JP2009044108A - Dielectric thin film thickness evaluation method of semiconductor element end surface - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dielectric thin film thickness evaluation method of a semiconductor element end surface, which measures with sufficient precision the reflection factor of the semiconductor element end surface, and at the same time, based on a measured value, makes it possible to obtain the semiconductor element of which the reflection factor of the semiconductor element end surface is made closer to a design value. <P>SOLUTION: The dielectric thin film thickness evaluation method is equipped with a TiO<SB>2</SB>film and an SiO<SB>2</SB>film, and the film thickness of an HR film provided in the light-emitting end surface of the semiconductor element is found by: measuring an optical beam induced current generated by a pumping light irradiated from the outside of the light-emitting end surface of the semiconductor element to the HR film; calculating the change rate I<SB>ph</SB>of the optical beam induced current based on the measured value of the optical beam induced current: comparing the relation of the calculated result of the change rate I<SB>ph</SB>of the optical beam induced current, and the change rate to the deviation between the design thickness of the TiO<SB>2</SB>film calculated previously as a reference value and the actual film thickness, and the deviation between the design thickness of SiO<SB>2</SB>film and the actual film thickness; and determining the region (I), (II), and (IV) which the actual film thickness of the TiO<SB>2</SB>film and the actual film thickness of the SiO<SB>2</SB>film can take. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a semiconductor element end face that enables optimization of the reflectance of a coating film used in an active element such as a semiconductor laser, a semiconductor optical amplifier, or a semiconductor optical switch used in optical fiber communication, optical information processing, an optical disk, etc. The present invention relates to a dielectric thin film thickness evaluation method.

  The cleaved semiconductor laser automatically forms a mirror whose cleaved surface has a reflectivity of 0.3. When current is injected into this semiconductor laser, the optical gain increases. When the drive current is increased until the loss is compensated, laser oscillation occurs. Since this laser oscillation is caused by low current drive when the loss is lowered (increased reflectivity), a dielectric thin film (hereinafter referred to as a coating film) is coated on the end face of the semiconductor laser as a means for controlling the reflectivity. It has been broken. On the other hand, when antireflection (AR) films are coated on both end faces of the semiconductor laser, the Fabry-Perot mode by the end faces is suppressed, and a semiconductor optical amplifier or an optical switch can be manufactured. As described above, the end face coating of the semiconductor laser is an indispensable technique for producing a multifunctional optical device.

FIG. 7 shows a partially broken perspective view of a distributed feedback semiconductor laser (DFB laser). In the figure, 1 is a 1.3 μm composition InGaAsP active layer, 2 is a 1.1 μm composition lower InGaAsP guide layer, 3 is a 1.1 μm composition upper InGaAsP guide layer, 4 is a p-InP buffer layer, 5 is an n-InP cladding layer, 6 is an n ⁺ -InGaAsP cap layer, 7 is a p-InP substrate, 8 is a Ru-added semi-insulator (SI) -InP current blocking layer, 9 is a p-electrode, and 10 is an n-electrode. Note that a grating 11 is fabricated in the 1.1 μm composition InGaAsP guide layer 3 as a structure that realizes distributed feedback, enabling single longitudinal mode oscillation.

  In the semiconductor laser shown in FIG. 7, the coating film is coated with an AR film 12 on the front end face side and a high reflection (HR) film 13 on the rear end face side. Laser light can be efficiently coupled to an optical fiber (not shown). Furthermore, since the cleavage end face is coated with a dielectric film, deterioration of the cleavage end face is suppressed, and long-term reliability is ensured. As described above, the end surface coating is an essential technique from the viewpoint of the optical characteristics and reliability of the semiconductor element.

  The reflectance of the end face of the semiconductor laser, that is, the reflectance of the coating film coated on the end face of the semiconductor laser is determined from two parameters of the thickness and refractive index of the coating film, and appropriate design is required. However, these two parameters (film thickness and refractive index) vary depending on the production of the coating film. When the two parameters fluctuate, the reflectance of the coating film deviates from the design value. In order to correct the deviation between the reflectance of the coating film and the design value, it is necessary to precisely measure the reflectance of the coating film, and based on the measurement result, the above parameters can be brought close to the design value. Become.

  Hereinafter, as an example, a measuring method for measuring the end face reflectance of the semiconductor laser shown in FIG. 7 will be described. The reflectance of the coating film is related to the optical output P of the semiconductor laser and is expressed by the following equation (1).

Here, R _f and R _r are the front end face side reflectance and the rear end face side reflectance of the semiconductor laser (in the configuration shown in FIG. 7, the reflectance of the AR film 12 and the reflectance of the HR film 13), respectively. Because reflectance before coating is about 0.3, the reflectance of the semiconductor laser facet can be estimated from the ratio of front facet optical output P _f and the rear end face side optical output P _r before and after each coating.

The coating film can be analyzed based on the front end face side reflectance R _f and the rear end face side reflectance R _r estimated by the measurement method.

First, the HR film 13 on the rear end face side will be described. The HR film 13 on the rear end face side can be realized by using a pair of a high refractive index dielectric material and a low refractive index dielectric material each having a thickness of 1/4 wavelength, and the increase in the number of pairs is a coating. The reflectance of the film is increased (for example, see Non-Patent Document 1). As an example, a pair of high refractive index (for example, refractive index 2.3) TiO ₂ film and low refractive index (for example, refractive index 1.45) SiO ₂ film is used. The reflectance R _r of the HR film 13 is calculated to be about 0.61.

  As described above, the reflectance of the coating film changes depending on the film thickness and the refractive index. In general, a dielectric used for coating is a stable material, and the refractive index of the dielectric hardly fluctuates. For this reason, it can be seen that the deviation of the reflectance of the coating film from the design value is due to the deviation of the thickness of the coating film from the design value.

FIG. 8 shows design film thicknesses d _TiO2-0 and d, which are design values of the respective film thicknesses of TiO ₂ film thickness d _TiO2-x and SiO ₂ film thickness d _SiO2-x when laser light having a wavelength of 1310 nm is used. _Accompanying deviation from _SiO2-0 (hereinafter _referred to as deviation of TiO ₂ film thickness and deviation of SiO ₂ film thickness) (d _TiO2-x -d _TiO2-0 ) and (d _SiO2-x -d _SiO2-0 ) The change (calculated value) of the reflectivity of the HR film 13, that is, the rear end face side reflectivity R _r is shown.

As shown in FIG. 8, if the deviation of TiO ₂ film thickness (d _TiO2-x -d _TiO2-0 ) and the deviation of SiO ₂ film thickness (d _SiO2-x -d _SiO2-0 ) are in the range of ± 10 nm. The reflectance R _r is 0.60 or more and 0.61 or less, and in the HR film, the change in reflectance with respect to the deviation of the film thickness from the design value is small. However, the large film thickness tolerance makes it difficult to estimate the film thickness deviation from the measured reflectance.

Next, the AR film 12 on the front end face side will be described. In the AR film 12 on the front end face side, when the front end face side reflectance R _f is less than several percent in the measurement method using the equation (1), the increase in the front end face side light output P _f becomes small, and the front end face side is accurate. It becomes difficult to estimate the reflectance R _f . Therefore, when the front end face side reflectance R _f is several% or less, the ripple measurement of the wavelength spectrum before laser oscillation is performed. When the ratio between the maximum value and the minimum value of the ripple is m, the relationship between the gain G and the reflectances R _f and R _r is expressed by the following equation (2).

In the as-cleaved element, the front end face side reflectivity R _f and the rear end face side reflectivity R _r are each 0.3. Therefore, the ratio m between the maximum value and the minimum value of the ripple should be measured at a current below the threshold current. Thus, the gain G can be estimated. Next, after coating the AR film 12, using the ratio m of the maximum and minimum ripples measured at the same current and the gain G measured before coating the AR film 12, the front end face side reflectance R _f is calculated. Can be estimated.

In order to lower the reflectivity to about 0.1%, a two-layered AR film with no refractive index limitation is more suitable than a single-layered AR film. As an example, when a SiO ₂ film having a low refractive index (for example, refractive index 1.45) and a TiO ₂ film having a high refractive index (for example, refractive index 2.3) are used and light having a wavelength of 1310 nm is used, When the front end face side reflectance R _f is calculated, it becomes 0.0002 or less.

Figure 9, AR for shift in SiO ₂ film thickness (d _SiO2-x -d _SiO2-0) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0) in the case of using light having a wavelength 1310nm The change (calculated value) of the reflectance of the film, that is, the front end face side reflectance R _f is shown.

As shown in FIG. 9, when the respective film thicknesses deviate from the design value by about 10 nm, the front end face side reflectance R _f is different from the TiO ₂ film thickness deviation (d TiO _{2 -x-} d _{TiO 2-0} ) and the SiO ₂ film. The thickness deviation ( _{dSiO2 -x- dSiO2-0} ) is increased by 10 times or more as compared with the case where each thickness is zero. Further, the front end face side reflectance R _f changes substantially concentrically with respect to the SiO ₂ film thickness d _SiO2-x and the TiO ₂ film thickness d _TiO2-x . Accordingly, if the front end face side reflectance R _f = 0.0005 is estimated by the ripple measurement method, the SiO ₂ film thickness d _SiO2-x and the TiO ₂ film thickness d _{TiO2-x of} the prepared coating film are respectively It is impossible to determine how the values _{deviate from} the design values _dSiO2-0 and _dTiO2-0 .

In general, in the case of a Fabry-Perot type semiconductor laser, the front end face side reflectance R _f and the rear end face side reflectance R _r can be estimated by using the expressions (1) and (2). However, as described above, the DFB laser has a grating structure, and the front end face side reflectivity R _f is about 0.1%. Therefore, the front end face side reflectivity R is measured in the same manner as the Fabry-Perot semiconductor laser. It is difficult to estimate _f .

M. Ettenberg, "A new dielectronic facet reflector for semiconductor lasers", Appl. Phys. Lett., Vol. 32, 1978, p.724-726

Thus, for example, in a semiconductor laser as shown in FIG. 7, the film thickness control of the coating film is important for realizing antireflection / high reflection of the end face of the semiconductor laser. the measurement method using the ratio estimate accurately the front facet reflectivity R _f is not easy. Second, as shown in FIG. 8, in the HR film 13, the change in the rear end face side reflectance R _r due to the deviation in film thickness is small, so the thickness deviation from the measured value of the rear end face side reflectance R _r. It was difficult to estimate. Thirdly, in the AR film, since the grating characteristics of the DFB laser appear in particular, it is difficult to measure the reflectance of the semiconductor laser end face coated with the AR film 12. Finally, since the measurement using the ripple of the wavelength spectrum is near the design wavelength, as shown in FIGS. 8 and 9, the SiO ₂ film thickness deviation (d _SiO2-x -D _SiO2-0 ) and TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) appear as a solution to the concentric film thickness deviation, The difference could not be determined.

  In addition to the method for obtaining the reflectance from the characteristics of the semiconductor laser as described above, there is a method for producing a coating film on the monitor wafer, measuring the reflected power, and estimating the reflectance. However, since the equivalent refractive index of the light emitting area of the semiconductor laser end face and the monitor wafer are different, different reflectance characteristics are exhibited. Therefore, the region to be measured needs to be the light emitting end face of the semiconductor laser.

  The present invention solves such a problem, and accurately measures the reflectivity of the end face of the semiconductor element, and also makes the reflectivity of the end face of the semiconductor element closer to the design value based on the measured value. It is an object of the present invention to provide a method for evaluating the thickness of a dielectric thin film on an end face of a semiconductor element that makes it possible to obtain an active element such as an optical amplifier or a semiconductor optical switch.

  A dielectric thin film thickness evaluation method for a semiconductor element end face according to a first aspect of the present invention for solving the above-described problem includes a first thin film and a second thin film, and is provided on a light emitting end face of a semiconductor element. A method for evaluating a thickness of a dielectric thin film for determining a reflectance at a light emitting end face, wherein the dielectric thin film is irradiated with excitation light from outside a light emitting end face of the semiconductor element and transmitted through the emitting end face of the semiconductor element Measure the photoexcitation current generated by light, calculate the variation rate of the photoexcitation current with respect to the case where the dielectric thin film has a designed film thickness based on the measured value of the photoexcitation current, and calculate the variation rate of the photoexcitation current The results and the relationship between the fluctuation rate of the design thickness of the first thin film and the actual film thickness calculated as reference values in advance and the deviation of the design thickness of the second thin film from the actual film thickness. Compare the photoexcitation current And determining the actual film thickness and the actual thickness can take the region of the second thin film of said first thin film in accordance with the sign of the variation rate.

  According to a second aspect of the present invention, there is provided a method for evaluating a thickness of a dielectric thin film on an end face of a semiconductor element according to the first aspect, wherein the wavelength of the excitation light is an actual film thickness of the dielectric thin film and a design value of the dielectric thin film. The wavelength is in the range of a wavelength band in which the change in reflectance caused by the difference is large.

  According to a third aspect of the present invention, there is provided a method for evaluating a thickness of a dielectric thin film on an end face of a semiconductor element, in the second aspect, the reflectance of the dielectric thin film is 0.1% with respect to a change in wavelength. It is characterized by using a wavelength in a wavelength band that fluctuates at / nm or more or −0.1% / nm or less.

  According to a fourth aspect of the present invention, there is provided a method for evaluating a thickness of a dielectric thin film on an end face of a semiconductor element according to any one of the first to third aspects, wherein the calculation result of the fluctuation rate of the photoexcitation current and the reference value are calculated in advance. The relationship between the design thickness of the first thin film and the actual film thickness, and the relationship of the reflectance at the light emitting end surface with respect to the design film thickness of the second thin film and the actual film thickness, A region where the actual film thickness of one thin film and the actual film thickness of the second thin film can be determined is determined.

  A dielectric thin film thickness optimization method for a semiconductor element end face according to a fifth invention is the first method obtained by the dielectric thin film thickness evaluation method for a semiconductor element end face according to any one of the first to fourth inventions. Deriving the actual film thickness of the first thin film and the actual film thickness of the second thin film based on the actual film thickness of the thin film and the area that the actual film thickness of the second thin film can take, Based on the derived result, the thickness of the first thin film and the thickness of the second thin film are adjusted so as to be the designed film thickness.

  According to the above-described dielectric thin film thickness evaluation method for an end face of a semiconductor device according to the present invention, a highly reflective or antireflective coating film for a light emitting end face of a semiconductor laser, and further, light of an active element such as a semiconductor optical amplifier or a semiconductor optical switch. Regarding the optimization of the antireflection coating film on the emission end face, the photoexcitation current is measured using an excitation wavelength light source, and the actual film thicknesses of the first thin film and the second thin film are determined according to the positive / negative of the fluctuation rate of the photoexcitation current. By determining a region where there is a deviation from the design film thickness, the process of bringing the actual film thicknesses of the first thin film and the second thin film closer to the design film thickness can be shortened.

  Further, the calculation result of the fluctuation rate of the photoexcitation current, the deviation of the design thickness of the first thin film from the actual film thickness, and the fluctuation rate of the photoexcitation current with respect to the deviation of the design thickness of the second thin film from the actual film thickness. And the relationship between the design thickness of the first thin film and the actual film thickness, and the relationship of the reflectance at the light emitting end face with respect to the design film thickness of the second thin film and the actual film thickness. If the actual film thickness of the thin film and the area where the actual film thickness of the second thin film can be determined are determined, the solution of the film thickness that can be obtained can be narrowed down to two or one point. The approaching process can be further shortened.

  Embodiments of the present invention will be described with reference to the drawings. This embodiment is a method for evaluating the thickness of a dielectric thin film on an end face of a semiconductor element that enables optimization of the thickness of a coating film having a high reflectance / low reflectance on a light emitting end face of a semiconductor laser or the like. In addition to the optical reflection characteristic of the coating film, the photoexcitation current characteristic is newly taken into consideration, and the process for optimizing the film thickness of the coating film can be shortened. Hereinafter, an example in which the present invention is applied to a semiconductor laser will be described.

  FIG. 10A shows the wavelength dependence (calculated value) of the reflectance of the HR film as the dielectric thin film, and FIG. 10B shows the wavelength dependence of the reflectance of the AR film as the other dielectric thin film (calculated value). ) Respectively. As shown in both figures, there is a region where the reflectance of the HR film and the reflectance of the AR film change greatly as they move away from the design wavelength of 1310 nm.

  FIG. 11 shows the wavelength dependency (calculated value) of dR / dλ of the HR film and the AR film. As shown in FIG. 11, the region where the absolute value of dR / dλ of the coating film is 0.1% / nm or more includes the wavelength band of 820 to 1080 nm for the HR film, and the wavelength of 510 to 650 nm for the AR film. There are obi etc. Note that dR / dλ is a reflectance variation rate with respect to a change in wavelength.

  In these wavelength bands, as shown in FIGS. 10A and 10B, the change in the reflectance caused by the difference in film thickness, that is, the difference between the actual film thickness and the design value is also large. . Accordingly, when the light emission end face of the semiconductor laser is irradiated from the outside of the semiconductor laser as the excitation light source and the photoexcitation current generated by the light transmitted through the end face is measured, the difference in reflectance (transmittance) is found. Thus, the photoexcitation current largely fluctuates, and a minute reflectance variation can be estimated with high accuracy as a film thickness deviation.

  Therefore, in this embodiment, in order to measure the reflectance of the HR film, the absolute value of dR / dλ of the coating film described above is 0.1% / nm or more, and the change in reflectance caused by the film thickness deviation occurs. As an example, light with a wavelength of 940 nm is used as an excitation light source to irradiate the light emitting end face of the semiconductor laser from a large wavelength range of 820 to 1080 nm, and the resulting photoexcitation current is measured to determine the film thickness deviation.

  In addition, for measuring the reflectance of the AR film, the absolute value of dR / dλ of the coating film described above is 0.1% / nm or more, and the change in reflectance caused by the difference in film thickness is large at a wavelength of 510 to 650 nm. From the wavelength range, for example, light with a wavelength of 600 nm is used as an excitation light source to irradiate the light emitting end face of the semiconductor laser, and the resulting photoexcitation current is measured to determine the film thickness deviation.

  Further, by comparing the reflectance solution estimated from the photoexcitation current measurement of this embodiment with the reflectance solution estimated from the conventional ripple measurement, the applicable solutions are narrowed down to two or one point.

  According to the method for evaluating a thickness of a dielectric thin film on an end face of a semiconductor element according to the present embodiment, the reflectance of the end face of the semiconductor element can be accurately measured, and the reflectivity of the end face of the semiconductor element can be further increased based on the measured value. The design value can be approached.

Hereinafter, the first embodiment of the present invention will be described in detail with reference to FIGS. In this embodiment, a TiO ₂ film having a high refractive index (for example, a refractive index of 2.3) as a pair of first thin films and a low refractive index (for example, having a refractive index of 1. use HR film made of SiO ₂ film 45), so that the maximum reflectance at the design wavelength of 1310 nm, the design thickness d _TiO2-0 and d _SiO2-0 of the TiO ₂ film and the SiO ₂ film, respectively 142. 39 and 225.86 nm.

FIG. 1 shows the difference in TiO ₂ film thickness (d _TiO2−x− d _TiO2−0 ) and SiO ₂ film thickness when 940 nm is used as the wavelength at which the reflectance R varies greatly due to the film thickness difference of the HR film. The change (calculated value) of the reflectivity R of the HR film due to the deviation ( _{dSiO2 -x- dSiO2-0} ) is shown. As shown in FIG. 1, in this example, the TiO ₂ film thickness deviation (d TiO _{2 -x-} d _{TiO 2-0} ) and the SiO ₂ film thickness deviation (d _{SiO 2 -x} -d _{SiO 2-0} ). In the range of ± 10 nm, the reflectivity of the HR film changes in the range of 0.15 to 0.39.

TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) and SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ), which may give a certain value as the reflectivity of the HR film ) Is a linear locus, and as the TiO ₂ film thickness d _TiO2-x and the SiO ₂ film thickness d _SiO2-x increase / decrease, the reflectivity of the HR film decreases / increases, respectively. Further, TiO ₂ thickness deviation and (d _TiO2-x -d _TiO2-0) of SiO ₂ film thickness deviation in the case of using an excitation light source of wavelength 940nm shown in FIG. 1 (d _SiO2-x -d _SiO2-0 ) Is within a range of ± 10 nm, the reflectance variation is larger by one digit or more than when the laser beam having a wavelength of 1310 nm shown in FIG. 8 is used.

In FIG. 2, when the thickness of the HR film is a design value, that is, the deviation of the TiO ₂ film thickness (d TiO _{2 -x} -d _{TiO 2-0} ) and the deviation of the SiO ₂ film thickness (d _{SiO 2 -x} -d _{SiO 2- 0)} photoexcitation current (less if each 0, based on the) that photoexcitation current design values, the deviation of the TiO ₂ film thickness (d _TiO2-x -d _TiO2-0) of SiO ₂ film thickness deviation The result of having calculated the fluctuation rate of the photoexcitation current with respect to ( _{dSiO2 -x- dSiO2-0} ) is shown. When the transmitted light at the end face of the semiconductor laser changes, reflecting the change in reflectance, the photoexcitation current absorbed by the semiconductor changes. The designed photoexcitation current _Iph is expressed by the following equation (3).

Here, R is reflectance, η is quantum efficiency, q is electronic charge, h is Planck's constant, ν is frequency, Γ is optical confinement factor, α ₀ is internal loss, and P _i (x) is power. The power P _i (x) is expressed by the following equation (4).

P _i0 is the incident power.

As shown in FIG. 2, in this embodiment, the TiO ₂ film thickness deviation (d TiO _{2 -x-} d _{TiO 2-0} ) and the SiO ₂ film thickness deviation (d _{SiO 2 -x} -d _{SiO 2-0} ). In the range of ± 10 nm, the fluctuation rate of the photoexcitation current changes in the range of −16% to 16%, and as the TiO ₂ film thickness d _TiO2-x and the SiO ₂ film thickness d _SiO2-x increase / decrease, The photoexcitation current increases and decreases, respectively.

For example, at the point (a) in FIG. 2, the thickness of the HR film is designed by decreasing the SiO ₂ film thickness d _SiO2-x and increasing the TiO ₂ film thickness d _TiO2-x. The film thickness can be approached. Further, it is possible to approach the direction of the arrow in (b) point, i.e. by reducing the SiO ₂ film thickness d _SiO2-x and TiO ₂ film thickness d _TiO2-x, the design thickness of the film thickness of the HR film . Further, at the point (c), by increasing the direction of the arrow, that is, the SiO ₂ film thickness d _SiO2-x and decreasing the TiO ₂ film thickness d _TiO2-x , the film thickness of the HR film is brought close to the design film thickness. be able to.

As shown in FIG. 2, divided into four regions a line d _TiO2-x -d _TiO2-0 = 0 line and d _SiO2-x -d _SiO2-0 = 0 as the boundary (I) ~ (IV). That is, regions (I), (II), (III) and (IV) are respectively (i) _{dTiO2 -x- dTiO2-0} <0 and _{dSiO2 -x- dSiO2-0} > 0, (ii) _{dTiO2 -x- dTiO2-0} > 0 and _{dSiO2 -x- dSiO2-0} > 0, (iii) _{dTiO2 -x- dTiO2-0} <0 and _{dSiO2 -x- dSiO2-0} <0 (Iv) d _TiO2-x- d _TiO2-0 > 0 and _{dSiO2 -x- dSiO2-0} <0.

Here, paying attention to the cases where the fluctuation rate of the photoexcitation current is positive and negative, when the fluctuation rate of the photoexcitation current is positive, the deviation of the TiO ₂ film thickness (d _TiO2-x -d _TiO2-0 ) and SiO ₂ The film thickness deviation ( _{dSiO2 -x- dSiO2-0} ) may be distributed in the regions (I), (II), and (IV), but not in the region (III). On the other hand, when the fluctuation rate of the photoexcitation current is negative, the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) and the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) are in the region ( It may be distributed in I), (III) and (IV), but not in region (II).

That is, in order to make the SiO ₂ film thickness d _SiO2-x and the TiO ₂ film thickness d _TiO2-x close to the designed film thickness, respectively, in the case where the variation rate of the photoexcitation current is positive and negative, the SiO ₂ film the thickness d _SiO2-x / TiO ₂ film thickness d _TiO2-x, indicating that there is no solution to reduce / increase and decrease / increase.

  FIG. 3 shows a schematic configuration diagram of the photoexcitation current measuring apparatus in the present embodiment. Reference numeral 14 denotes an excitation light source, 15 denotes a beam splitter, 16 denotes a power meter, 17 denotes a galvanometer, 18 denotes a relay lens, 19 denotes a mirror, 20 denotes a measurement sample, 21 denotes a photoexcitation current measuring instrument, and 22 denotes a computer. In this example, a laser diode (LD) was used as the measurement sample 20. A coating film is formed on the end face of the LD, and the surface is irradiated with light from the excitation light source 14. The LD has electrodes on the front surface and the back surface, and measures the photoexcitation current by connecting the wiring from each to the photoexcitation current measuring device 21.

In this example, when the excitation wavelength light source of 940 nm was used and the photoexcitation current at the LD light exit end face was measured, it increased by 4% compared to the photoexcitation current of the standard sample. That is, the area shown in FIG. 2 (I), (II) and the actual TiO ₂ film thickness deviation of 4% of the linear trajectory in the _{(IV) (d TiO2-x} -d TiO2-0) SiO 2 film There is a solution that may satisfy the relationship of thickness deviation ( _{dSiO2 -x- dSiO2-0} ), and there is a deviation from the actual design film thickness somewhere in this linear locus. This is to change the SiO ₂ film thickness d _SiO2-x / TiO ₂ film thickness d _TiO2-x to decrease / increase as shown in FIG. 2 (a), and to decrease / decrease as shown in (b). It is shown that the film thickness of the HR film can be brought close to the design film thickness by changing so as to increase or decrease as shown in (c).

As a result of eliminating the region (III) in this way, the film thickness was optimized based on the remaining regions (I), (II), and (IV). First, as a result of decreasing the SiO ₂ film thickness d _SiO2-x and increasing the TiO ₂ film thickness d _TiO2-x so as to approach the design film thickness based on the region (I), a result corresponding to the design film thickness is obtained. I couldn't. Next, as a result of decreasing the SiO ₂ film thickness d _SiO2-x and reducing the TiO ₂ film thickness d _TiO2-x so as to approach the design film thickness based on the region (II), the result corresponding to the design film thickness is It was not obtained. Finally, as a result of increasing the SiO ₂ film thickness d _SiO2-x and decreasing the TiO ₂ film thickness d _TiO2-x so as to approach the design film thickness based on the region (IV), a result corresponding to the design film thickness is obtained. Obtained. As a result of forming the HR film on the end face of the LD under the condition that the designed film thickness was obtained, good LD characteristics with a reflectance of 0.61 with respect to light having a wavelength of 1310 nm were obtained.

As described above, the photoexcitation current measurement using the excitation wavelength light source according to the present embodiment shows that the deviation of the TiO ₂ film thickness (d _TiO2−x −d _{TiO2−) in} the case where the variation rate of the photoexcitation current is positive and negative. ₀ ) and SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) can be eliminated in the regions (III) and (II), respectively, and the coating film thickness can be reduced to the designed film thickness. The process of approaching can be shortened. In this embodiment, 1310 nm is used as the design wavelength. However, it is obvious that the same effect can be obtained even if other wavelengths are used.

Hereinafter, a second embodiment of the present invention will be described in detail with reference to FIGS. In this embodiment, an AR film composed of a pair of low refractive index (for example, refractive index 1.45) SiO ₂ film and high refractive index (for example, refractive index 2.3) TiO ₂ film as a coating film. The designed film thicknesses d _SiO2-0 and d _TiO2-0 of the SiO ₂ film and the TiO ₂ film were set to 168.86 and 101.67 nm, respectively, so that the minimum reflectance was obtained at a wavelength of 1310 nm.

FIG. 4 shows the SiO ₂ film thickness shift (d _{SiO 2−x} −d _{SiO 2-0} ) and TiO ₂ when light having a wavelength of 600 nm is used as an example of the wavelength at which the reflectance varies greatly due to the film thickness shift of the AR film. The change (calculated value) in the reflectivity of the AR film due to the film thickness deviation (d _TiO2-x -d _TiO2-0 ) is shown. As shown in FIG. 4, in this embodiment, the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ). In the range of ± 10 nm, the reflectance of the AR film changes in the range of 0.06 to 0.18.

The SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ), which may give a certain value as the reflectance of the AR film relationship) becomes linear trajectory, in accordance with the film thickness of SiO ₂ and TiO ₂ increases or decreases, the reflectivity of the AR film, respectively, it is increasing or decreasing. Further, SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0) and the TiO ₂ film thickness deviation in the case of using an excitation light source of wavelength 600nm shown in FIG. 4 (d _TiO2-x -d _TiO2-0 ) In the range of ± 10 nm, the change in reflectance is larger by one digit or more than in the case of the wavelength of 1310 nm shown in FIG.

5, photoexcitation of the design value when the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0) and TiO ₂ thickness deviation (d _TiO2-x -d _TiO2-0) is 0, respectively Based on the current, the rate of variation of the photoexcitation current with respect to the SiO ₂ film thickness deviation (d _SiO2-x −d _SiO2-0 ) and the TiO ₂ film thickness deviation (d _TiO2-x −d _TiO2-0 ) The calculated result is shown. Reflecting the reflectance variation, the light transmitted through the LD end face varies, and as a result, the photoexcitation current absorbed by the semiconductor changes.

As shown in FIG. 5, in the present embodiment, SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0) and TiO ₂ thickness deviation (d _TiO2-x -d _TiO2-0) In the range of ± 10 nm, the fluctuation rate of the photoexcitation current changes in the range of 6% to -8%, and the TiO ₂ film thickness d _TiO2-x and the SiO ₂ film thickness d _SiO2-x increase / decrease. As a result, the photoexcitation current decreases and increases, respectively.

5, divided into four regions a line d _SiO2-X -d _SiO2-0 = 0 line and d _TiO2-X -d _TiO2-0 = 0 as the boundary (I) ~ (IV). That is, regions (I), (II), (III) and (IV) are respectively (i) _{dSiO2 -X- dSiO2-0} <0 and _{dTiO2 -X- dTiO2-0} > 0, (ii) _{dSiO2 -X- dSiO2-0} > 0 and _{dTiO2 -X- dTiO2-0} > 0, (iii) _{dSiO2 -X- dSiO2-0} <0 and _{dTiO2 -X- dTiO2-0} <0 a and _{(iv) d SiO2-X -d} SiO2-0> 0 and d _TiO2-X -d _TiO2-0 <0 region.

Here, paying attention to the cases where the fluctuation rate of the photoexcitation current is positive and negative, when the fluctuation rate of the photoexcitation current is positive, the SiO ₂ film thickness deviation (d _SiO2−x −d _SiO2-0 ) and TiO ₂ The value of the film thickness deviation ( _{dTiO2 -x- dTiO2-0} ) may be distributed in the regions (I), (III) and (IV), but not in the region (II). On the other hand, when the fluctuation rate of the photoexcitation current is negative, the values of the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) are It may be distributed in regions (I), (II) and (IV), but not in region (III).

That is, in the case variation rate of the excitation current for positive and negative, in order to approximate the TiO ₂ film thickness d _TiO2-x and SiO ₂ film thickness d _SiO2-x on the design thickness, respectively, TiO ₂ thickness This shows that there is no solution to decrease / decrease and increase / increase the d _TiO2-X / SiO ₂ film thickness _dSiO2-X .

In this example, when the photoexcitation current measurement system shown in FIG. 3 and described above was used to measure the photoexcitation current of the LD light emitting end face with an excitation wavelength light source of 600 nm, it increased by 2% compared to the photoexcitation current of the standard sample. That is, the actual SiO ₂ film thickness deviation (d _SiO2−x −d _SiO2-0 ) and the TiO ₂ film in the 2% line-like locus in the regions (I), (III) and (IV) shown in FIG. There is a solution that may satisfy the relationship of thickness deviation (d _TiO2-x -d _TiO2-0 ), and there is a deviation from the actual design film thickness somewhere in this linear locus. This is to change the TiO ₂ film thickness d _TiO2-x / SiO ₂ film thickness d _SiO2-x to decrease / increase as shown in FIG. 5 (d), and to increase / increase as shown in (e). It is shown that the film thickness of the AR film can be brought close to the design film thickness by changing so as to increase or decrease as shown in (f).

In the case where the reference light excitation current design values according to the present invention in FIG. 6, SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0) and TiO ₂ thickness deviation (d _TiO2-x -d volatility characteristics of the excitation current by _TiO2-0) (calculated) (shown in FIG. 6 solid line), and, of SiO ₂ film thickness deviation and (d _SiO2-x -d _SiO2-0) of TiO ₂ thickness deviation The change (calculated value) (indicated by a broken line in FIG. 6) of the reflectance (wavelength 1310 nm) of the AR film due to ( _{dTiO2 -x- dTiO2-0} ) is shown.

From the measurement of ripple, the reflectance when the set wavelength was 1310 nm was 0.0005. As a result, the solution of the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) has a reflectance of 0.0005 as shown in FIG. It can be seen that it is on a concentric locus. Further, the optical excitation current measurement wavelength 600 nm, the solution of the SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0) and the TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0) 2% It is on the linear trajectory. Therefore, the point (A) and the point (B) where the concentric circle with a reflectance of 0.0005 and the line with the fluctuation rate of 2% of the photoexcitation current intersect are both points, and either point is the actual SiO ₂ film thickness. it is a deviation (d _SiO2-x -d _SiO2-0) and TiO ₂ thickness deviation _{(d TiO2-x -d TiO2-0)} . This means that the TiO ₂ film thickness d _TiO2-x / SiO ₂ film thickness d _SiO2-x can be brought close to the design film thickness by the method of (A) decrease / increase or (B) increase / decrease. Yes.

Thus, the regions (II) and (III) were excluded, and the film thickness was optimized based on the point (A) in the region (I) and the point (B) in the region (IV). First, as a result of decreasing the SiO ₂ film thickness d _SiO2-x and increasing the TiO ₂ film thickness d _TiO2-x so as to approach the design film thickness based on the point (A), a result corresponding to the design film thickness is obtained. I couldn't. On the other hand, as a result of increasing the SiO ₂ film thickness d _SiO2-x and decreasing the TiO ₂ film thickness d _TiO2-x so as to approach the design film thickness based on the point (B), a result corresponding to the design film thickness is obtained. It was. As a result of forming an AR film on the LD end face under the condition where the designed film thickness was obtained, a favorable characteristic of a reflectance of 0.0002 at a set wavelength of 1310 nm was obtained.

  As described above, the photoexcitation current measurement using the excitation wavelength light source according to the present invention can eliminate the region (II) and the region (III), respectively, in the case where the fluctuation rate of the photoexcitation current is positive and negative, and the coating The process of bringing the film thickness closer to the designed film thickness can be shortened. Further, by incorporating the reflectance solution estimated from the photoexcitation current measurement of the present invention into the reflectance solution estimated from the conventional ripple measurement, the possible solutions are narrowed down to 2 or 1 point (in this embodiment, 2 points). In addition, the process of bringing the coating film thickness closer to the design film thickness can be further shortened.

  In this embodiment, 1310 nm is used as the design wavelength, but it is obvious that the same effect can be obtained even when other wavelengths are used. In this embodiment, the semiconductor laser is used as the semiconductor element to be coated with the AR film. However, it is obvious that the same effect can be obtained even if an active element such as a semiconductor optical amplifier or a semiconductor optical switch is used.

  The present invention relates to a semiconductor element end face that enables optimization of the reflectance of a coating film used in an active element such as a semiconductor laser, a semiconductor optical amplifier, or a semiconductor optical switch used in optical fiber communication, optical information processing, an optical disk, etc. It is suitable for application to a dielectric thin film thickness evaluation method.

TiO ₂ thickness deviation and (d _TiO2-x -d _TiO2-0) of SiO ₂ film thickness deviation in the case of the 940nm wavelength of the excitation light in the first embodiment of the present invention (d _SiO2-x -d _{SiO2- 0} ) is a graph showing the HR reflectivity (calculated value). TiO ₂ thickness deviation and (d _TiO2-x -d _TiO2-0) of SiO ₂ film thickness deviation in the case of the 940nm wavelength of the excitation light in the first embodiment of the present invention (d _SiO2-x -d _{SiO2- 0} ) is a graph showing the fluctuation rate (calculated value) of the photoexcitation current. It is a schematic block diagram of the photoexcitation current measuring apparatus which concerns on Example 1 of this invention. In Example 2 of the present invention, when the wavelength of the excitation light is 600 nm, the SiO ₂ film thickness deviation (d SiO _{2 −x} −d _{SiO 2-0} ) and the TiO ₂ film thickness deviation (d TiO _{2 −x} −d _{TiO 2− 0} ) is a graph showing the AR reflectance (calculated value). SiO ₂ film thickness of the displacement (d _SiO2-x -d _SiO2-0) and the TiO ₂ film thickness deviation in the case of the 600nm wavelength of the excitation light in the second embodiment of the present invention (d _TiO2-x -d _{TiO2- 0} ) is a graph showing the fluctuation rate (calculated value) of the photoexcitation current. In Example 2 of the present invention, when the wavelength of the excitation light is 600 nm, the SiO ₂ film thickness deviation (d SiO _{2 −x} −d _{SiO 2-0} ) and the TiO ₂ film thickness deviation (d TiO _{2 −x} −d _{TiO 2− 0} ) is a graph showing the fluctuation rate (calculated value) and reflectance of the photoexcitation current. 1 is a perspective view showing a distributed feedback semiconductor laser (DFB laser) partially cut away. FIG. TiO ₂ film thickness deviation in the case of a set wavelength _{1310nm (d TiO2-x -d TiO2-0} ) and HR reflectivity for SiO ₂ thickness of the displacement _{(d SiO2-x -d SiO2-0)} ( calculated) It is a graph which shows. AR reflectivity (calculated value) for SiO ₂ film thickness deviation (d _SiO2-x -d _SiO2-0 ) and TiO ₂ film thickness deviation (d _TiO2-x -d _TiO2-0 ) when setting wavelength is 1310 nm It is a graph which shows. FIG. 10A is a graph showing the wavelength dependence of the reflectance of the HR film, and FIG. 10B is a graph showing the wavelength dependence of the reflectance of the AR film. It is a graph which shows the wavelength dependence of the fluctuation rate of the reflectance with respect to the change of the wavelength of HR film | membrane and AR film | membrane.

Explanation of symbols

1 1.3 μm composition InGaAsP active layer 2 1.1 μm composition lower InGaAsP guide layer 3 1.1 μm composition upper InGa AsP guide layer 4 p-InP buffer layer 5 n-InP cladding layer 6 n ⁺ -InGaAsP cap layer 7 p-InP Substrate 8 Ru-added SI-InP current blocking layer 9 p electrode 10 n electrode 11 grating 12 antireflection (AR) film 13 high reflection (HR) film 14 excitation light source 15 beam splitter 16 power meter 17 galvanometer 18 relay lens 19 mirror 20 measurement Sample 21 Photoexcitation current measuring instrument 22 Computer

Claims (5)

A dielectric thin film thickness evaluation method comprising a first thin film and a second thin film, provided on a light emitting end face of a semiconductor element, and determining a reflectance at the light emitting end face,
Irradiating the dielectric thin film with excitation light from the outside of the light emitting end face of the semiconductor element,
Measure the photoexcitation current generated by the light transmitted through the emission end face of the semiconductor element,
Based on the measured value of the photoexcitation current, the rate of change of the photoexcitation current with respect to the case where the dielectric thin film has a design thickness is calculated,
The calculation result of the fluctuation rate of the photoexcitation current, the deviation between the design thickness and the actual film thickness of the first thin film calculated as a reference value in advance, and the deviation between the design thickness and the actual film thickness of the second thin film And the relationship of the variation rate of the photoexcitation current with respect to
An area where the actual film thickness of the first thin film and the actual film thickness of the second thin film can be taken is determined according to the sign of the rate of change of the photoexcitation current. Thin film thickness evaluation method. The wavelength of the excitation light is a wavelength in a range of a wavelength band in which a change in reflectance caused by a difference between an actual thickness of the dielectric thin film and a design thickness of the dielectric thin film is large. The dielectric thin film film thickness evaluation method of the semiconductor element end surface according to claim 1. As the wavelength of the excitation light, a wavelength in a wavelength band in which the reflectance of the dielectric thin film varies by 0.1% / nm or more or −0.1% / nm or less with respect to a change in wavelength is used. The dielectric thin film film thickness evaluation method of the semiconductor element end surface according to claim 2. The calculation result of the fluctuation rate of the photoexcitation current, the reference value, the deviation between the design thickness and the actual film thickness of the first thin film calculated in advance, and the design film thickness and the actual film thickness of the second thin film. And the relationship of the reflectance at the light exit end face with respect to the deviation of
4. The semiconductor element according to claim 1, wherein an area where the actual film thickness of the first thin film and the actual film thickness of the second thin film can be determined is determined. 5. Method for evaluating the thickness of the dielectric thin film on the end face.   The actual film thickness of said 1st thin film and the actual film thickness of said 2nd thin film obtained by the dielectric thin film film thickness evaluation method of the semiconductor element end surface concerning any one of Claims 1 thru | or 4 The actual film thickness of the first thin film and the actual film thickness of the second thin film are derived based on the area that can be taken, and the film thickness of the first thin film and the second film thickness are derived based on the derived result. A method for optimizing the thickness of a dielectric thin film on an end face of a semiconductor device, wherein the thickness of the thin film is adjusted to be a designed film thickness.

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