JP4530962B2 - Manufacturing method and manufacturing apparatus for semiconductor laser device - Google Patents

Description

  The present invention relates to a manufacturing method and a manufacturing apparatus for a semiconductor laser device.

In a semiconductor laser device, a semiconductor laser element mounted on a package is hermetically sealed by a cap because a resonator end face deteriorates when exposed to outside air. For example, Patent Document 1 proposes a hermetic sealing method of a semiconductor laser element in which a package in which a semiconductor laser element is mounted in a chamber into which oxygen is introduced is hermetically sealed with an inert gas after being irradiated with ultraviolet rays. As a result, a very small amount of organic matter remaining inside the package that causes deterioration of the resonator end face of a semiconductor laser device having a high output, for example, an optical output of 100 mW or more and an oscillation wavelength of the laser light of 980 nm band or 1020 nm band is decomposed. Can be removed.
Japanese Patent Laid-Open No. 2000-133736 (pages 5-7, FIG. 1)

  However, as a result of investigations by the inventors, in a semiconductor laser element having an oscillation wavelength of laser light of 390 nm to 500 nm (near ultraviolet to green), such as a GaN semiconductor laser element or a ZnO semiconductor laser element, a 980 nm band or a 1020 nm band Since the oscillation wavelength is as short as half or less and the energy hν of the laser light is as large as twice or more, the method proposed in Patent Document 1 deteriorates the semiconductor laser element as described later. Can not be suppressed.

  Accordingly, an object of the present invention is to provide a semiconductor laser device manufacturing method and a manufacturing apparatus capable of improving the durability of a semiconductor laser device using a semiconductor laser element having an oscillation wavelength of 390 nm to 500 nm. .

  In order to achieve the above object, a method of manufacturing a semiconductor laser device according to the present invention includes a dry air introducing step of introducing dry air into a sealed tank, and a semiconductor laser element provided in the sealed tank into which the dry air is introduced. In the sealed tank into which the dry air is introduced, the semiconductor laser element irradiated with the ultraviolet light by the cap irradiated with the ultraviolet light on the inner surface together with the dry air. And a sealing step for sealing.

  According to the present invention, in the method of manufacturing a semiconductor laser device having the above-described configuration, the stem and the cap are placed at a temperature higher than the boiling point of water in the sealed tank between the ultraviolet irradiation step and the sealing step. It has the heating process which heats, It is characterized by the above-mentioned.

  According to the present invention, in the method of manufacturing the semiconductor laser device having the above-described structure, the method further includes a cooling step of cooling the stem and the cap heated in the heating step to room temperature before the sealing step.

  According to the present invention, in the method of manufacturing a semiconductor laser device having the above-described configuration, the dew point of the dry air is -30 ° C or lower.

  According to the present invention, in the method of manufacturing a semiconductor laser device having the above-described configuration, the heating temperature in the heating step is 300 ° C. or higher.

  The semiconductor laser device manufacturing apparatus of the present invention includes a sealed tank whose interior is maintained in a dry air atmosphere, an ultraviolet ray generator that irradiates ultraviolet rays onto the inner surface of the cap and a stem provided with the semiconductor laser element, the stem, A heating device that heats the cap at a temperature higher than the boiling point of water; and a sealing device that attaches the cap to the stem and seals the semiconductor laser element, the ultraviolet ray generator, a war record heating device, The sealing device is disposed inside the sealed tank. As this sealing device, for example, a welding machine can be used.

  Further, the present invention provides a semiconductor laser device manufacturing apparatus having the above-described configuration, comprising a plurality of the sealed tanks maintained in a dry air atmosphere having the same dew point, the ultraviolet ray generating means, the heating means, and the sealing device. At least one of the plurality of sealed tanks is arranged in another sealed tank, and the stem and the cap move between the sealed tanks in an atmosphere maintained at the same dew point as the dry air. It is characterized by that. For example, a desiccator can be used to move the package and the cap.

  According to the present invention, a dry air atmosphere is sealed inside a package part constituted by a cap and a stem, and the semiconductor laser device sealed in this package part keeps the driving voltage constant for a long period of time. The durability of the semiconductor laser device can be increased. Further, since the organic impurities attached to the inner portions of the cap and stem package portions are decomposed by the ultraviolet rays, it is possible to suppress the deterioration of the semiconductor laser element due to the impurities.

  In addition, according to the present invention, the deterioration of the semiconductor laser device can be further suppressed by removing moisture by heating the stem and the cap. Furthermore, after heating, the inside of the package part can be made more difficult to condense by cooling before attaching the cap to the stem.

  Further, according to the present invention, the semiconductor laser device manufacturing apparatus is provided in the sealed tank in which the ultraviolet ray generator, the heating device, and the sealing device are maintained in a dry air atmosphere. A semiconductor laser device in which the semiconductor laser element is sealed with dry air can be obtained.

  In addition, according to the present invention, since there may be a plurality of sealed tanks, a semiconductor laser device manufacturing apparatus can be freely arranged.

  Embodiments of the present invention will be described with reference to the drawings. 1 is a schematic configuration diagram of a semiconductor laser device according to the present invention, FIG. 2 is a schematic configuration diagram of a semiconductor wafer, FIG. 3 is a schematic configuration diagram of a semiconductor wafer in which a ridge stripe is formed, and FIG. 4 is a perspective view of a semiconductor laser device. FIG. 5 is a schematic plan view of a hermetic sealing device which is a semiconductor laser device manufacturing apparatus according to the present invention. In the drawings of the present application, dimensional relationships such as length, width, thickness, and depth are appropriately changed for the sake of clarity and simplification of the drawings, and do not represent actual dimensional relationships.

  As shown in FIG. 1, the semiconductor laser device 20 is provided with a heat sink 22 and electrode pins 23 on the upper surface of a stem 21, and two electrode lead wires 25 are provided on the lower surface of the stem 21. A semiconductor laser element 29 is provided in the heat sink 22, and the semiconductor laser element 29 and the electrode pin 23 are electrically connected by a wiring 24.

  A cap 26 is provided on the upper surface of the stem 21, and the stem 21 and the cap 26 constitute a package portion 28. A window 27 made of glass is attached to the cap 26 so that the laser light emitted from the semiconductor laser element 29 can be extracted to the outside. The package portion 28 surrounds the heat sink 22, the semiconductor laser element 29, the electrode pin 23, and the wiring 24, and the sealed atmosphere 28a is sealed.

  The semiconductor laser element 29 is a chip-shaped element that is formed by bonding a p-type semiconductor to which an acceptor impurity is added and an n-type semiconductor to which a donor impurity is added. When a voltage is applied between the p-type semiconductor and the n-type semiconductor and a drive current is passed, the pn junction layer, which is the junction, emits light, and two parallel electrodes with different reflectivities formed on the cleavage plane are produced. The light resonates and is amplified between the resonance surfaces that are mirror surfaces, and laser light is emitted from the resonance surface with the lower reflectance. Laser light emitted from the semiconductor laser element 13 is emitted outside the package section 28 through the window 18. In the present embodiment, the semiconductor laser element 29 is made of a GaN-based semiconductor, and the oscillation wavelength of the laser light is 390 to 500 nm.

Next, the semiconductor laser element 29 will be described. FIG. 2 is a schematic configuration diagram showing the structure of the semiconductor wafer 30 before forming the semiconductor laser device according to the embodiment of the present invention. The semiconductor wafer 30 has a semiconductor growth layer 32 formed on an n-type GaN substrate 31. The semiconductor growth layer 32 includes an n-type GaN layer 32a having a thickness of 0.2 μm, an n-type Al _0.05 Ga _0.95 N clad layer 32b having a thickness of 2.2 μm, and an n-type having a thickness of 20 nm in this order from the n-type GaN substrate 31 side. A GaN guide layer 32c, an InGaN / GaN-3 MQW (Multi Quantum Well) active layer 32d in which three layers of InGaN layers having a thickness of 4 nm and GaN layers having a thickness of 8 nm are alternately stacked, and a p-type having a thickness of 20 nm. An Al _0.3 Ga _0.7 N evaporation prevention layer 32e, a 0.3 μm thick p-type Al _0.05 Ga _0.95 N cladding layer 32f, and a 0.1 μm thick p-type GaN contact layer 32g are laminated.

Note that the composition, configuration, and thickness of each layer are examples, and may be changed as appropriate. In the present embodiment, the n-type GaN substrate 31 is used as a semiconductor substrate. However, the semiconductor substrate is not limited to this material. A p-type GaN substrate, a semi-insulating GaN substrate, Al _{1− An x} Ga _x N (0 ≦ x ≦ 1) substrate or the like may be used.

  Next, a method for manufacturing the semiconductor growth layer 32 will be described. In the following description, a case where the MOCVD (Metal Organic Chemical Vapor Deposition) method is used is shown, but the growth method is not limited to the MOCVD method as long as it is an epitaxial growth method, and MBE ( Other vapor phase growth methods such as a molecular beam epitaxy (HV) method and a HVPE (hydride vapor phase epitaxy) method may be used.

The n-type GaN substrate 31 is placed on a predetermined susceptor in the growth furnace of the MOCVD apparatus, and the susceptor temperature is raised to 1050 ° C. while flowing H ₂ and NH ₃ as carrier gases at a rate of 5 l / min. When the temperature rise is completed, trimethylgallium ((CH ₃ ) ₃ Ga; TMG) is supplied into the growth furnace as a Ga raw material at a flow rate of 130 μmol / min and SiH ₄ at a flow rate of 70 nmol / min, and the n-type GaN layer 32a is set to 0. Grow 2 μm. Thereafter, TMG is set to 100 μmol / min, trimethylaluminum ((CH ₃ ) ₃ Al; TMA) is supplied to the growth reactor at 5 μmol / min as an Al raw material, and n-type Al _0.05 Ga _0.95 N clad layer 32b is added to 2 Grow 2 μm. When the growth of the n-type Al _0.05 Ga _0.95 N cladding layer 32b is completed, the supply of TMA is stopped, TMG is increased to 130 μmol / min, and the n-type GaN guide layer 32c is grown to 20 nm.

Thereafter, the supply of TMG and SiH ₄ is stopped, the carrier gas is changed from H ₂ to N ₂ , and the susceptor temperature is lowered to 800 ° C. After the temperature is lowered, first, 10 μmol / min of TMG is supplied into the growth furnace to grow a GaN layer having a thickness of 20 nm. Next, trimethylindium ((CH ₃ ) ₃ In; TMI) as an indium raw material is supplied into the growth reactor while maintaining 5 μmol / min and TMG at 10 μmol / min to grow an InGaN layer having a thickness of 4 nm. . Next, the supply of TMI is stopped, and an 8 nm GaN layer is grown. Thereafter, TMI is again supplied to the growth furnace at 5 μmol / min, an InGaN layer having a thickness of 4 nm is grown, and the same operation is repeated, so that the InGaN / GaN-3MQW active layer 32d is GaN / InGaN / GaN / InGaN / GaN. / InGaN / GaN in this order.

When the InGaN / GaN-3MQW active layer 32d is formed, the supply of TMI and TMG is stopped, the susceptor temperature is raised to 1050 ° C., the carrier gas is changed from N ₂ to H ₂ , and TMG is 30 μmol / Min, TMA is 10 μmol / min, and bisethylcyclopentadienylmagnesium ((C ₂ H ₅ C ₅ H ₄ ) ₂ Mg; EtCp ₂ Mg) is grown at 0.2 μmol / min as a raw material for Mg which is a p-type dopant. The p-type Al _0.3 Ga _0.7 N evaporation preventing layer 32e is grown to 20 nm by supplying into the furnace. When the growth of the p-type Al _0.3 Ga _0.7 N evaporation prevention layer 32e is completed, TMG is supplied into the growth furnace at 100 μmol / min and TMA at 4 μmol / min, and the p-type Al _0.05 Ga _0.95 N clad layer 32f is 0.35 μm. Grow.

After growing the p-type Al _0.05 Ga _0.95 N clad layer 32f, the supply of TMA is stopped, EtCp ₂ Mg is increased to 1 μmol / min, and TMG is maintained at 100 μmol / min. The semiconductor growth layer 32 is completed by growing 32 g of 0.1 μm, and the growth of the semiconductor multilayer film of the semiconductor laser element is completed. Thereafter, the supply of TMG and EtCp ₂ Mg is stopped and the temperature is lowered. In this way, the semiconductor wafer 30 having the semiconductor growth layer 32 formed on the surface is obtained. In addition, the thickness of each of these layers is a target value and may differ from an actual value.

After the semiconductor growth layer 32 is completed, a p-type GaN contact layer 32g and a p-type Al _0.05 Ga _0.95 N clad are formed so as to form a ridge stripe 39 having a width of about 2 μm by using a normal photolithography technique and a dry etching technique. Etching is stopped by etching a part of the layer 32f. In this embodiment, SiCl ₄ is used as a reactive gas used for dry etching, but the present invention is not limited to this, and other gases such as BCl ₃ may be used.

  After the ridge stripe 39 is formed, a process for processing the semiconductor wafer 30 into individual semiconductor laser elements 29 is performed. Since the outline is general, the details are omitted, but an insulating film 33 for current confinement and p-electrode pads 34 and 35 are formed on the surface of the semiconductor wafer 30 on which the ridge stripe 39 is formed, and the n-type is formed. The GaN substrate 31 is ground and polished from the surface on which the semiconductor growth layer 32 is not formed, and an n-electrode 36 is produced on the ground and polished surface, as shown in FIG. Thereafter, the semiconductor wafer 30 is cleaved perpendicularly to the ridge stripe direction and divided into bars. The distance between the resonator end surface 37a and the resonator end surface 37b formed by this cleavage is in the range of 300 μm to 1200 μm. Next, as shown in FIG. 4, a low-reflectance end-coat film 38a is formed on the resonator end face 37a, and a high-reflectance end-coat film 38b is formed on the resonator end face 37b. Finally, the semiconductor wafer 30 on which the end face coat films 38a and 38b are formed is divided into individual chips by dicing or cleaving, and the semiconductor laser element 29 is completed.

Each of the p electrode pads 34 and 35 is formed by vapor-depositing a metal, and is a multilayer film of a metal such as Mo / Au, Mo / Pt / Au, Mo / Pd / Au, or Mo / Pt / Al. Or it may be a single layer of Au only. The insulating film 33 is made of SiO ₂ , ZrO, TiO ₂ , Si ₂ N ₄ or the like. Further, the low-reflectance end face coating film 38a is formed of, for example, Al ₂ O ₃ , TiO ₂ , SiO ₂ , Y ₂ O ₅ , Nb ₂ O ₅ , Ta ₂ O ₅ , ZrO ₂ , AlN, Si ₃ N ₄ , MgF. An end face coating film 38b having a high reflectivity, for example, a film composed of 9 layers of SiO ₂ / TiO ₂ / SiO ₂ /... / TiO ₂ / SiO ₂ can be used.

  Next, a hermetic sealing device, which is a semiconductor laser device manufacturing apparatus according to an embodiment of the present invention, will be described with reference to the schematic plan view of FIG. The hermetic sealing device 10 includes a sealing tank 11, an ultraviolet ray generator 12, a heating device 13, and a welding machine 14 disposed therein.

  A plurality of work doors 15 sealed with a glove (not shown) are provided on the outer wall of the sealed tank 11 so that an operator can put a hand into the sealed tank 11 from here and work. Yes. Further, the outer wall of the sealed tank 11 is provided with a dry air introduction port 11a and an exhaust port 11b for introducing dry air generated by a dry air generator (not shown), and the inside of the sealed tank 11 has a constant humidity. It is kept in a dry air atmosphere.

  The ultraviolet ray generator 12 generates short wavelength ultraviolet rays having a wavelength of 254 nm. The ultraviolet ray generator 12 is provided with an ozone exhaust port 12a, and ozone generated by the ultraviolet ray irradiation can be discharged out of the sealed tank 11. The heating device 13 can heat and evacuate the furnace, and can remove moisture from the stem 21 and the cap 26 placed in the furnace by heating in a vacuum atmosphere.

  A cap cooling base 17a and a stem cooling base 17b made of a material having high thermal conductivity such as a metal plate are provided adjacent to the heating device 13, and the cap 26 and the stem 21 heated by the heating device 13 are placed thereon. It can cool by doing. Adjacent to the welder 14, a cap supply base 18a, a stem storage base 18b, and a stem supply base 18c are provided. The cap supply base 18a and the stem supply base 18c are used to place the cap 26 and the stem 21 automatically supplied to the welding machine 14 by a robot arm or the like, and the stem storage base 18b is placed on the stem supply base 18c. The previous cooled stem 21 is accommodated.

  A material storage 16 is provided between the heating device 13 and the welding machine 14. This is to keep the cooled cap 26 until it is moved to the cap supply base 18a. A finished product storage 19 is provided near the stem storage base 18b, and a cap 26 is attached to the stem 21 by the welding machine 14 so that the completed semiconductor laser device 20 can be stored.

  Next, a method for manufacturing the semiconductor laser device 20 will be described. First, the semiconductor laser element 29 is provided on the heat sink 22, and the heat sink 22 provided with the semiconductor laser element 29 is provided on the upper surface of the stem 21. The stem 21 and the cap 26 provided with the heat sink 22 are placed at a position where the worker irradiates ultraviolet rays of the ultraviolet ray generator 12 in the sealed tank 11 from the work door 15, and the stem 21, the heat sink 22, and the semiconductor laser element. 29 and the inner surface of the cap 26 are irradiated with ultraviolet light having a wavelength of 254 nm. At this time, the inside of the sealed tank 11 is a dry air atmosphere having a dew point of −30 ° C.

  By irradiating with ultraviolet light having a wavelength of 254 nm, the organic impurity gas adhering to the inner surface of the stem 21, the heat sink 22, the semiconductor laser element 29, and the cap 26, that is, the inner portion of the package portion 28 is decomposed and removed. be able to.

  The stem 21 and the cap 26 from which the organic impurities have been removed are moved into the furnace of the heating device 13 and heated in vacuum to remove moisture. At this time, the surface and internal moisture of the end face coating films 38a and 38b of the semiconductor laser element 29 can be removed by heating, and the semiconductor laser element 29 can be modified to have high durability. At this time, the heating temperature is preferably higher than the boiling point of water in order to remove more moisture, and more preferably 300 ° C. or higher.

  The stem 21 and the cap 26 taken out from the heating device 13 are placed on the cap cooling table 17a or the stem cooling table 17b, respectively, and cooled to room temperature. Next, the stem 21 is moved to the stem storage stand 18b and the cap 26 is moved to the material storage place 16, and the necessary portions are placed on the stem supply stand 18c or the cap supply stand 18a, respectively.

  The stem 21 and the cap 26 that are automatically supplied from the stem supply base 18c and the cap supply base 18a are welded by the welding machine 14 so that the cap 26 seals the heat sink 22 and the semiconductor laser element 29. Is completed and temporarily stored in the finished product storage 19. At this time, since the inside of the sealed tank 11 is a dry air atmosphere, the sealed atmosphere 28a is dry air. For this reason, the possibility of dew condensation inside the package portion 28 is low, but the possibility of dew condensation is further reduced by cooling the stem 21 and the cap 26 before welding.

  In this invention, it is preferable that the dew point of dry air is -30 degrees C or less. In general, the degree of drying of dry air is controlled by the dew point. As a dry air generator that generates dry air having a dew point of −30 ° C. or less, for example, a compressed air produced by a screw compressor (KST22AD-6) manufactured by Kobe Steel is used as a receiver tank and a refrigeration air dryer (manufactured by Orion). RAX-22F-SE), line filter (Orion OLF-700), micromist removal filter (Orion OMF-700), adsorption air dryer (Orion QSQ-420A-E) Can be used. The dew point of this dry air can be measured with a dew point thermometer DM70 manufactured by VAISALA.

  An accelerated aging test was performed on the semiconductor laser device 20 thus produced. The aging condition was ACC (Automatic Current Control) at a sample temperature of 70 ° C. and 120 mA, and the forward voltage (Vf) was measured. The result is shown in FIG. FIG. 6 (a) is a graph with the horizontal axis representing time and the vertical axis representing forward voltage, and shows the measurement results of seven samples, but there is almost no change in Vf after 50 hours. I understand that. The sealed atmosphere at this time is dry air having a dew point of -37 ° C.

  Further, as a comparative example, the same applies to a semiconductor laser device in which the sealing atmosphere in the above-described semiconductor laser device is changed from dry air to nitrogen gas having a dew point of −10 to −30 ° C. as the inert gas used in Patent Document 1. An accelerated aging test was conducted. FIG. 6B shows the results of measurement for 10 samples. As can be seen from FIG. 6B, there is a case where Vf increases after 25 hours have elapsed. Therefore, it can be seen that by using dry air as the sealing atmosphere, Vf of the semiconductor laser device 20 is stabilized, that is, durability and reliability are improved. This is presumably because a semiconductor laser device having a short oscillation wavelength has a different degradation mode from that having a long oscillation wavelength.

  In this embodiment, the semiconductor laser element 29 is made of a GaN-based semiconductor. However, the present invention is not limited to this. Or the like.

  In the present embodiment, the hermetic sealing device 10 includes the ultraviolet ray generator 12, the heating device 13, and the welding machine 14 in one sealed tank 11, but the ultraviolet ray generator 12, the heating device 13, and the You may provide the welding machine 14 in another sealed tank maintained in the dry atmosphere of the same dew point, respectively. In this case, the stem 21 and the cap 26 may be accommodated in a desiccator or the like maintained in a dry atmosphere with the same dew point between the sealed tanks. Thereby, the hermetic sealing device 10 can be freely arranged. Further, when the ultraviolet ray generator 12 irradiates the ultraviolet ray and then accommodates it in the desiccator, there is no problem even if it is exposed to the atmosphere for a short time.

  In addition, the process of heating the stem 21 and the cap 26 by the heating device 13 and then cooling the cap 21 with the cap cooling table 17a or the stem cooling table 17b is intended to move to the next process quickly, and may be omitted. Similarly, the step of heating the stem 21 and the cap 26 by the heating device 13 is for removing moisture even in a dry air atmosphere, but may be omitted.

1 is a schematic configuration diagram of a semiconductor laser device according to an embodiment of the present invention. 1 is a schematic configuration diagram of a semiconductor wafer according to an embodiment of the present invention. Schematic configuration diagram of a semiconductor wafer formed with a ridge stripe according to an embodiment of the present invention 1 is a perspective view of a semiconductor laser device according to an embodiment of the present invention. 1 is a schematic plan view of an airtight sealing device according to an embodiment of the present invention. (A) The semiconductor laser device which sealed dry air concerning the embodiment of the present invention, (b) The graph which shows the accelerated aging test result of the semiconductor laser device which sealed nitrogen gas

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Airtight sealing apparatus 11 Sealing tank 11a Dry air introduction port 12 Ultraviolet generator 13 Heating apparatus 14 Welding machine 17a Cap cooling stand 17b Stem cooling stand 20 Semiconductor laser device 21 Stem 26 Cap 28 Package part 28a Enclosed atmosphere 29 Semiconductor laser element

Claims (7)

  A step of introducing dry air into the sealed tank, a step of irradiating ultraviolet light onto the inner surface of the stem and cap provided with the semiconductor laser element in the sealed tank into which the dry air has been introduced, and the drying A method of manufacturing a semiconductor laser device, comprising: a sealing step of sealing together with the dry air the semiconductor laser element irradiated with ultraviolet rays by the cap irradiated with ultraviolet rays on the inner surface in the sealed tank into which air has been introduced.   The heating process of heating the stem and the cap in the sealed tank at a temperature higher than the boiling point of water between the ultraviolet irradiation process and the sealing process. Manufacturing method of the semiconductor laser device.   3. The method for manufacturing a semiconductor laser device according to claim 2, further comprising a cooling step of cooling the stem and the cap heated in the heating step to room temperature before the sealing step.   The method for manufacturing a semiconductor laser device according to claim 1, wherein a dew point of the dry air is −30 ° C. or lower.   The method for manufacturing a semiconductor laser device according to claim 2, wherein a heating temperature in the heating step is 300 ° C. or higher.   A sealed tank whose inside is kept in a dry air atmosphere, an ultraviolet ray generator for irradiating ultraviolet rays onto the inner surface of the cap and the stem provided with the semiconductor laser element, and the stem and the cap are heated at a temperature higher than the boiling point of water. A heating device, and a sealing device that seals the semiconductor laser element by attaching the cap to the stem, and the ultraviolet ray generator, the war record heating device, and the sealing device are located inside the sealed tank. An apparatus for manufacturing a semiconductor laser device.   A plurality of the sealed tanks maintained in a dry air atmosphere having the same dew point, and at least one of the ultraviolet ray generating means, the heating means, and the sealing device is another sealed of the plurality of sealed tanks. 7. The semiconductor laser device according to claim 6, wherein the semiconductor laser device is disposed in a tank, and the stem and the cap move between the sealed tanks in an atmosphere maintained at the same dew point as the dry air. apparatus.

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