124 together direct light and emit a focused laser through the topmost lens layer 124t.

FIG. 2 illustrates a cross-sectional view of some other embodiments of a VCSEL device 200 having a self-aligned microlens coupled to a transistor structure.

In some embodiments, a transistor 228 is disposed over a semiconductor substrate 101 and coupled to an interconnect structure 220. The transistor 228 may be a metal oxide semiconductor field-effect transistor (MOSFET), for example, and comprise a gate electrode overlying a gate dielectric, and source/drain regions disposed on opposite ends of the gate electrode. In some embodiments, the interconnect structure 220 comprises interconnect wires 226, interconnect vias 224, and contact pads 222 embedded within a lower inter-level dielectric (ILD) structure 212. In some embodiments, the first conductive layer 110 is bonded to a bottom electrode 218, and the bottom electrode 218 is coupled to one of the contact pads 222 and interconnect wires 226. The contact pads 222 and the interconnect wires 226 may couple the bottom electrode 218 to some other semiconductor device (e.g., transistor). In some embodiments, a top electrode contact 204 is coupled to the uppermost one 116u of the second reflector layers 116. In some embodiments, the top electrode contact 204 surrounds the microlens stack 104. The top electrode contact 204 may be coupled to the transistor 228 through conductive vias 206 and the interconnect structure 220. In some embodiments, the conductive vias 206 are disposed within an upper ILD structure 208. During operation, a bias may be applied across the reflector stack 102 through the bottom electrode 218 and the top electrode contact 204 to generate light within the optically active region 118.

FIGS. 3-19 illustrate cross-sectional views 300-1900 of some embodiments of a method of forming a VCSEL device comprising a self-aligned microlens. Although FIGS. 3-19 are described in relation to a method, it will be appreciated that the structures disclosed in FIGS. 3-19 are not limited to such a method, but instead may stand alone as structures independent of the method.

As shown in cross-sectional view 300 of FIG. 3, a second conductive layer 112 is formed over a first conductive layer 110 on a carrier substrate 302. In some embodiments, the second conductive layer 112 comprises a conductive material such as, for example, nickel, gold, copper, or the like, and the first conductive layer 110 may comprise a conductive material such as, for example, gold. Thus, in some embodiments, the first and second conductive layers 110, 112 may comprise a same material, whereas in other embodiments, the first and second conductive layers 110, 112 may comprise different materials. In some embodiments, the first conductive layer 110 may have a thickness in a range of between, for example, approximately 50 nanometers and approximately 800 nanometers. In some embodiments, the second conductive layer 112 may have a thickness in a range of between, for example, approximately 50 nanometers and approximately 500 nanometers.

In some embodiments, over the second conductive layer 112, a lower DBR region 106 is formed by alternatively forming a first reflector layer 114 and a second reflector layer 116. An optically active region 118 may be formed over the lower DBR region 106, and an upper DBR region 108 may be formed over the optically active region 118, such that the lower DBR region 106, the optically active region 118, and the upper DBR region 108 form a reflector stack 102. In some embodiments, each of the first reflector layers 114 and the second reflector layers 116 have a thickness in a range of between, for example, approximately 4 nanometers and approximately 100 nanometers. In some embodiments, the thickness of the first reflector layers 114 and the second reflector layers 116 are about equal.

In some embodiments, the first reflector layers 114 may each comprise a first material, such as, for example, aluminum gallium arsenide or indium gallium arsenide, whereas the second reflector layers 116 may comprise gallium arsenide. In some alternative embodiments, different ones of the first reflector layers 114 may comprise different materials, such as, for example, aluminum gallium arsenide and indium gallium arsenide. The first and second reflector layers 114, 116 may be formed, in some embodiments, through an epitaxial growth process. In some embodiments, the mole fraction of aluminum or indium in the first reflector layers 114 may be in a range of between approximately 40 percent and approximately 100 percent, for example. In some embodiments, each of the first reflector layers 114 in the reflector stack 102 may have a concentration of the first element (e.g., aluminum or indium) that is approximately the same. In some embodiments, the optically active region 118 may comprise the same material as the first reflector layers 114, except with an increased concentration in the first element (e.g., aluminum or indium). For example, in some embodiments, the optically active region 118 comprises at least 98 percent aluminum, and the remaining concentration of gallium and arsenide may be at most 2 percent. In some embodiments, the optically active region 118 may have a thickness in a range of between approximately 1 nanometer and approximately 20 nanometers. In some embodiments, the first and second reflector layers 114, 116 are transparent to allow the passage of light throughout the reflector stack 102.

In some embodiments, the reflector stack 102 may comprise a focusing layer 120 that defines an optical aperture 121 above the optically active region 118, and thus, within the upper DBR region 108. In other embodiments, the focusing layer 120 may be within the lower DBR region 106 and below the optically active region 118. In yet other embodiments, the focusing layer 120 may be omitted. In embodiments where the focusing layer 120 is present, the focusing layer 120 may surround one of the first and/or second reflector layers 114, 116 to guide reflected light through the optical aperture 121 in the focusing layer 120.

In some embodiments, a microlens stack 104 is formed over the reflector stack 102 and may comprise alternating layers of lens layers 124 and additional second reflector layers 125. In some embodiments, the additional second reflector layers 125 may comprise the same second material as the second reflector layers 116. In other embodiments, the additional second reflector layers 125 may comprise a different material than the second reflector layers 116. For example, in some embodiments, the additional second reflector layers 125 may comprise gallium arsenide, whereas the second reflector layers 116 may comprise indium arsenide. In some embodiments, the lens layers 124 comprise the same first material as the first reflector layers 114. The lens layers 124 may also be transparent to allow the passage of reflected light. In other embodiments, the lens layers 124 may comprise a different material than the first reflector layers 114. For example, in such embodiments, the lens layers 124 may comprise aluminum gallium arsenide with a varying concentration of aluminum, whereas the first reflector layers 114 comprise indium gallium arsenide. The first element (e.g., aluminum) in the lens layers 124 may be susceptible to oxidation to form an oxide (e.g., aluminum oxide). Nevertheless, the lens layers 124 each have an equal thickness in a range of between approximately 4 nanometers and approximately 100 nanometers, for example, in some embodiments. The concentration of aluminum in the lens layers 124 may vary in a range of between approximately 60 percent and approximately 100 percent. The lens layers 124 may also be formed by epitaxial grown, for example, such that the lens layers 124, the additional second reflector layers 125, and the first reflector layers 114 are formed using a same process method (e.g., epitaxial growth) without the use of a bonding material between the reflector stack 102 and the microlens stack 104.

In some embodiments, each of the lens layers 124 may have a different average concentration of the first element from one another. The average concentration of each of the lens layers 124 may increase from a bottommost lens layer 124b to a topmost lens layer 124t. Each individual lens layer 124 may also have a concentration gradient of the first element that increases from a bottom surface of each lens layer 124 to a top surface of each lens layer 124. The change in concentration between a bottom surface and a top surface of each lens layer 124 may be equal amongst each lens layer 124. For example, in some embodiments, the bottommost lens layer 124b and the topmost lens layer 124t may each have a change in concentration over its thickness of approximately 2 percent, for example. In some embodiments, the concentration of the first element (e.g., aluminum) may be adjusted by adjusting the associated source material for the first element during the epitaxial growth process. In some embodiments, a topmost layer of the microlens stack 104 may comprise a topmost one 125t of the additional second reflector layers 125.

In some embodiments, the change in concentration of the first element in the lens layers 124 may change the refractive index in the lens layers 124. For example, in some embodiments, the lens layers 124 may comprise aluminum gallium arsenide, wherein aluminum is the first element. The refractive index for aluminum gallium arsenide may be in a range of between approximately 3.0 and approximately 3.5. A high concentration of aluminum in aluminum gallium arsenide corresponds to a low refractive index, whereas a low concentration of aluminum in aluminum gallium arsenide corresponds to a high refractive index. Thus, as the concentration of aluminum increases from a bottom surface of each lens layer 124 to a top surface of each lens layer 124, the refractive index may decrease from a bottom surface of each lens layer 124 to a top surface of each lens layer 124.

As shown in cross-sectional view 400 of FIG. 4, a masking layer 402 may be formed over the topmost one 125t of the additional second reflector layers 125. In some embodiments, the masking layer 402 may comprise a photosensitive material (e.g., photoresist) formed by a spin coating process. Using photolithography, the photosensitive material may be patterned to form the masking layer 402 over a center region of the microlens stack 104 and the reflector stack 102. In some embodiments, the masking layer 402 is centered over the optical aperture 121 in the focusing layer 120, and outer portions 404 of the microlens stack 104 are uncovered. In some embodiments, the masking layer 402 may comprise a photoresist, or some other polymer, that is clear.

As shown in cross-sectional view 500 of FIG. 5, a first vertical etch process 502 may be performed to remove the outer portions (404 of FIG. 4) of the microlens stack 104. In some embodiments, the first vertical etch process 502 is a dry etch process and may be controlled to remove each layer in the outer portions (404 of FIG. 4) of the microlens stack 104. Thus, in some embodiments, the first vertical etch process 502 may be controlled to stop at an uppermost one 116u of the second reflector layers 116 in the reflector stack 102. For example, in some embodiments, the first vertical etch process 502 may comprise a dry etching processes that utilizes chlorine and fluorine gases to remove outer portions of each lens layer 124 and outer portions of each additional second reflector layer 125 in the microlens stack 104. After the first vertical etch process 502, the remaining microlens stack 104 may be substantially centered over the optical aperture 121 in the focusing layer 120.

As shown in cross-sectional view 600A of FIG. 6A, an oxidation process 602 may be performed to form oxidized lens layers 126 surrounding the lens layers 124. In some embodiments, the oxidation process 602 may be a wet oxidation process which uses water at a high temperature. In some embodiments, the high temperature is at least 400 degrees Celsius. In other embodiments, the oxidation process 602 may be a deep hydrofluoric wet oxidation process. The oxidation process 602 utilizes wet solutions to effectively oxidize the lens layers 124 in the lateral direction. Because the lens layers 124 comprise the first element that is susceptible to oxidation, but the additional second reflector layers 125 do not comprise the first element or any element susceptible to oxidation, peripheral portions of the lens layers 124 oxidize to form oxidized lens layers 126, whereas the additional second reflector layers 125 remain substantially unaffected by the oxidation process 602.

In some embodiments, the oxidation process 602 may effectively oxidize the lens layers 124 at the same time, but at different rates. The rate of oxidation increases from the bottommost lens layer 124b to the topmost lens layer 124t due to the concentration gradient of the first element in the lens layers 124. Further, because each lens layer 124 has a concentration gradient from a bottom surface to a top surface of each lens layer 124, the bottom surface is wider than the top surface of each lens layer 124. In some embodiments, sides of the lens layers 124 may be substantially curved or planar from the perspective of the cross-sectional view 600A, for example.

In some embodiments, the rate of oxidation of the first element exponentially increases with the concentration of the first element, whereas in other embodiments, the rate of oxidation of the first element linearly increases with the concentration of the first element, depending on the relationship between the first element and the wet solution. The additional second reflector layers 125 act as a barrier to prevent the oxidation process 602 from oxidizing top and bottom surfaces of the lens layers 124. Further, the uppermost one 116u of the second reflector layers 116 protects the upper and lower DBR regions 108, 106 from the oxidation process 602. In some embodiments, the additional second reflector layers 125 and the lens layers 124 are clear or transparent. In some embodiments, the oxidized lens layers 126 may be clear, and in other embodiments, the oxidized lens layers 126 may be translucent. The oxidized lens layers 126 may comprise aluminum oxide after the oxidation process 602, which has a refractive index of approximately 1.8. The oxidized lens layers 126 may also comprise gallium arsenide, in some embodiments, such that the composition of the oxidized lens layers 126 may be aluminum oxide gallium arsenide. The refractive index of the oxidized lens layers 126 may be different than the additional second reflector layers 125 and the lens layers 124 to effectively guide the light and produce a focused laser.

Cross-sectional view 600B of FIG. 6B illustrates some embodiments of the oxidized lens layers 126 and the additional second reflector layers 125 that may correspond to box A of FIG. 6A. In some embodiments, the oxidized lens layers 126 may laterally protrude outward past outermost sidewalls 125s of the additional second reflector layers 125 due to expansion from the oxidation process (602 of FIG. 6A). In some embodiments, an upper one of the oxidized lens layers 126 may have an outermost sidewall that protrudes outward past outermost sidewalls 125s of the additional second reflector layers 125 by a first distance d₁. A middle one of the oxidized lens layers 126 may have an outermost sidewall that protrudes outward past outermost sidewalls 125s of the additional second reflector layers 125 by a second distance d₂that is less than the first distance d₁. A lower one of the oxidized lens layers 126 may have an outermost sidewall that protrudes outward past outermost sidewalls 125s of the additional second reflector layers 125 by a third distance d₃less than the first distance d₁and the second distance d₂. Thus, the expansion of the oxidized lens layers 126 may increase with a rate of oxidation of each lens layer (124 of FIG. 6A). In some embodiments, the outer sidewalls of the oxidized lens layers 126 may be rounded.

Cross-sectional view 600C of FIG. 6C illustrates some alternative embodiments of the oxidized lens layers 126 and the additional second reflector layers 125 that may correspond to box A of FIG. 6A. In some embodiments, the oxidized lens layers 126 may have outermost sidewalls 126s that are concave upward with respect to a bottom surface of the oxidized lens layers 126. In other words, in some embodiments, an upper one of the oxidized lens layers 126 may have an upper surface that protrudes past the outermost sidewalls 125s of the additional second reflector layers 125 by the first distance d₁, whereas an upper one of the oxidized lens layers 126 may have a lower surface that protrudes past the outermost sidewalls 125s of the additional second reflector layers 125 by a fourth distance d₄that is less than the first distance d₁. Because of the concentration gradient of the first element in the lens layers (124 of FIG. 6A), the upper surface of each oxidized lens layer 126 may protrude past the outermost sidewalls 125s of the additional second reflector layers 125 more than the lower surface.

As shown in cross-sectional view 700 of FIG. 7, in some embodiments, a first spacer layer 702 may be deposited over the reflector stack 102, the microlens stack 104, and the masking layer 402. In some embodiments, the first spacer layer 702 comprises a dielectric, such as, for example, silicon nitride, silicon carbide, or the like. In some embodiments, the first spacer layer 702 may be deposited by a deposition technique (e.g., physical vapor deposition (PVD), chemical vapor deposition (CVD), PE-CVD, atomic layer deposition (ALD), etc.).

As shown in cross-sectional view 800 of FIG. 8, a second vertical etch process 804 may be performed in some embodiments that is selective to the first spacer layer (702 of FIG. 7) to form a first upper sidewall spacer 802. In some embodiments, the first upper sidewall spacer 802 has curved outer sidewalls because of the second vertical etch process 804.

As shown in cross-sectional view 900 of FIG. 9, second upper sidewall spacers 902 may be formed beside the first upper sidewall spacers 802. In some embodiments, the second upper sidewall spacers 902 may be formed using similar steps as in FIGS. 7 and 8 regarding the formation of the first upper sidewall spacers 802. In some embodiments, the second upper sidewall spacers 902 are thicker than the first upper sidewall spacers 802. Further, in some embodiments, the second upper sidewall spacers 902 may comprise a different material than the first upper sidewall spacers 802. For example, in some embodiments, the second upper sidewall spacers 902 may comprise silicon dioxide, silicon oxynitride, or the like.

As shown in cross-sectional view 1000 of FIG. 10, in some embodiments, a third vertical etch process 1002 may be performed in some embodiments to remove peripheral portions of the reflector stack 102 that are uncovered by the microlens stack 104, the first upper sidewall spacers 802, and the second upper sidewall spacers 902. Thus, in some embodiments, the third vertical etch process 1002 may use the masking layer 402 and the second upper sidewall spacers 902 as a mask. The third vertical etch process 1002 may leave the second conductive layer 112 substantially unchanged.

As shown in cross-sectional view 1100 of FIG. 11, lower sidewall spacers 122 may be formed over the reflector stack 102 and along the second upper sidewall spacers 902. The lower sidewall spacers 122 may be formed using similar steps as in FIGS. 7 and 8 regarding the formation of the first upper sidewall spacers 802. In some embodiments, the lower sidewall spacers 122 may comprise a dielectric material, such as silicon nitride, silicon carbide, or the like. Thus, in some embodiments, the lower sidewall spacers 122 may comprise a same material as the first upper sidewall spacers 802.

As shown in cross-sectional view 1200 of FIG. 12, in some embodiments, a molding layer 1202 may be formed around the lower sidewall spacers 122. In some embodiments, the molding layer 1202 may have a top surface that is substantially coplanar with a top surface of the reflector stack 102. In other embodiments, the molding layer 1202 may have a top surface that is above or below a top surface of the reflector stack