On-chip terahertz double-optical-comb device and manufacturing method thereof

Abstract

The invention relates to an on-chip terahertz double-optical-comb device and a manufacturing method thereof, that is, two terahertz quantum cascade lasers with the same size and opposite end faces are manufactured on the same substrate, the two lasers can both work in an optical frequency comb mode, the optical frequency comb repetition frequency difference is small, and the optical frequency comb repetition frequency difference is small. Terahertz lasers output by the end face of the laser are coupled and interacted, and a terahertz double-optical-comb spectrum can be generated. The end faces of the lasers are formed through the self-alignment deep etching technology, the edges of the upper electrode metal strips are aligned with the end faces of the lasers, the size consistency of the two lasers is good, the end face direction is not restrained by the crystal orientation, and on-chip terahertz double optical combs opposite to each other at any angle can be achieved. And a deep etching gap between the two lasers is convenient for placing a sample to carry out terahertz double-optical-comb spectrum detection. The on-chip terahertz double-optical-comb device has the advantages of being compact in structure and high in stability, and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor devices, and particularly relates to an on-chip terahertz dual-comb device and a manufacturing method thereof. Background Art

[0002] An optical frequency comb is a highly stable laser source with a comb-shaped spectrum. In recent years, various precision spectroscopy and metrology based on optical frequency combs have been developed. Among them, the dual-comb spectroscopy technology has received extensive attention due to its advantages of high spectral resolution, high sensitivity, and fast detection. In the terahertz (THz) band, a THz quantum cascade laser (QCL) is a solid-state semiconductor device based on intersubband transitions, which can emit lasers with frequencies in the range of 1 - 5 THz. In particular, a long-cavity terahertz quantum cascade laser operating in continuous-wave mode is very suitable for use as a terahertz optical frequency comb source due to its characteristics of high power, multi-mode, and wide spectrum. By using two terahertz quantum cascade lasers with almost exactly the same size and operating them in the optical frequency comb mode, since the repetition frequencies of the two optical frequency combs differ very little, the interaction between the two optical frequency combs can achieve a terahertz dual comb.

[0003] In existing terahertz dual-comb technologies, some dual-comb systems use two discrete terahertz quantum cascade lasers operating in the optical frequency comb mode. The two lasers are separated in space and their output end faces are opposite. By introducing a sample into the laser optical path between the two lasers, fast multi-heterodyne spectroscopy detection can be achieved. However, this discrete device structure is not compact enough, and the dual-comb system is easily affected by external factors such as device packaging and system installation. Some technologies use an on-chip dual-comb system, where two terahertz quantum cascade laser optical frequency combs are arranged in parallel on the same substrate, and the laser end faces are formed by cleavage. The optical coupling between them is either through the bottom metal layer of the double-sided metal waveguide structure or through the semi-insulating substrate of the semi-insulating surface plasmon waveguide structure. However, the optical coupling of this on-chip dual-comb device is not through the end-face output terahertz laser for coupling, and it is difficult to conveniently detect the interaction between the measured sample, the terahertz laser, and the dual comb. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide an on-chip terahertz dual-comb device and a manufacturing method thereof, which have the advantages of compact structure, good anti-interference ability, and high stability.

[0005] The present invention provides an on-chip terahertz dual-comb device;

[0006] Provide a substrate;

[0007] Two terahertz quantum cascade lasers with the same size and opposite end faces are provided above the substrate;

[0008] The terahertz quantum cascade laser includes a lower contact layer at the bottom, lower electrode metal strips at both ends above the lower contact layer, and a ridge strip in the middle above the lower contact layer; an upper electrode metal strip is also provided above the ridge strip.

[0009] Both ends of the two terahertz quantum cascade lasers are etched deeply into the substrate, and a deep etching gap is formed between the two lasers.

[0010] Preferably, the cavity length of the terahertz quantum cascade laser is 4 - 20 mm.

[0011] Preferably, the length of the upper electrode metal strip is 2 - 1000 μm longer than the cavity length of the terahertz quantum cascade laser, and the width is 50 - 300 μm.

[0012] Preferably, the ridge strips of the two terahertz quantum cascade lasers are on the same straight line and / or form an included angle relative to each other, and the angle is 1 - 180°.

[0013] Preferably, the length of the lower electrode metal strip is 0 - 50 μm less than the cavity length of the terahertz quantum cascade laser, and the distance from the ridge strip is 10 - 50 μm.

[0014] Preferably, the width of the deep etching gap is 0.1 - 20 mm, and the depth is 30 - 100 μm below the interface of the lower contact layer extending to the substrate.

[0015] The present invention also provides a preparation method of an on-chip terahertz dual optical comb device, including the following steps:

[0016] Provide a terahertz quantum cascade laser epitaxial wafer, which from bottom to top is successively a substrate, a lower contact layer, an active region, and an upper contact layer.

[0017] S1: Fabricate two upper electrode metal strips with the same size on the epitaxial wafer, and the two upper electrode metal strips are oppositely arranged to form an included angle.

[0018] S2: Fabricate a first etching mask to cover the upper electrode metal strip and extend to cover a part of the epitaxial wafer region, and dry-etch the region not covered by the first etching mask until the lower contact layer to form two ridge strips with the same width, and then remove the first etching mask; S3: Fabricate lower electrode metal strips on the surfaces of the lower contact layer exposed by etching on both sides of the ridge strips.

[0019] S4: Fabricate a second etching mask to cover part of the upper electrode metal strip and laterally extend to cover all of the lower electrode metal strip and part of the epitaxial wafer region. Use wet etching or ion beam etching to remove the upper electrode metal strip not covered by the second etching mask, and then deeply etch to remove the upper contact layer, active region, lower contact layer and part of the substrate not covered by the second etching mask to form an end face, and form a deep etching gap between two terahertz quantum cascade lasers; then remove the second etching mask;

[0020] S5: Thinning, polishing and gold plating on the back side to complete the device process;

[0021] S6: Finally, dicing, chip mounting and wire bonding to complete the device packaging.

[0022] Preferably, the manufacturing methods of the upper electrode metal strip and the lower electrode metal strip both include photolithography, evaporation of a metal film, solvent degumming and / or other stripping processes.

[0023] Preferably, the material of the first etching mask in step S2 is one or more of photoresist, silicon nitride, and silicon dioxide; the width of the first etching mask is 0.1 - 40 μm greater than that of the upper electrode metal strip.

[0024] Preferably, the material of the second etching mask in step S4 is one or more of photoresist, silicon nitride, and silicon dioxide; the length of the second etching mask is equal to the cavity length of the terahertz quantum cascade laser.

[0025] The manufacturing method of the present invention does not adopt a cleavage process, but uses a self-aligned deep etching technology to form the laser end face. The edge of the upper electrode metal strip can also be aligned with the laser end face. The manufacturing dimensions of the two lasers are in good consistency, and the direction of the end face is not restricted by the crystal orientation, and an on-chip terahertz dual optical comb device with arbitrary relative angles can be realized.

[0026] Beneficial effects

[0027] The present invention provides an on-chip terahertz dual optical comb device, that is, two terahertz quantum cascade lasers with the same size and opposite end faces are fabricated on the same substrate. Both lasers can operate in the optical frequency comb mode, and the repetition frequencies of the optical frequency combs are close. The terahertz lasers output from the laser end faces interact through coupling to generate a terahertz dual optical comb spectrum. The end face of the laser is formed by a self-aligned deep etching technology. The edge of the upper electrode metal strip is aligned with the laser end face. The two lasers have good size consistency, and the direction of the end face is not restricted by the crystal orientation, and an on-chip terahertz dual optical comb with arbitrary relative angles can be realized. The deep etching gap between the two lasers is convenient for placing samples for terahertz dual optical comb spectrum detection. The on-chip terahertz dual optical comb device of the present invention has the advantages of compact structure and high stability, and has good application prospects. Description of the Drawings

[0028] Figure 1 It is a schematic structural diagram of the epitaxial wafer of the terahertz quantum cascade laser of the present invention;

[0029] Figure 2 It is a top view schematic diagram of the epitaxial wafer after manufacturing step S1 of the present invention;

[0030] Figure 3 It is a top view schematic diagram of the epitaxial wafer after manufacturing step S2 of the present invention;

[0031] Figure 4 It is a top view schematic diagram of the epitaxial wafer after manufacturing step S3 of the present invention;

[0032] Figure 5 It is a top view schematic diagram of the terahertz dual-comb device on the wafer after manufacturing step S4 of the present invention;

[0033] Figure 6 It is the terahertz dual-comb device on the wafer after manufacturing step S4 of the present invention at Figure 5 the cross-sectional schematic diagram at the dashed line;

[0034] Figure 7 It is the dual-comb spectrum characterization of the terahertz dual-comb device on the wafer manufactured in the embodiment of the present invention.

[0035] In the figure: 1 - substrate, 2 - lower contact layer, 3 - active region, 4 - upper contact layer, 5 - upper electrode metal strip, 6 - ridge, 7 - lower electrode metal strip, 8 - end face, 9 - deep etching gap. Specific Embodiments

[0036] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0037] As an embodiment of the present invention, a terahertz dual-comb device on a wafer is provided, which consists of Figure 5 and Figure 6 as shown;

[0038] Provide the substrate 1;

[0039] Two terahertz quantum cascade lasers with the same size and opposite end faces 8 are provided above the substrate 1;

[0040] The terahertz quantum cascade laser includes a lower contact layer 2 at the bottom, lower electrode metal strips 7 at both ends above the lower contact layer 2, and a ridge 6 in the middle above the lower contact layer 2; an upper electrode metal strip 5 is also provided above the ridge 6;

[0041] Both ends of the two terahertz quantum cascade lasers are etched deep into the substrate, and a deep etching gap 9 is formed between the two lasers.

[0042] As an implementation manner of the present invention, a preparation method of an on-chip terahertz dual optical comb device is further provided, including the following steps:

[0043] Provide an epitaxial wafer of a terahertz quantum cascade laser, and the epitaxial wafer sequentially includes a substrate 1, a lower contact layer 2, an active region 3, and an upper contact layer 4 from bottom to top, as Figure 1 shown;

[0044] S1: Spin-coat photoresist on the epitaxial wafer, form patterned photoresist after exposure and development by a lithography machine, then electron beam evaporate a TiAu metal film, and remove the photoresist with an organic solvent to fabricate two laser upper electrode metal strips 5 on the epitaxial wafer. This method of fabricating patterned metal is also called the lift-off method. The widths of the two upper electrode metal strips 5 are the same, and the length is greater than the laser cavity length, as Figure 2 shown. S2: Fabricate a patterned first etching mask on the epitaxial wafer, and the material can be photoresist, silicon nitride, silicon oxide, etc. The first etching mask is wider than the upper electrode metal strip 5, completely covers the upper electrode metal strip 5 and extends to cover a part of the epitaxial wafer region, and then inductively coupled plasma etching is used to remove the region not covered by the first etching mask until the lower contact layer 2 to form two ridges 6 with the same width. The two ridges 6 are on the same straight line and / or form an included angle relative to each other, and the angle is 1-180°. Then remove the first etching mask with an organic solvent or reactive ion etching, as Figure 3 shown.

[0045] S3: Fabricate patterned photoresist on the epitaxial wafer, then electron beam evaporate a GeAuNiAu metal film, and remove the photoresist with an organic solvent to fabricate lower electrode metal strips 7 on the surfaces of the lower contact layer 2 on both sides of the ridge 6 and exposed by etching. The length of the lower electrode metal strip 7 is slightly less than the laser cavity length. The distances of the lower electrode metal strips 7 from the ridge 6 are the same, as Figure 4 shown.

[0046] S4: Fabricate a patterned second etch mask on the epitaxial wafer, covering part of the upper electrode metal strip 5 and laterally extending to cover all of the lower electrode metal strip 7 and part of the epitaxial wafer region. Wet etch or ion beam etch to remove the upper electrode metal strip 5 not covered by the second etch mask. Then, inductively coupled plasma deep etch to remove the upper contact layer 4, active region 3, lower contact layer 2, and part of the substrate 1 not covered by the second etch mask to form the laser end face 8, and form a deep etch gap 9 between the two lasers. The deep etch gap 9 facilitates placing samples for terahertz dual-comb detection. Optional materials for the second etch mask are photoresist, silicon nitride, and silicon dioxide. The length of the second etch mask is equal to the laser cavity length. The depth of the deep etch gap 9 at both ends of the laser needs to reach deep into the substrate to ensure strong enough laser reflection at the end face. This self-aligned etch method can ensure the alignment of the edge of the upper electrode metal strip and the end face, avoiding absorption losses introduced by the inward shrinkage of the upper electrode metal strip or uneven etching of the end face caused by the protrusion of the upper electrode metal strip. Then, remove the second etch mask with an organic solvent or reactive ion etching.

[0047] S5: Complete the device process by back thinning, polishing, and gold plating.

[0048] S6: Finally, complete the device packaging by dicing, chip bonding, and wire bonding.

[0049] Example 1

[0050] Provide a terahertz quantum cascade laser epitaxial wafer with a center frequency of 4.2 THz for the wide gain spectrum. The epitaxial materials from the substrate 1 upwards are a 400-nm-thick lower contact layer 2, a 10-μm-thick active region 3, and a 50-nm-thick upper contact layer 4. The manufacturing method steps of the terahertz dual-comb device on the wafer are as follows:

[0051] S1: Spin coat a double-layer photoresist of LOR10B and AZ5214 on the epitaxial wafer. First, spin coat LOR10B at a speed of 3000 rpm and bake at 150 °C for 3 min. Then, spin coat AZ5214 photoresist at a speed of 3000 rpm and bake at 100 °C for 3 min. Expose with a MA6 mask aligner for 6 s and develop with AZ300 developer for 40 s. Electron beam evaporate Ti / Au (20 / 400 nm). Then, remove the photoresist with NMP to fabricate two upper electrode metal strips 5 on the epitaxial wafer. The width of the upper electrode metal strip 5 is 150 μm, and the length is 6.1 mm.

[0052] S2: Use a silicon nitride hard mask for ridge etching. First, grow 1 μm of silicon nitride by inductively coupled plasma chemical vapor deposition (ICPCVD) on the epitaxial wafer at a deposition temperature of 130 °C. Then spin-coat AZ5214 photoresist at a speed of 3000 rpm and bake it at 100 °C for 3 min. The thickness of the photoresist is about 1.5 μm. Expose it for 6 s using MA6 and develop it for 40 s using AZ300 developer. Then use reactive ion etching to remove the silicon nitride not covered by the photoresist to fabricate a patterned silicon nitride mask. The width of the silicon nitride mask is 156 μm, which completely covers the upper electrode metal strip 5 and covers about 3 μm of the epitaxial material on both sides. Then use an Oxford ICP-180 inductively coupled plasma etcher to remove the epitaxial material on both sides of the upper electrode metal strip 5 that is not covered by silicon nitride until the lower contact layer 2, forming a laser ridge 6 with a width of 156 μm. Finally, use reactive ion etching to remove the remaining silicon nitride mask. A total of 2 groups of laser ridges 6 were made. One group of laser ridges 6 is in a straight line, and the included angle of the other group of laser ridges 6 is 170°.

[0053] S3: Spin-coat a double-layer film of LOR10B + AZ4620 on the epitaxial wafer. First, spin-coat LOR10B at a speed of 3000 rpm and bake it at 150 °C for 3 min. Then spin-coat a thick film of AZ4620 at a speed of 4000 rpm and bake it at 100 °C for 3 min. Expose it for 16 s using MA6 and develop it for 150 s using AZ300 developer. Electron beam evaporate a GeAuNiAu (13 / 33 / 30 / 300) nm metal film, and use NMP to remove the photoresist on the surface of the lower contact layer exposed by etching on both sides of the laser ridge to fabricate the lower electrode metal strip 7 of the laser, and perform rapid annealing to form a good ohmic contact. The length of the lower electrode metal strip 7 is 5996 μm, and the lower electrode metal strip 7 is 25 μm away from the edge of the laser ridge 6.

[0054] S4: End-face etching is carried out using a double mask of thick photoresist and silicon nitride. First, 1.5 μm of silicon nitride is grown on the epitaxial wafer by inductively coupled plasma chemical vapor deposition (ICPCVD) at a deposition temperature of 130 °C. Then, AZ4620 thick photoresist is spin-coated at a rotational speed of 4000 rpm and baked at 100 °C for 3 min. The thickness of the photoresist is about 5.8 μm. It is exposed for 16 s using MA6 and developed for 150 s using AZ300 developer. The silicon nitride not covered by the photoresist is removed by reactive ion etching, and the remaining patterned thick photoresist and silicon nitride are used as a deep etching mask. The length of the mask is 6 mm, covering part of the upper electrode metal strip 5 and extending laterally to cover all of the lower electrode metal strip 7 and part of the epitaxial material. The length of the upper electrode metal strip 5 is 6.1 mm, the length of the mask is 6 mm, and 50 μm at each end of the upper electrode metal strip 5 is not covered. The Au and Ti of the upper electrode metal strip not covered by the mask are removed by wet etching with an iodine: potassium iodide solution and hydrofluoric acid. Then, an Oxford ICP-180 inductively coupled plasma etching machine is used to remove the epitaxial material and part of the substrate not covered by the second etching mask, forming the laser end-face 8 and creating a deep etching gap 9 between the two lasers. The deep etching gap 9 facilitates placing samples for terahertz dual-comb detection. The depth of the deep etching gap 9 reaches about 40 μm below the interface between the lower contact layer and the substrate. The cavity length of both lasers is 6 mm, and the width of the deep etching gap 9 between the two lasers is 1 mm. Then, the remaining silicon nitride mask is removed by reactive ion etching;

[0055] S5: The back side is thinned and polished to a thickness of 170 μm, and TiAu (20 / 200 nm) is sputtered on the back side;

[0056] S6: Finally, dicing is performed. There is no need to cleave the end-face. The device is packaged by pasting and wire bonding.

[0057] The on-chip terahertz dual-comb device obtained above is installed in a liquid helium cryocooler. At a working temperature of 25 K, the two lasers of the dual-comb can operate continuously and operate in the optical frequency comb mode at a certain driving current. The dual-comb spectrum is measured using a spectrum analyzer. Figure 7 This is the dual-comb spectrum of the on-chip terahertz dual-comb device with a 170° included angle measured. There are 15 dual-comb spectral lines, and the spectral line spacing is about 29.1 MHz. This terahertz dual-comb spectrum can be used for fast multi-heterodyne spectroscopic detection.

Claims

1. A on-chip terahertz dual-comb device, characterized in that: A substrate (1) is provided; Above the substrate (1), there are two terahertz quantum cascade lasers with the same size and opposite end faces; The terahertz quantum cascade laser includes a lower contact layer (2) at the bottom, lower electrode metal strips (7) at both ends above the lower contact layer (2), and a ridge (6) in the middle above the lower contact layer (2); an upper electrode metal strip (5) is also provided above the ridge (6); Both ends of the two terahertz quantum cascade lasers are etched deeply into the substrate, and a deep etching gap (9) is formed between the two lasers.

2. The on-chip terahertz dual optical frequency comb device according to claim 1, wherein: The cavity length of the terahertz quantum cascade laser is 4 - 20 mm.

3. The on-chip terahertz dual optical frequency comb device according to claim 1, characterized in that: The length of the upper electrode metal strip (5) is 2 - 1000 μm longer than the cavity length of the terahertz quantum cascade laser, and the width is 50 - 300 μm.

4. The on-chip terahertz dual optical frequency comb device according to claim 1, characterized in that: The ridges (6) of the two terahertz quantum cascade lasers are on the same straight line and / or form an angle with each other, and the angle is 1 - 180°.

5. The on-chip terahertz dual optical comb device according to claim 1, characterized in that: The length of the lower electrode metal strip (7) is 0 - 50 μm shorter than the cavity length of the terahertz quantum cascade laser, and the distance from the ridge (6) is 10 - 50 μm.

6. The on-chip terahertz dual optical comb device according to claim 1, characterized in that: The width of the deep etching gap (9) is 0.1 - 20 mm, and the depth is 30 - 100 μm below the interface of the lower contact layer (2) extending to the substrate (1).

7. A preparation method of a on-chip terahertz dual-comb device, comprising the following steps: Provide a terahertz quantum cascade laser epitaxial wafer, which from bottom to top is successively a substrate (1), a lower contact layer (2), an active region (3), and an upper contact layer (4); S1: Fabricate two upper electrode metal strips (5) with the same size on the epitaxial wafer, and the two upper electrode metal strips (5) are oppositely arranged to form an angle; S2: Fabricate a first etching mask to cover the upper electrode metal strip (5) and extend to cover part of the epitaxial wafer region, and dry-etch the region not covered by the first etching mask until the lower contact layer (2) to form two ridges (6) with the same width, and then remove the first etching mask; S3: On the surfaces of the lower contact layer (2) exposed by etching on both sides of the ridge (6), fabricate lower electrode metal strips (7); S4: Fabricate a second etching mask to cover part of the upper electrode metal strip (5) and laterally extend to cover all the lower electrode metal strips (7) and part of the epitaxial wafer region, wet-etch or ion beam etch the upper electrode metal strip (5) not covered by the second etching mask, and then deeply etch to remove the upper contact layer (4), the active region (3), the lower contact layer (2), and part of the substrate (1) not covered by the second etching mask to form an end face (8), and form a deep etching gap (9) between the two terahertz quantum cascade lasers; then remove the second etching mask; S5: Thinning, polishing, and gold plating on the back to complete the device process; S6: Finally, dicing, chip mounting, and wire bonding to complete the device packaging.

8. The preparation method according to claim 7, characterized in that: The fabrication methods of the upper electrode metal strip (5) and the lower electrode metal strip (7) both include photolithography, evaporation of a metal film, solvent degluing, and / or other stripping processes.

9. The preparation method according to claim 7, wherein: The material of the first etching mask in the step S2 is one or several of photoresist, silicon nitride, and silicon dioxide; the width of the first etching mask is 0.1 - 40 μm greater than that of the upper electrode metal strip (5).

10. The preparation method according to claim 7, characterized in that: The material of the second etching mask in the step S4 is one or several of photoresist, silicon nitride, and silicon dioxide; the length of the second etching mask is equal to the cavity length of the terahertz quantum cascade laser.

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