A terahertz quantum cascade laser ridge waveguide etching method

Abstract

This invention relates to a method for etching a ridge waveguide of a terahertz quantum cascade laser. The method includes: (1) providing a terahertz quantum cascade laser material; (2) etching a groove (7) with the same depth and thickness as the lower contact layer on the surface of the material; (3) further etching the material using a dry etching system equipped with a laser interferometer to etch a ridge structure (8), with the groove (7) being etched simultaneously, and the laser interferometer performing online detection on the bottom of the groove; (4) when the intensity of the oscillation signal of the laser interferometer changes abruptly, the bottom of the groove (7) is just etched to the interface between the lower contact layer (4) and the etching stop layer (3), while at this time, both sides of the ridge structure (8) are just etched to the interface between the multi-quantum well active region (5) and the lower contact layer (4). This method can effectively determine the optimal etching depth by adding only a simple groove pre-etching process, thus improving the accuracy of the etching depth of the laser ridge waveguide.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser technology, and specifically relates to a method for etching the ridge waveguide of a terahertz quantum cascade laser. Background Technology

[0002] Terahertz quantum cascade lasers are unipolar multi-quantum-well semiconductor lasers. Their basic working principle involves electrons radiating and releasing photons in the conduction band of the multi-quantum-wells, with photon frequencies ranging from 1 to 5 THz. Terahertz quantum cascade lasers offer advantages such as easy solid-state integration and high conversion efficiency, making them a hot research topic in terahertz radiation sources. Two waveguide structures for terahertz quantum cascade lasers are semi-insulating surface plasmon waveguides and double-sided metallic waveguides. The semi-insulating surface plasmon waveguide structure is relatively simple to fabricate, offers good laser directionality, and provides high coupling output power; therefore, it is commonly used in high-power terahertz quantum cascade lasers.

[0003] Semi-insulating surface plasmonic waveguide (SPL) fabrication involves sputtering (evaporating) the upper electrode metal layer, ridge waveguide etching, evaporating the lower electrode metal layer, substrate thinning, and cleavage bonding. Ridge waveguide etching is a critical process, requiring the etching depth to reach the lower contact layer without exceeding it; otherwise, the device cannot achieve electrical conduction. Furthermore, to reduce parasitic series resistance, the thicker the retained lower contact layer, the better. Therefore, optimal electrical performance is achieved when the etching depth reaches precisely at the interface between the multi-quantum-well active region and the lower contact layer.

[0004] Etching depth is typically measured using a profilometer. During etching, the sample must be repeatedly removed from the etching system to measure the steps and determine if the etching depth has been reached, a cumbersome process. Terahertz quantum cascade lasers have an active region thickness of up to 10 μm and a lower contact layer of only 400–600 nm, meaning the etching depth accuracy cannot exceed 4%–6%, otherwise the lower electrode will have poor contact. Furthermore, dry etching also etches the mask, requiring consideration of the remaining mask thickness to estimate the actual etching depth; the accuracy of this method cannot meet the process requirements of terahertz quantum cascade lasers.

[0005] Mainstream dry etching systems can be equipped with laser interferometers. A laser interferometer illuminates the sample surface with a laser beam and detects the reflected signal using a detector. If the sample surface contains a heterogeneous thin film material, the laser beam is reflected from the two interfaces of the film, creating interference. As the film thickness changes during etching, the reflected laser signal exhibits approximately sinusoidal oscillation characteristics. Therefore, laser interferometers can be used to determine the endpoint of thin film etching. Conventional laser interferometers use visible red light, which has good transmittance for dielectric films such as silicon dioxide, resulting in a strong interferometer signal. For GaAs materials, red light transmittance is poor; only when the GaAs material is as thin as two to three hundred nm does the laser interferometer show an oscillating signal. Even with longer wavelength laser interferometers, due to the thickness of the multi-quantum-well active region material and its very similar refractive index to the underlying contact layer material, the interface between the multi-quantum-well active region and the underlying contact layer cannot be distinguished from the laser interferometer signal. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for etching the ridge waveguide of a terahertz quantum cascade laser, so as to solve the problem that the etching depth of the ridge waveguide of the terahertz quantum cascade laser is difficult to control and easily causes poor contact of the lower electrode in the prior art.

[0007] This invention provides a method for etching the ridge waveguide of a terahertz quantum cascade laser, comprising:

[0008] (1) A terahertz quantum cascade laser material is provided, wherein the material comprises, from bottom to top, a substrate, a buffer layer, an etch stop layer, a lower contact layer, a multi-quantum well active region, and an upper contact layer;

[0009] (2) Perform the first etching on the material surface to etch a groove;

[0010] (3) A dry etching system equipped with a laser interferometer is used to perform a second etching on the material to etch out a ridge structure with a convex cross-section. The top of the ridge structure is never etched, while the two sides of the ridge structure, including the aforementioned groove, are etched simultaneously to ensure that the bottom of the groove is always deeper than the two sides of the ridge structure by the thickness of the lower contact layer. The laser interferometer performs online detection on the bottom of the groove, and the signal intensity of the laser interferometer oscillates smoothly over time.

[0011] (4) Since the refractive indices of the lower contact layer and the etch stop layer are different, the intensity of the reflected signal of the laser interferometer on these two materials is different. Therefore, when the oscillation signal of the laser interferometer no longer changes smoothly and the intensity jumps, it means that the bottom of the groove is just etched to the interface between the lower contact layer and the etch stop layer. At this time, the two sides of the ridge structure are just etched to the interface between the active region of the multi-quantum well and the lower contact layer, and the etching depth reaches the optimal level.

[0012] Preferably, in step (1), the substrate material is a semi-insulating GaAs single crystal; the thickness of the buffer layer is 100nm to 500nm, and the material is undoped GaAs.

[0013] Preferably, in step (1), the thickness of the etching stop layer is 100 nm to 300 nm, and the material is high-aluminum undoped Al. x Ga 1-x As, x is 0.5 to 0.6.

[0014] Preferably, in step (1), the thickness of the lower contact layer is 400 nm to 600 nm, the material is n-type Si-doped GaAs, and the Si doping concentration is 2 × 10⁻⁶. 18 cm -3 ~3×10 18 cm -3 .

[0015] Preferably, in step (1), the thickness of the multi-quantum-well active region is 10 μm to 12 μm, and the material is partially n-type Si lightly doped GaAs / Al. x Ga 1-x As multiple quantum wells, x is 0.15–0.25, and the Si doping concentration in some GaAs layers is 1 × 10⁻⁶. 16 cm -3 ~1×10 17 cm -3 .

[0016] Preferably, in step (1), the thickness of the upper contact layer is 50 nm to 100 nm, the material is n-type Si-doped GaAs, and the Si doping concentration is 3 × 10⁻⁶. 18 cm -3 ~5×10 18 cm -3 .

[0017] Preferably, in step (2), the first etching of the material surface to etch a groove is performed by: making a first patterned mask on the material surface, ensuring that the area to be etched is not covered by the mask, and then etching out the groove.

[0018] Preferably, the first etching in step (2) is performed using wet etching or inductively coupled plasma dry etching.

[0019] Preferably, in step (2), the groove depth is the same as the thickness of the lower contact layer, and the groove width is 50-300 μm.

[0020] Preferably, in step (3), the second etching of the material using a dry etching system equipped with a laser interferometer is performed by: removing the first mask after etching the material surface, remaking a second patterned mask, ensuring that the area to be etched, including the groove, is not covered by the second mask, and then using a dry etching system equipped with a laser interferometer to etch the material.

[0021] Preferably, the etching in step (3) is performed using an inductively coupled plasma dry etching system, and the etching gas includes one or more of Cl2, Ar, and BCl3.

[0022] Preferably, in step (4), tens of nanometers can be added to the etching process to ensure uniformity.

[0023] Given that conventional ridge waveguide etching processes are difficult to control in terms of etching depth, which can easily lead to poor contact of the lower electrode in lasers, this invention proposes an improved ridge waveguide etching method for terahertz quantum cascade lasers. The ridge waveguide etching is performed in two steps: first, a groove is etched with the same depth as the lower contact layer thickness; then, a ridge structure is etched simultaneously with the groove, ensuring that the bottom of the groove is always deeper than the sides of the ridge structure by the thickness of the lower contact layer. A laser interferometer is used to monitor the etching depth at the groove in real time and terminates the etching at appropriate times. When the intensity of the laser interferometer's oscillation signal changes abruptly, the bottom of the groove is precisely etched to the interface between the lower contact layer and the etching stop layer, while the sides of the ridge structure are precisely etched to the interface between the multi-quantum well active region and the lower contact layer, achieving the optimal etching depth.

[0024] Beneficial effects

[0025] This invention effectively determines the optimal etching depth and improves the accuracy of laser ridge waveguide etching depth by simply adding a simple groove pre-etching process and introducing a laser interferometer into the etching system. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the cross-sectional structure of the terahertz quantum cascade laser material after the first etching step in this invention.

[0027] Figure 2 This is a schematic diagram of the cross-sectional structure of the terahertz quantum cascade laser material after the second step of etching in this invention.

[0028] Figure 1-2 The numbers in the middle are labeled as follows: 1—substrate, 2—buffer layer, 3—etch stop layer, 4—lower contact layer, 5—multiple quantum well active region, 6—upper contact layer, 7—groove, 8—ridge structure.

[0029] Figure 3 This is the laser interferometer signal in the later stage of the etching process. Detailed Implementation

[0030] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0031] Example 1

[0032] This embodiment provides a method for etching the ridge waveguide of a terahertz quantum cascade laser, the steps of which are as follows:

[0033] (1) Provide a terahertz quantum cascade laser material, the material comprising, from bottom to top, a 350 μm thick semi-insulating GaAs substrate, a 250 nm thick undoped GaAs buffer layer, and a 250 nm thick undoped Al layer. 0.55 Ga 0.45 As etch stop layer, 400nm thick Si-doped GaAs contact layer (doping concentration 3×10⁻⁶) 18 cm -3 ), 10μm thick Si-doped GaAs / Al 0.15 Ga 0.85 As multi-quantum-well active region (partial GaAs layer doping concentration 1.6 × 10⁻⁶) 16 cm -3 ), 50nm thick Si-doped GaAs upper contact layer (doping concentration 5×10⁻⁶) 18 cm -3 );

[0034] (2) Photoresist is used as the etching mask. AZ5214 photoresist is spin-coated onto the material surface, with a thickness of approximately 1.5 μm. The patterned photoresist is then exposed and developed. After development, the area to be etched is not covered by the photoresist.

[0035] (3) Wet etching was used to create grooves with a width of 150 μm. The etching solution was H3PO4:H2O2:H2O (1:1:25). The etching rate was approximately 170 nm / min. The depth of the etched grooves was measured using a profilometer until it reached approximately 400 nm.

[0036] (4) Remove the photoresist by heating in an acetone water bath. Then spin-coat AZ4620 photoresist onto the material surface as a mask, with a thickness of approximately 8 μm. Expose and develop to pattern the photoresist. The areas to be etched, including the grooves, should not be covered by the photoresist.

[0037] (5) The material was etched using a SENTECH SI500D inductively coupled plasma etching system equipped with a laser interferometer, creating a ridge structure with a convex cross-section. The top of the ridge structure remained unetched, while both sides, including the aforementioned groove, were etched simultaneously to ensure that the bottom of the groove was always 400 nm deeper than the sides of the ridge structure. The etching gases were Cl2, Ar, and BCl3 (flow rates of 8, 20, and 16 sccm, respectively). The laser interferometer was used to detect the bottom of the groove. The laser wavelength was 670 nm. When the bottom of the groove was etched close to the etching stop layer, the interferometer signal oscillated, and the amplitude increased with further etching. Figure 3 As shown, when the interferometer oscillation signal intensity jumps, it indicates that the bottom of the groove has just been etched to the interface between the lower contact layer and the etch stop layer. At this time, both sides of the ridge structure have just been etched to the interface between the multi-quantum well active region and the lower contact layer, and the etching depth has reached the optimal level. Considering etching uniformity, etching is stopped after another 7 seconds (approximately 50 nm) to ensure that both sides of the ridge structure are etched to the lower contact layer.

[0038] (6) Remove the photoresist by heating with an acetone water bath. The profilometer measured that the middle of the ridge structure was about 10089 nm higher than the two sides, which confirmed that the etching depth on both sides of the ridge reached the vicinity of the interface between the active region and the lower contact layer (about 39 nm deeper into the lower contact layer).

[0039] In summary, this invention provides an etching method for the ridge waveguide of a terahertz quantum cascade laser. This method can effectively determine the optimal etching depth and improve the accuracy of the etching depth of the laser ridge waveguide by adding a pre-etching process for the groove and introducing a laser interferometer into the etching system.

[0040] Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

Claims

1. A method for etching the ridge waveguide of a terahertz quantum cascade laser, characterized in that, Includes the following steps: (1) A terahertz quantum cascade laser material is provided, wherein the material comprises, from bottom to top, a substrate (1), a buffer layer (2), an etch stop layer (3), a lower contact layer (4), a multi-quantum well active region (5), and an upper contact layer (6); the etch stop layer (3) has a thickness of 100 nm to 300 nm and is made of high-aluminum undoped Al. x Ga 1-x As, x is 0.5 to 0.6; the thickness of the lower contact layer (4) is 400 nm to 600 nm, the material is n-type Si-doped GaAs, and the Si doping concentration is 2 × 10⁻⁶. 18 cm -3 ~3×10 18 cm -3 ; (2) A first etching is performed on the material surface to etch a groove (7); the depth of the groove (7) is the same as the thickness of the lower contact layer (4); (3) The material is etched a second time using a dry etching system equipped with a laser interferometer to etch out a ridge structure (8) with a convex cross section. The top of the ridge structure (8) is never etched, and the two sides of the ridge structure (8), including the above-mentioned groove (7), are etched simultaneously to ensure that the bottom of the groove (7) is always deeper than the two sides of the ridge structure (8) by the thickness of a lower contact layer (4). The laser interferometer performs online detection on the bottom of the groove, and the signal intensity of the laser interferometer oscillates smoothly over time. (4) When the oscillation signal of the laser interferometer no longer changes smoothly and the intensity jumps, the bottom of the groove (7) is just etched to the interface between the lower contact layer (4) and the etching stop layer (3), and at this time the two sides of the ridge structure (8) are just etched to the interface between the multi-quantum well active region (5) and the lower contact layer (4), and the etching depth reaches the optimal.

2. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (1), the substrate (1) is a semi-insulating GaAs single crystal; the buffer layer (2) has a thickness of 100nm to 500nm and is made of undoped GaAs.

3. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (1), the thickness of the multi-quantum-well active region (5) is 10 μm to 12 μm, and the material is partially n-type Si lightly doped GaAs / Al. x Ga 1-x As multiple quantum wells, x is 0.15–0.25, and the Si doping concentration in some GaAs layers is 1 × 10⁻⁶. 16 cm -3 ~1×10 17 cm -3 The thickness of the upper contact layer (6) is 50 nm to 100 nm, and the material is n-type Si-doped GaAs with a Si doping concentration of 3 × 10⁻⁶. 18 cm -3 ~5×10 18 cm -3 .

4. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (2), the first etching is performed on the material surface to etch a groove (7) as follows: a first patterned mask is made on the material surface, the area to be etched is not covered by the mask, and then the groove is etched.

5. The terahertz quantum cascade laser ridge waveguide etching method according to claim 1, characterized in that, In step (2), the first etching is performed using wet etching or inductively coupled plasma dry etching.

6. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (2), the groove (7) has a width of 50-300 μm.

7. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (3), the material is etched a second time using a dry etching system equipped with a laser interferometer: the first mask after etching is removed from the material surface, a second patterned mask is remade, the area to be etched, including the groove, is not covered by the second mask, and then the material is etched using a dry etching system equipped with a laser interferometer.

8. The etching method for the ridge waveguide of a terahertz quantum cascade laser according to claim 1, characterized in that, In step (3), the etching is performed using an inductively coupled plasma dry etching system, and the etching gas includes one or more of Cl2, Ar, and BCl3.

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