Retraction step.

<p>(a): Overview of a V-groove after etching of the SiRN layer, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4(h)</a>. (b): Zoom in the extremity of the groove after stripping away the polySi top layer, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4(i)</a>. Retraction occurs in all concave corners, including the vertical planes. The retraction length is the same in every direction.</p

Difficulties arising when implementing corner lithography in an upright mold.

<p>We consider the case where the process flow depicted in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4</a> has been followed up to step (i) inclusive. (a): Dry etching is impossible, a wet etching strategy must be considered using a masking layer. A polySi layer is deposited followed by short oxidation. (b): Photoresist is spun over the wafer. The use of a thick photoresist dedicated to high-aspect ratio structures protection is necessary to protect the molds. (c): Directional nature of UV illumination makes impossible the proper patterning of photoresist under the thick SiRN plate. (d): Consequently, the masking layer is not etched away everywhere. (e): SiRN is still present all around the molds at their bottoms.</p

SEM images of fabrication steps for 90° complex hinges.

<p>(a): Retraction of polySi is visible under SiO₂, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4(g)</a>. The polySi plate is not exactly straight. These irregularities are exact replicas of the defects of the silicon mold, which have been magnified through corner lithography. (b): Thick SiRN with patterned masking partially oxidized polySi layer on top. (c): Same as (b) right after wet etching of SiRN. Stress in SiO₂ mask causes the curtain-like overhanging thin film. (d): Final structure before release after the second wet etching procedure was applied.</p

Folding results of 90° stop-programmable hinges.

<p>(a): The thick part of the complex hinge is too short (1.8 ± 02) μm) and fails to stop the folding. Consequently, the flap is 180° folded. (b): For this sample, the thick part is slightly longer (3.2 ± 02) μm). This forces folding to stop at an intermediate position, but fails to stop the folding at 90° as was intended.</p

Corner lithography in a rounded corner.

<p>(a): When a material layer of thickness <i>t</i> is conformally deposited over a mold with radius of curvature <i>R</i> that is greater than <i>t</i>, the thickness at the tip is unchanged. (b): On the other hand, when <i>R</i> is less than <i>t</i>, a concave corner is created and the effective thickness is </p><p></p><p><mi>c</mi><mo>=</mo><mi>R</mi><mo>+</mo></p><p></p><p><mo stretchy="false">(</mo><mi>t</mi><mo>−</mo><mi>R</mi><mo stretchy="false">)</mo></p><p>sin<mo stretchy="false">(</mo></p><p><mi>α</mi><mn>2</mn></p><mo stretchy="false">)</mo><p></p><p></p><p></p><p></p>.<p></p

Stop-programmable folding principle.

<p>The design of the complex hinges is such that once folded, the flap forms a predefined angle with the planar support. Self-folding of the structure is enabled through evaporation of water and decrease of the liquid–air interface of the meniscus. (a) 70.6° stop-programmable hinge. (b): 90° stop-programmable hinge. In both cases, the flaps adhere due to a sufficiently large stiction area and there is no need for a locking mechanism.</p

Fabrication of a tetrahedron folding pattern, extra steps.

<p>(a): Since only one of the hinges lies on the correct intersection of the planes, standard lithography is used to define flat hinges for the other faces. Faces 2 and 3 are designed with small appendices on their sides to allow them to lock onto face 1 while folding. (b): Under-etching of Si by semi-isotropic etching of silicon (SF₆ etchant). Etching is stopped when the hinges are free and the central flap rests on a silicon pillar.</p

Folded structure arrested by 70.6° stop-programmable hinges.

<p>(a): Side view of an extruded-2D structure. The flap is 100 μm wide and the complex hinge has an original width of 40 μm. The flap of the left hand side, connected with a flat hinge, reopened after folding because of an insufficient bonding area. (b): Folded tetrahedron. The faces have sides of 200 μm, the complex hinges are 20 μm wide and the flat hinges are 10 μm wide.</p

Corner lithography in a sharp corner.

<p>(a): When a conformal layer of thickness <i>t</i> is deposited over a concave corner of opening <i>α</i>, the effective thickness of material at the corner is </p><p></p><p><mi>a</mi><mo>=</mo></p><p><mi>t</mi></p><p>sin<mo stretchy="false">(</mo></p><p><mi>α</mi><mn>2</mn></p><mo stretchy="false">)</mo><p></p><p></p><mo>></mo><mi>t</mi><p></p><p></p> [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.ref042" target="_blank">42</a>]. (b): After isotropic etching by an amount of <i>r</i>, material with thickness <i>b</i> = <i>a</i> − <i>r</i> remains in the corner.<p></p

Illustration of the design criterion described in Fig 3.

<p>Figs. (a) and (b) show samples with different radiis of curvature for the initial mold after performing corner lithography and partially etching the bottom SiRN layer, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4(i)</a>. The thickness <i>t</i> of the first deposited SiRN layer is 1.1 μm. (a): When <i>R</i> > <i>t</i>, the entire surface of the polySi layer was oxidized during step (f), consequently the retraction, step (g), had no effect. (b): For <i>R</i> < <i>t</i>, the polySi layer is opened and the underlying SiRN layer can be etched, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0125891#pone.0125891.g004" target="_blank">Fig 4(i)</a>.</p