1993) 2599–2603.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Patterning of pyramidal recesses in (1 0 0)InP substrate P. Eliáš a,⇑, I. Kosticˇ b, J. Šolty´s a a b
Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic Institute of Informatics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 07 Bratislava, Slovak Republic
a r t i c l e
i n f o
Article history: Received 17 February 2010 Received in revised form 30 June 2010 Accepted 18 August 2010 Available online 22 August 2010 Keywords: Inverted pyramids InP Wet-etching
a b s t r a c t Symmetrical pyramidal recesses were etched into (1 0 0)InP substrate in 3HCl:1H3PO4 at (16 ± 0.05) °C via 20 lm 20 lm square windows opened in InGaAs. The windows had sides aligned in h0 0 1i and cor 1. The recesses had each of the four sides at the square edges comners tapered along [0 1 1] and ½0 1 posed of a large ordinary facet (called pyramidal) and a small re-entrant facet. The pairs were 1 0g (pyramidal pairs), i.e. the ordinary pyramidal facets were identified to be initially close to {1 1 0}/f1 0Þ, and ð1 0 1Þ. However, they deviated towards planes with higher initially close to (1 1 0), (1 0 1), ð1 1 Miller indices with etching duration. The recesses were also confined to etch-stop {0 1 1} and fast-etch 1 taper edges. The ing {1 1 1}B facets at the [0 1 1] taper edges and etch-stop {2 1 1}A ones at the ½0 1 recesses evolved into sharp inverted pyramids with sub-100 nm extremities at the bottom if etched at least 30 min. The sharpening is possible thanks to the elimination of the etch-stop {2 1 1}A facets via a self-limited etching of the pyramidal pairs. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
The artificial structuring of semiconductor substrates, realized by localized dry- and wet-etching, lies at the core of any device technology. In III–Vs, such as InP and GaAs, mesas, ridges and grooves are the traditional patterns used for the definition and isolation of devices and in patterned epitaxy. The patterns are confined to sides whose profiles can range from rounded, containing ensembles of crystallographic facets, to straight ones, defined by a single facet [1–3]. The geometry of the latter is carefully defined to lead during epitaxy to the self-organized growth of quantum wells [4], wires [5,6], and dots [7]. This paper reports on pyramidal recesses exposed in (1 0 0)InP. The InP crystal contains twelve {1 1 0} crystallographic planes that have identical atomic arrangements. Four of them, (1 1 0), 0Þ, and ð1 0 1Þ, intersect (1 0 0) at 45° and theoretically (1 0 1), ð1 1 define inverted and upright pyramids based in (1 0 0). It was demonstrated previously that mesas confined to pyramidal facets 0Þ and (1 0 1)/ð1 0 1Þ pairs can be exposed in related to (1 1 0)/ð1 1 HCl via linear etching mask patterns aligned close to h0 0 1i [8– 10]. Below is a demonstration showing that flat-bottom and sharp inverted pyramids confined to such facets can be exposed in (1 0 0)InP by etching in HCl via square windows opened in InGaAs etching mask.
The recesses were etched into standard (1 0 0) semi-insulating InP:Fe substrate via a lattice-matched InGaAs layer, prepared by organometallic vapour phase epitaxy. The etching mask material exhibits superior masking properties because InGaAs (1) forms excellent interface with InP and (2) is inert to non-oxidizing HCl solutions, used to etch InP [11]. The patterning of InP substrates through such mask is well defined and highly repeatable [12,13]. The InGaAs etching mask was 120 nm thick, which allowed the observer to evaluate the amount of the InGaAs mask undercutting. We assume that as the mask was being undercut, edges of the released (free-standing) sections of the mask may have been subjected to some stress. However, we expect it had a negligible influence on the pyramidal recess formation. Windows in the InGaAs layer were opened with complete selectivity over InP in 1H3PO4 (85% w/w):1H2O2 (30% w/w):8H2O at 25 °C through openings in resist, defined by contact mode optical lithography. The windows were nominally 20 lm 20 lm squares with their sides aligned along h0 0 1i (Fig. 1(a)). The squares had 1. their corners tapered along [0 1 1] and ½0 1 Prior to the patterning, samples of the processed substrate were cleaned in acetone, isopropylalcohol and in oxygen plasma, then stripped off of surface oxides in 1HF(40% w/w):4H2O for 1 min at room temperature, and dried up in nitrogen flow. They were glued to GaAs holder chips (GaAs is not etched in HCl). The samples were etched in 3HCl:1H3PO4 at (16 ± 0.05) °C during 1, 2, 3, 5, 10, 15, 20, 30, 40, 60, and 80 min in light without agitation. (It was not crucial to keep the etching bath temperature fluctuation so small for this
⇑ Corresponding author. Tel.: +421 2 5922 2695; fax: +421 2 5477 5816. E-mail address: [email protected] (P. Eliáš). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.08.016
P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 1. (a) InGaAs etching mask: 20 lm 20 lm square windows with the sides of 1; (b) length s in h0 0 1i and corners of length t (3 lm) tapered in [0 1 1] and ½0 1 plane-view SEM micrograph of a recess etched for 1 min (InGaAs was etched off). {1 1 0} pyramidal facets and re-entrant facets f1 (appear bright) were exposed at the square sides. Re-entrant facets f2 (appear bright) linked with bottom ordinary {1 1 1}B-related ones were revealed at the [0 1 1] taper edges and ordinary {2 1 1}A 1 taper edges. related facets at the ½0 1
experiment. It will be more important for the patterning of the recesses with smaller size and pitch than those presented here. Such experiments are being in progress at our laboratory.) The solution was prepared by mixing electronic grade aqueousbased HCl (37% w/w) and H3PO4 (85% w/w) by volume ratio. To stop the etching process, the samples were dipped in H3PO4. They were rinsed with water and blown dry in nitrogen. The pyramidal recesses were studied with optical, scanning electron and atomic force microscopes (SEM and AFM, respectively). The SEM micrographs were taken with an S-800 HITACHI ultra-high vacuum (10–7 Pa) computer-controlled scanning electron microscope with a cold field emission electron source (single crystal tungsten, tip accelerating voltage between 1 and 30 keV, a 2 nm secondary electron image resolution). The AFM micrographs were obtained in non-contact mode with a Topometrix Explorer atomic force microscope from Veeco Instruments (maximum scanning field of 100 lm by 100 lm, maximum lift and resolution in the z axis of 10 lm and 0.2 nm, respectively). 3. Results Fig. 1(b) exemplifies a flat-bottom recess revealed via a 20 lm 20 lm square during 1 min. Its inverted pyramidal shape is evident: the sides along the square edges are confined to ordin 0Þ, and ð1 0 1Þ ary pyramidal facets close to (1 1 0), (1 0 1), ð1 1 ({1 1 0}) and smaller re-entrant facets f1. Other facets showed at the taper edges: top re-entrant facets f2 linked with bottom ordinary {1 1 1}B-related ones ({1 1 1}B) along [0 1 1]; and ordinary 1. The f2 and {2 1 1}A {2 1 1}A-related ones ({2 1 1}A) along ½0 1 facets did not undercut the mask. Fig. 2 captures the recess shape evolution during the first 3 min. It was the most dynamic period thanks to the pair of {1 1 1}B facets and (1 0 0)-related surface that disappeared after 2 and 3 min,
Fig. 2. Recesses in (a), (b), (c) etched for 1, 2, and 3 min, respectively; (b) {1 1 1}B facets disappear in 2 min and f3 facets emerge (black arrows); (c) (1 0 0)-related bottom surface etches off in 3 min and the {2 1 1}A facets intersect.
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respectively. With the fast-etching {1 1 1}B facets eradicated, the 0Þ and (1 1 0)/ð1 0 1Þ pairs of pyramidal facets be(1 0 1)/ð1 1 came closer to one another. However, they remained separated by four emerging f3 facets (Fig. 2(b), black arrows), which slightly enlarged with etching duration (Fig. 3(b), black arrows). As the (1 0 0)-related bottom surface etched off, the {2 1 1}A facets intersected (Fig. 2(c), black arrow). All the bottom intersections 1. Fig. 3 shows a recess cleaved projected onto (1 0 0) along ½0 1 1 view of the cleaved in the main orthogonal directions: (a) a ½0 1 recess showing the re-entrant f1 and f2, and emergent f3 facets (along with sections of the {1 1 0} facets); (b) a [0 1 1] view of the cleaved recess showing that f1 and f2 are re-entrant. The recess shape evolved slowly further on. The {2 1 1}A facets narrowed, started to become eliminated at the bottom, and let the neighbouring pyramidal facets intersect after 10, 20, and 40 min, respectively (Fig. 4). As a result, a sub-100 nm extremity formed at the bottom of the recesses (Fig. 5). Fig. 6 shows that the pyramidal facets were initially close to {1 1 0}, but they deviated towards planes with higher Miller indices with etching duration. Although the facets remained at 45° to (1 0 0) (see Figs. 8 and 9), their edges in (1 0 0) turned away from 1. It led to the undercutting of the InGaAs mask, h0 0 1i to ½0 1 which decelerated with etching duration. Fig. 7 shows a SEM micrograph of a recess with the InGaAs mask on. The black arrows mark the position at which the undercut data was taken for the graph in Fig. 6. The position lies at spot where the pyramidal facet intersected f1 in the (1 0 0) surface. Figs. 8(a) and 9(a) show AFM micrographs of a 6.20 lm deep flat-bottom recess and a 11.85 lm deep sharp-bottom one etched during 2 and 30 min, respectively. Figs. 8(b) and 9(b) depict sets of perpendicular cross-sections of the recesses along the top square edges at steps of 875 and 963.5 nm, respectively. The line 80 cross-section (Fig. 8(b)) shows a profile composed of lines related to (1 0 1)|1?2 (between points 1 and 2), (2 1 1)A|2?3, (1 0 0)|3?4, 4?5, and f1|5?6. Facets f1 appear steep (artifact), as the tip ð1 0 1Þ| cannot trace re-entrant surfaces. They remain present in the side up to a line between lines 95 and 100 (distance between lines 80 and 100 is 3.5 lm). The situation repeats with an inverse symmetry at line 140 diagonally close to the other corner. Fig. 8(c) shows the tilt of the ð1 0 1Þ-related pyramidal facet to (1 0 0) vs. distance along the top edge. It remains 45.35° between lines 80 and 140. Fig. 8(d) shows the pyramidal facet width from top to bottom. The facet occupied an area close to 49 lm2. The line 90 cross-section in Fig. 9(b) has two additional features vs. the one in Fig. 8(b): (1) a slanted 3 ? 4 section cut via the adjacent pyramidal facet, and (2) V-shaped groove between two f3 facets along section 4 ? 5. The 3 ? 4 line is not parallel to the x axis because the pyramidal facet turned away from (1 1 0). Fig. 9(b) shows a line 190 profile composed almost only of the pyramidal facet sections.
1 view of f1, f2, f3, and {1 1 0}; Fig. 3. Recess cleavages observed by SEM: (a) a ½0 1 (b) a [0 1 1] view showing that f1 and f2 are re-entrant. The recess was etched for 20 min (marker represents 5 lm).
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P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 4. Recesses observed by SEM in (a), (b), (c) etched for 10, 20, and 40 min, respectively; the {2 1 1}A were gradually eliminated (black arrows in (c)) and a sub-100 nm extremity was formed at the bottom.
Fig. 5. Sub-100 nm extremity, observed by SEM, at the bottom of a recess etched for 40 min.
Fig. 6. Full circles: the InGaAs mask undercut rate of the square sides vs. etching duration. Empty circles: pyramidal facet edges as seen in the top (1 0 0) surface 1 with etching duration. deviate from h0 0 1i to ½0 1
Fig. 9(c) shows the tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge. Fig. 9(d) depicts the width of the pyramidal facet from top to bottom. The facet occupied an area close to 77 lm2. 4. Discussion Unlike the ideal four-sided pyramidal recess confined only to four {1 1 0} crystallographic planes, the real recesses had each of the four sides composed of an ordinary {1 1 0} pyramidal facet and a re-entrant f1 facet. This can be explained considering observations published before. It was observed the facet revelation at
Fig. 7. Recess, observed by SEM, etched for 5 min with the InGaAs mask left on (marker represents 12 lm). The black arrows mark the spot at which the InGaAs mask undercut data was taken. The spot lies at the intersection between the (1 0 0), f1 and pyramidal facet.
etching mask patterns aligned in exactly h0 0 1i was ambiguous. 1, ordinary If the patterns were oriented slightly off h0 0 1i to ½0 1 {1 1 0} facets were exposed [8,9]. We showed ordinary {1 1 0} facets tilted at between 45° and 45.5° to (1 0 0) were revealed at 1 and re-entrant facets patterns aligned within h0.5°, 5°i off to ½0 1 0 0Þ were revealed at those tilted at between 55° and 45° to ð1 aligned within h0.5°, 5°i off to [0 1 1]. At 5° off h0 0 1i each way, the ordinary facet closed 45° to (1 0 0) and the re-entrant one 0 0Þ [14]. One can thus assume that facets f1 are similar 45° to ð1 in atomic configuration to the {1 1 0} pyramidal facets. This leads to a conclusion that the sides of the pyramidal recesses are initially 1 0Þ, (1 0 1)/ð1 0 1Þ, ð1 1 0Þ/ confined to pairs of (1 1 0)/ð1 ð1 1 0Þ, and ð1 0 1Þ/ð1 0 1Þ (all four pairs can be called 1 0g). {1 1 0}/f1 The pyramidal facets, being very close to {1 1 0} at early stages of etching, deviated to planes with higher Miller indices with etching duration. The longer a recess was etched, the more its pyramidal facets turned away from {1 1 0} and the slower they did so. Evidence of it is the deceleration of the undercutting rate of the InGaAs mask (Fig. 6). We showed previously that ordinary facets 1 revealed at linear patterns aligned beyond 5° off h0 0 1i to ½0 1 were etched at decreasing rates eventually dropping to zero [10]. The more the pyramidal facet pairs are turned away from {1 1 0}/ 1 0g, the slower they etch, which leads to an assumption that f1 the recess etching is self-limiting, i.e. proceeding at ever decreasing rates given sufficient etching duration. In practice, however, the recesses etched longer than 80 min had a worsened surface morphology, which suggests there is an etching mechanism through which HCl attacks the revealed InP facets if given excessive time. Apart from the pyramidal facets and f1 facets, the recesses were also confined to facets exposed along the taper edges: Re-entrant etch-stop f2 and fast-etching {1 1 1}B along [0 1 1] and etch-stop 1. Their exposure agrees with observations {2 1 1}A along ½0 1
P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
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Fig. 8. (a) AFM micrograph of a 6.20 lm deep flat-bottom recess etched for 2 min; (b) perpendicular cross-sections of the recess along the top square edge; (c) tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge. It is 45.35° for cross-sections between lines 80 and 140; (d) pyramidal facet occupied an area close to 49 lm2.
published in the literature: (1) facets f2 and {2 1 1}A were etchstop if revealed via InGaAs [12,15]; (2) facets f2 were re-entrant 1 and ½0 1 1 to ½1 0 0 [16]; and (3) fast-etching at 2–5° off ½0 1 {1 1 1}B facets etched almost as fast as (1 0 0) [12,15]. Considering the above results from the literature, f2 can be assumed to be 1Þ and ð0 1 1Þ crystallographic planes. We believe that close to ð0 1 it was useful to study the facetting at the blunt corners because sharp ones are hard to achieve for very small squares due to proximity effects in lithography. The evolution of the recesses with etching duration can be summarized as follows. Stage 1: the recess is at first confined to (1 0 0), 1ÞB, f2 ð0 1 1Þ and ð0 1 1Þ, {1 1 0}/f1 1 0g ð1 1 1ÞB and ð1 1 1ÞA. pairs, and (2 1 1)A and ð2 1 Stage 2: the fast-etching {1 1 1}B and bottom (1 0 0) surface become eliminated within 3 min. This ends the rapid period of the recess development and leads to the emergence of four f3 facets, which intersect along 1 and keep the ordinary {1 1 0} pyramidal facets apart. The ½0 1 1 0g pairs, recess is then confined to the slow-etching {1 1 0}/f1 slow-etching f3 facets, etch-stop f2 ð0 1 1Þ and ð0 1 1Þ, and the 1 0g pairs and f3 are etch-stop {2 1 1}A facets. The {1 1 0}/f1 etched further at diminishing rates. This is evident at the decelerated undercutting of the etching mask (Fig. 6). Interestingly, the etch-stop {2 1 1}A facets do not prevail in the recess shape but become partially eliminated. This leads to a pointed extremity at the bottom of the recess within 30 min. The etch-stop {2 1 1}A facets thus become eradicated whether between (1 1 0) facets at concave corners or between (1 1 0) facets at convex corners [14]. We believe that it is possible thanks to the 1 0g pairs having the freedom to be gradually etched. {1 1 0}/f1 The recess shape evolution can be expressed using the following inequalities on the etching rates: r ð100Þ ; r f111gB rf110g=f110g ; rf3 > r f211gA 0; rf2 0, with r f110g=f110g ! 0 for 3HCl:1H3PO4 at (16 ±
0.05) °C. Etching duration necessary to have such a recess pointed at the bottom depends on square size: the smaller the window, the shorter the etching time. Also, the smaller the window, the closer the pyramidal facets are to the ideal {1 1 0} sides. This experiment has provided an insight into the formation of pyramidal recesses in HCl for a limited set of parameters. To understand it in greater detail, one needs to explore the influences of HCl concentration, etching solution temperature, light, and type of substrate doping and conductivity. Some predictions can be made considering the literature. One may expect that the pyramidal recess formation in 3HCl:1H3PO4 will probably not depend on substrate conductivity or light. This assumption is based on the claim that InP is dissolved in undissociated HCl molecules through a purely chemical reaction of exchange type [17], which does not require the participation of free charge carriers at the etching front [18]. Huo et al. reported that the formation of V-grooves confined to (1 1 1)B-related facets in 5HCl:1H3PO4 was identical for Fe-, Znand Cd-doped InP layers on n-type InP(0 0 1):S substrate [19]. Neves and De Paoli observed that InP(0 0 1):Sn and InP(0 0 1):S substrates were etched in water and ethanolic HCl solutions (>5 M) at identical rates [20]. However, the [1 1 0]-oriented grooves etched into the substrates had side profiles that exhibited irregularities. The profiles were composed of variable length sections related to (1 1 1)B and (1 1 0). The density of such irregularities depended on the type of doping. It was correlated with the density of crystal defects associated with the type of dopant [20]. One may thus expect that the pyramidal recess shape can slightly vary with variables, such as the concentration of undissociated HCl and etching solution temperature. Also, it may be more profoundly changed or distorted due to different etching close to crystal defects. Although the present experiment did not show such irregularities in the pyramidal shape of the recesses, they may occur if
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P. Eliáš et al. / Microelectronic Engineering 88 (2011) 36–40
Fig. 9. (a) AFM micrograph of a 11.85 lm deep sharp-bottom recess etched for 30 min; (b) perpendicular cross-sections of the recess along the top square edge; (c) tilt of the ð1 0 1Þ-related facet to (1 0 0) vs. distance along the top edge; (d) pyramidal facet occupied an area close to 77 lm2.
different HCl concentrations and etching solution temperatures are used. 5. Conclusion Symmetrical pyramidal recesses were etched into (1 0 0)InP substrate in 3HCl:1H3PO4 at (16 ± 0.05) °C via 20 lm 20 lm square windows opened in InGaAs. The windows had sides aligned 1. The recesses in h0 0 1i and corners tapered along [0 1 1] and ½0 1 had each of the four sides at the square edges composed of a large ordinary facet (called pyramidal) and a small re-entrant facet. The 1 0g (pyramipairs were identified to be initially close to {1 1 0}/f1 dal pairs), i.e. the ordinary pyramidal facets were initially close to 0Þ, and ð1 0 1Þ. However, they deviated towards (1 1 0), (1 0 1), ð1 1 planes with higher Miller indices with etching duration. The recesses were also confined to etch-stop {0 1 1} and fast-etching {1 1 1}B facets at the [0 1 1] taper edges and etch-stop 1 taper edges. The recesses evolved into {2 1 1}A ones at the ½0 1 sharp inverted pyramids with sub-100 nm extremities at the bottom if etched at least 30 min. The sharpening is possible thanks to the elimination of the etch-stop {2 1 1}A facets via a self-limited etching of the pyramidal pairs. Acknowledgements This research was supported by VEGA Agency Grants 2/0081/09 and 2/0214/09.
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