Mechanically stable conjugate and suspended lasing membranes of bridged nano-cylinders

We present two different fabrication approaches for a suspended lasing membrane with intricate sub-micron patterning for an InGaAsP/InP platform. One approach involves a hydrogen silsesquioxane (HSQ) electron beam lithography resist as a dry etch hard mask and another with an added chromium (Cr) hard mask. The Cr hard mask process allows for fine control over patterned dimensions in comparison to the HSQ mask. This is crucial to both membrane stability and device performance. Both approaches are heavily susceptible to dry etch requirements and the etching window used for membrane release. The techniques presented here are of practical interest to the design of membrane based devices with applications in microfluidic biosensors and flexible laser membranes. © 2017 Optical Society of America OCIS codes: (220.4241) Nanostructure fabrication; (140.5960) Semiconductor lasers; (250.5230)


Introduction
Membrane structures have attracted ample interest over the years from many disciplines with applications in micro-electro-mechanical systems (MEMS) [1], cavity opto-mechanics [2][3][4], and optical lasing [5,6].Depending on the application, diverse membrane materials are used with most applications resorting to silicon (Si) or dielectrics such as silicon nitride (Si 3 N 4 ) and silicon dioxide (SiO 2 ) [7][8][9][10][11].The fabrication process differs for each.Nevertheless, the processes are established and standard membrane types are commercially available.However, applications pertaining to lasing require intricately patterned membranes and typically employ alloyed semiconductor gain material such as InGaAsP with quantum wells [12].These membranes are periodically patterned into photonic crystals (PhCs) to highly confine light of certain wavelengths or, in other words, sustain high quality factor (Q) modes required for lasing [13,14].Thus, these membrane PhCs serve as optical cavities for lasers.Here, we construct photonic crystal cavities with InGaAsP that employ bound states in the continuum (BIC) for lasing [15][16][17][18][19]. Bound states in the continuum (BICs) are waves that exist within a continuum of radiating waves and yet do not radiate or decay.Contrary to conventional wisdom, these states remain localized or bound to the cavity [20].
Unlike conventional PhCs that most often take the complimentary form, i.e. periodic array of holes, these suspended PhC cavities are composed of periodically spaced and interconnected cylinders.Furthermore, because the cylinders' radii are critical to the lasing mode in BIC lasers, stringent requirement is placed on fabrication precision.Hence, the fabrication of these new devices offers its own unique challenges which include maintaining pattern dimensionality while realizing a fully suspended and mechanically stable membranes.Here, we describe two different approaches of fabricating these BIC membrane lasers and their effect on device performance.One approach involves a hydrogen silsesquioxane (HSQ) electron beam chromium (C fabrication qu intended.Mo optimization etching the su openings whi etch window given the sam

Device fa
In this work, We choose th material lattic (1.5-1.6 μm).layers of 10 n 20 nm thickn total height of periodic cylin c).The pattern of a hard etc different etch beam lithogra resistance of the patterns.

A. Method I (HSQ hard mask)
Figure 2 shows the process flow for the fabrication of BIC membrane lasers using a HSQ negative tone resist acting as the hard mask for reactive-ion etching (RIE).Starting with the epitaxially grown wafer (Fig. 2(a)), 6% HSQ in methyl isobutyl ketone (MIBK) is spincoated at 2500 rpm for 60 s and soft-baked at 180 °C for 60 s to yield a film thickness of 130 nm.Subsequently, the HSQ is exposed at 100 kV and 3 nA beam current with 800 μC/cm 2 dose using a Vistec EBPG5200 electron beam lithography machine.The sample is developed using 25% tetramethylammonium hydroxide (TMAH) in water developer for 60 s (Fig. 2(b)).In step c, a purely RIE process is performed at a base pressure of 30 mTorr and a temperature of 35 °C with a RF power of 150 W using a Trion RIE/ICP Dry Etcher.The etch is carried out with 10 sccm (standard cubic centimeter per minute) of methane (CH 4 ) flow, 40 sccm of hydrogen (H 2 ), and 7 sccm of argon (Ar) combination for 680 s with an estimated etch rate of 70 nm/min to yield an etch depth of 800 nm [21].Here, the etch time is chosen such that all of the 300nm of InGaAsP would be etched in addition to a considerable thickness of InP for easy membrane release.Next, residual organic contamination and polymer buildup during RIE are removed with a microwave oxygen (O 2 ) plasma treatment with an O 2 flow rate of 120 sccm (150 W) for 15 min.The HSQ layer is removed with 30 s of buffered oxide etchant (BOE) with a ratio of 6:1 (H 2 O:HF) (Fig. 2(d)).Next, with the help of photolithography and a hydrochloric acid (HCl) based wet-etching solution, we remove a substantial amount of InP substrate below InGaAsP [22][23][24].In step e, the areas to be wet-etched are opened in the negative-tone NR9-1500PY photoresist spun at 3500 rpm for 40 s to yield a thickness of 1.5 μm.After a 20 s UV exposure with the Karl Suss MA6 Mask Aligner and a reversal bake at 100 °C for 60 s, the resist is developed for 35 s with RD6 developer.Lastly, a diluted solution of hydrochloric acid (HCl:H 2 O::3:1) with three parts acid to one part water by volume is used to selectively and anisotropically etch InP while minimally etching InGaAsP for a total etch time of 3 min (Fig. 2f-g).Radii of the final cylindrical resonators are smaller than the radii defined after e-beam lithography mainly due to the eroding (i.e.narrowing) HSQ hard mask during RIE [25,26].Consequently, the InGaAsP sidewalls are also eroded.This reduction in dimensions is exacerbated with increased etch depth or etch time.The reduction in radii for an etch depth of 800 nm was ~70 nm.The reduction in dimensions also applies to the interconnecting bridges, and consequently, the mechanical stability of the membrane is drastically weakened.To compensate for this reduction, the dimensions can be over defined by the e-beam lithography mask.However, for a periodic structure the maximum radii of the cylinders defined by the lithography mask is limited to half the period (P/2).Hence, dimensions close to P/2 cannot be realized with the current fabrication process with HSQ as the etch mask.Therefore, a tougher etch mask is needed to preserve the dimensions defined by e-beam lithography during the dry etching process.It is worth noting that in addition to etching PMMA, the oxygen plasma extensively undercuts the PMMA below the Cr (black arrows in Fig. 3(d)).Thus, the etch needs to be tightly controlled such that all the PMMA is removed from the InGaAsP surface and yet the undercut is minimal [29].The PMMA undercut ultimately contributes to rough sidewalls in the patterned InGaAsP.Both the Cr and PMMA dry etch steps were thoroughly optimized.Following the Cr/PMMA etch, we dry etch the InGaAsP/InP as described in method I except with a longer etch time of 16 min for a total etch depth of 1200 nm (Fig. 3(e)).This etch depth is deeper compared to method I (800 nm).Here, in contrast to method I, the higher etch resistance of the metal hard mask allows for a deeper etch into the InP substrate with minimal reduction in pattern dimensions.The deeper etch is required and is conducive for an easy membrane release discussed below.Next, the HSQ/Cr/PMMA stack is removed (Fig. 3(f)).Starting with top layer, HSQ is easily removed with the help of BOE (6:1).To lift-off Cr, the sample is submerged in acetone for 2 hours with slight sonication so all the PMMA is attacked and the Cr layer is lifted off.Following the lift-off, the same wet etching recipe described in method I is used to suspend the membranes (Fig. 2(e)-2(g)).
However, in comparison with the sole HSQ hard mask, we now etch for a shorter 2 min 18 sec for complete membrane suspension due to the deeper dry etch of the III-V material.
Ultimately, the reduction of cylinders' radii is minimized with the use of the Cr metal mask.
Fig. 3. Device fabrication process involving a metal hard mask, starting with the epitaxially grown multiple quantum wells on InP substrate (a-f).The subsequent membrane release process is the same for both processes: with and without a metal hard mask (Fig. 2e-g).Note that the bridges connecting the cylinders are intentionally drawn thinner as a guide to the eye.Both the bridges and the cylinders are of the same thickness.the appropriate dry etch depth is reached.The less efficient the geometry of the etch window, the more the dry etch depth required.In our case, the rectangular etch window is the least efficient.It is worth highlighting that a longer etch depth leads to a reduction in lateral dimensions even with a Cr metal mask.Therefore, it is prudent to optimize the geometry of the wet etch window for a quick membrane release.For a given geometry of the etch window such as the rectangular one, we see both a suspended array which subsequently collapsed (Fig. 6c.i) and an etch-halted array (Fig. 6(c).ii).The two samples were processed together where both are dry-etched for an etch depth of 600 nm and subsequently wet-etched for 1 min 30 sec in HCl:H 2 O (3:1) solution.The only difference being the cylindrical resonators in one array have smaller radii than the other.The array in Fig. 6(c).i with a measured radius of 440 nm, allows for larger dry-etched openings in-between the cylinders compared to the array in Fig. 6(c).ii with a larger radius of 540 nm.The larger openings make it easier for the neighboring {111}-InP planes to meet for a given dry etch depth.Hence, the smaller radii membrane is easily released whereas the other is halted by the formation of etch pits.It is worth noting that only a HSQ hard mask was used for these two samples.Therefore, the released membrane collapsed due to the reduction of the supporting bridge widths past their breaking point during the dry etch process.Similarly, wet etch of an array with a trapezoidal geometry is halted by the formation of etch pits due to thick supporting arms as seen in Fig. 6(d).Even prolonging the wet etch to 4 min did not break the etch pits.However, a trapezoidal etch window with thin supporting arms allows for a quick (2 min 18 sec) membrane release leaving a visibly large V-groove that runs underneath as seen in Fig. 6(e).Larger arrays were also fabricated and, as expected, required longer wet-etching times for complete suspension.Both meth functional de dimensions of with HSQ as Similarly, a 1 Fig. 7(d).As dimensions w bridges shrin roughness wh with an added s a tighter co (see Fig. 7).A and a magnif en in Fig. 7c    HSQ hard ma d 10x10 array of 67 μW.Sim Cr hard mask b ys are optically Hz (Fig. 8(c

Results and discussion
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Figure 4
Figure4is a direct comparison of the dry etch quality of InGaAsP/InP with a HSQ hard mask (Method I) and an added Cr hard mask (Method II).A control pattern was etched for both cases while aiming for similar etch depths.With HSQ serving as the hard mask and its narrowing during the dry etch, the InGaAsP sidewalls have eroded inward directly resulting in the reduction of lateral dimensions as seen in Fig.4(a) and in 4(b) with the HSQ removed.However, with the added Cr layer, the patterns etched in InGaAsP experience minimal reduction in the lateral dimensions as seen in Fig.4(c) and in 4(d) with the mask removed.This comparison is further apparent from the analysis of bridge reduction of the finished devices shown in Fig.5.In Fig.4(c), the undercut in PMMA due to the oxygen plasma is clearly visible underneath the Cr layer.After the initial etch over the PMMA thickness, the undercut rate was estimated to be 20 nm/min[29].A PMMA layer that is not undercut below Fig. 5 nomin schem Right the m with a the cy is mo increa Following selectively re suspension of geometry of crystallograph 6a, the etch i InGaAsP form of neighborin etch pits.The and fully susp the radius of t in addition to we employ t opening width (3P) and open widths (4P) w 6. Etch requiremen depth on the crys est set of etch plan dry-etch depth (h AsP membrane o ows for membrane orting arms (2.5P) supporting arms ( upporting arms (P es of completed f 0 arrays with recta nm and subseque psed array with alent array with la long {111} plane ck supporting arm rneath indicated by drical resonators.A essfully suspended e arrows) which w rneath the fully rele hod I with a evices.Howev f the pattern, i. s the hard mas 10x10 array wi s seen, the cy with straighter nk drastically w hen using a Cr/P nts and optimizatio stallographic selec nes, indium (In)-ri ) required for adj f thickness (t) is e release: i. rectang in blue, both ind 3P) and trapezoid P) and opening wid fabrication for dif angular etching win ently wet-etched measured cylindr arger measured rad of InP.(d) Array ms of InGaAsP in y second pair of w All the etch pits r d array with trapez was wet-etched for eased membrane rrows, ii.trapezoi s (4P), iii.trapezoi = 1.2 μm.(c-f) Ele dows and etch con h are dry-etched fo ec in HCl:H 2 O (3 0 nm after wet halted etch due to etch window with white arrows an ere are visible etch a prolonged wetw due to the thinne 8 sec.A visibly lar irection. an e Cr hard ma e HSQ mask b re is also som ue to the etch q ndence of InP dryh is halted by the 55°.Therefore, the fully suspend the different wet-etch s (2P) in white and idal windows with idal windows with ectron micrograph nditions.(c) Two or an etch depth of 3:1) solution.(i.) etching and (ii.) o formation of etch h a halted etch due nd halted InP etch h pits between the etch of 4 min.(e) er supporting arms rge V-groove runs Fig. 7 resona substr two cy mask both t 4. Device pe In Fig. 8, we In Fig. 8(a), w hard mask em lasing from a with a thresho laser with 12 By design within the las mode and thu at different w etch windows used for the H hard mask.Th the release of the dimension the trapezoid

Table 1 . Original R5
R5 4. Electron micro ns.Method I: Side Method II: Side v val (d).Both were radii defined for H