GB2035684A - Subdividing semiconductor wafers - Google Patents

Abstract

A substrate 10 having epitaxial layers 11, 12, 13, 14 forming double heterostructure lasers is subdivided by anisotropically etching V-grooves 22 in the lower face of the substrate and then mechanically cleaving or breaking along lines 20. The resulting bars are scribed and broken along lines 23 to form individual laser diodes. When the wafer is 3 to 5 mils thick the V-grooves preferably extend to 1 to 2 mils less than thickness of the wafer. When the wafer is 6 to 10 mils thick preferably a parallel-sided channel is first formed followed by V-grooving. The method results in cleaved faces with little mechanical damage in the active area and produces diode lasers of uniform length. <IMAGE>

Description

SPECIFICATION Method of cleaving a semiconductor into individual diode lasers The invention relates to a method of cleaving a semiconductor wafer into individual diode lasers.

Coherent light-emitting diodes having a GaAs-(AI,Ga) As double heterostructure, such as described in "Semiconductor Lasers and Heterojunction LED's" by H. Kressel and J. K. Butler, Academic Press, New York, 1977, are known to be efficient light sources for optical communications systems.

As is well-known, such diode lasers comprise layers of GaAs and (AI,Ga)As on an n-GaAs substrate. The final layer is a cap layer of p-GaAs. Metallized stripes, parallel to the intended direction of lasing, are deposited on the p-side of the wafer. Gold contact pads, somewhat smaller in area than the intended size of the diode laser, are deposited on the n-side of the wafer. The stripes and pads are for subsequent connection to an external electrical source.

The wafer is then cut in two mutually orthogonal directions to form the individual diodes. First, the wafer is cut perpendicular to the intended lasing facets into bars of diodes. Then the bars of diodes, following passivation of lasing facets, are cut into individual diodes.

Cutting of the wafer into bars is generally accomplished by cleaving the wafer through the substrate side, using an instrument such as a razor blade, knife, scalpel blade or the like. Control over length of the diode laser is consequently poor, and variation of diode laser length is great, with the result that longitudinal mode distribution and threshold currrent vary considerably from one diode laser to the next. Further, the gold contact pads must be kept thin in order to permit reasonably clean cleaving. Also, the thickness of the substrate is constrained in order to promote better cleaving. This often limits useful wafer thicknesses to about 3 to 5 mils. yet, such thin wafers are susceptible to breaking during handling.Finally, striations generated by the mechanical cleaving, if across the active lasing regions, affect device yield since such devices are consequently prone to degradation.

In accordance with the invention, a wafer comprising a semiconductor substrate, at least a portion of one surface of which is metallized, and a plurality of semiconductor layers deposited on at least a portion of the opposite surface, at least one of which layers when appropriately biased generates coherent electromagnetic radiation, is cleaved into bars of diodes by a process which comprises (a) etching into the substrate with an anisotropic etchant to form V-grooves in the wafer and (b) mechanically cleaving into bars of diodes.

Preferably for separating thin wafers (normally those which are 3 to 5 mils thick), prior to the anisotropic etching, the wafers are processed by forming an array of exposed lines on the metallized substrate by photolithography to define lasing ends of the diodes and etching through the exposed metallized portions to expose portions of the underlying substrate. The exposed portions of the wafer are then anisotropically etched to for the V-grooves to a depth of 1 to 2 mils less than the total thickness of the wafer.

Preferably for separating thicker wafers (normally those which are 6 to 10 mils thick), prior to the anisotropic etching, the wafers are processed by forming channels of substantially parallel sidewalls and 1 to 4 mils deep in the surface of the substrate. The wafer is then anisotropically etched to a depth sufficient to form V-grooves in the bottom of the channels, suitably of depth 1 to 3 mils deeper than the bottom of the originally parallel channels, but the total depth being less than the thickness of the wafer.

As a consequence of the process of the invention, good cleavage control, substantially damage-free facets along the plane of cleaving and substantially uniform definition of diode laser length are obtained. Further, cleaving in accordance with the invention increases device yield by at least 50%, as compared with prior art techniques.

Brief description of the drawing Figures la and lb, in perspective, depict a portion of a thin wafer following etching of V-grooves in accordance with the invention prior to final mechanical cleaving; Figures 2a and 2b are photomicrographs of lasing facets of, respectively, a diode laser formed by cleaving in accordance with prior art procedures, showing striations (damage) resulting from cleavage, and a diode laser formed by cleaving a thin wafer in accordance with the invention, showing substantial absence of striations; Figures 3a and 3b, in perspective, depict a portion of a thick wafer following formation of channels and etching of V-grooves in accordance with the invention prior to mechanical cleaving into diode bars; and Figures 4a and 4b depict a portion of a thick wafer in cross-section, following formation of channels and V-grooves, respectively.

Detailed description of the invention The description that follows is given generally in terms of double heterostructure (DH) (Al, Ga)As diode lasers having a stripe geometry. However, it will be appreciated that other configurations and other geometries of both gallium arsenide diode lasers, as well as other semiconductor diode lasers, may also be beneficially processed following the teachings herein. Specific configurations of devices may generate coherent electromagnetic radiation in the UV, visible or IR regions.

Figures 1 a, 1 b, 3a and 3b depict a portion of a wafer, considerably enlarged for purposes of illustration, from which a plurality of DH diode lasers are to be fabricated. Figures 1a and 3a show the wafer n-side down, while Figures 1 b and 3b show the wafer p-side down. The wafer includes an n-type GaAs substrate 10, on at least a portion of which are normally grown four successive layers 11, 12, 13, and 14, respectively, of n-(Al,Ga)As, p-GaAs, p-(AI,Ga)Asand p-GaAs. Layers 11 and 12 from a p-n junction region 15, with central areas 16 in layer 12 providing light-emitting areas. The layers are conveniently formed one over the other in one run by liquid phase epitaxy, using conventional diffusion techniques and a horizontal sliding boat apparatus containing four melts, as is well-known.Metal electrodes 17 in the form of stripes parallel to the intended direction of lasing are deposited through conventional photolithography techniques onto top layer 14 and provide means for external contact. A metal layer 18 is deposited on at least a portion of the bottom of the substrate 10. Gold pads 19, somewhat smaller in area than the intended device, are formed on layer 18, and provide means for external contact. When cleaved into individual devices, as shown by dotted lines 20, planar mirror facets are formed along (110) planes. When current above a threshold value from a battery 21 is sent through a selected electrode 17, light L is emitted from the facet on the p-n junction 16, such p-n junction lying in a plane that is perpendicular to the direction of current flow from electrode 17to electrode 18.That is, the cavity of the laser structure is bounded by the two cleaved facets, and the laser light is emitted from the facets in a direction approximately perpendicular to the direction of current flow. The necessary reflectivity at the cavity facets is provided by the discontinuity of the index of refraction between the semiconducting materials and air.

In the typical fabrication of DH (Aì,Ga)As diode lasers, the wafer comprises a substrate 10 of n-GaAs, typically about 3 to 5 mils thick and having a carrier concentration ranging from about 1 to 3 x 1018 cm~3, usually doped with silicon. Alternatively, the wafer comprises a substrate 10a of n-GaAs at least about 6 mils thick, and preferably 6 to 10 mils thick, having the indicated carrier concentration.

Layers 11 and 13 of n-(Al,Ga)As and p-(Al,Ga)As, respectively, are typically about 0.75 to 2 um thick, with both layers having a value of x (AIxGa7 xAs) of about 0.30 to 0.35. Layer 11 is typically doped with tin, while layer 13 is typically doped with germanium. Active layer 12, of either p-GaAs or p-(AI,Ga)As, is typically about 0.1 to 0.3um thick and is undoped. If layer 12 is p-(Al,Ga)As, then the value of y (AI,Gal,As) ranges from about 0.05 to 0.10. Cap layer 14 of p-GaAs is typically about 0.2 to 0.Sum thick and provides a layer to which ohmic contact may be made.The carrier concentration of layer 14, provided by germanium, is typically about 1 to 3 x 1019 cm~3. Metallic ohmic contacts 17 in stripe form are deposited onto layer 14 by conventional photolithographic techniques employing electroless nickel plating having a thickness ranging from about 0.05 to 0.07,um, followed by about 1000 A of electroplated gold. Ohmic contact 18 is formed by evaporation of, e.g., 3% silver/97% tin alloy onto the bottom of substrate 10 and typically has a thickness ranging from about 0.18 to 0.20cm. Gold pads 19, formed by electroplating through a photoresist mask, typically are about 2 to 3lim thick.

Following the foregoing procedure, the wafer is first cleaved into bars of diodes by cleaving the wafer through the substrate side, perpendicular to lasing facets, along lines 20, which are between gold pads 19.

However, the regions covered by the gold pads are locally strained, and cleavage is unpredictable, with the consequence that prior art mechanical cleaving techniques such as a knife, razor blade or other instrument, result in diode bars of uneven length. Variations in diode laser length affect longitudinal mode distribution and threshold current, with the result that these values can differ considerably for diode lasers taken from different locations on the same wafer. Further, the diode bars are subsequently placed in a fixture for evaporation of a film of Al203 of about 1200 A in thickness to passivate lasing facents. If the diode bars are too long (as measured between lines 20 in Figures 1a and 3a), then the diode bars cannot be placed in the fixture.

If too short, then, due to a shadowing effect, the lasing facets are not properly passivated.

Another consequence of prior art mechanical cleaving is that the gold pads must be kept thin, as must the substrate, in order to maximize yield of diode lasers. Yet, thin gold pads are bonded to only with difficulty when connecting one end of an external lead, and thin substrates render handling of the wafer difficult.

Further, such mechanical cleavage often generates striations (damage), which, when formed across the active region, can lead to increased degradation of the devices, with consequent low device yield. Such striations are shown in Figure 2a, which is photomicrograph of a facet cleaved in accordance with prior art techniques.

In accordance with one aspect of the invention, variation in diode laser length in thin wafers (e.g., 3 to 5 mils thick) is minimized by the following procedure. An array of exposed lines on the n-side of the wafer is formed by conventional photolithographic techniques. The lines or channels expose n-side metallized contact layer 18. The exposed portions of the metallized layer are then etched through with an etchant which selectively etches the metal without etching the semiconductor material. For example, for a contacting layer 18 of 3% Ag-97% Sn having a thickness of about 0.18 to 0.20 F m, etching is conveniently performed in about 10 minutes employing concentrated HCI. Grooves are then etched into the exposed portions of the substrate with a preferential etchant that forms V-grooves 22.Where the substrate is gallium arsenide, an example of such an etchant comprises a solution of H2SO4, H202 and H2O. The exact details of a successful etchant for producing a V-groove 22 as shown in Figures 1 a and 1 b are described in a paper entitled "Selective Etching of Gallium Arsenide Crystals in H2SO4-H2O2-H2O Systems" by S. lida et al in Volume 118, Electrochemical Society Journal, pages 768-771(1971) and forms no part of this invention. An example of an etchant that produces a V-groove is 1 H2SO4-8H2O2-1 H2O, in which the concentration of H2SO4 is a 98% solution by weight and the concentration of H202 is a 30% solution by weight, whereas the formula concentration is by volume.

The 1-8-1 solution, at a temperature of 25 C, is able to etch through the GaAs layer at a rate of about 41l m/min. The etchant in this concentration produces a V-shaped channel in GaAs with sidewalls having an angle of 54 44' with respect to the plane of the wafer when the etch is performed on the (001) surface along the < 011 > direction which gives V-grooves. The etching solution is quenched as soon as the desired amount of etching has taken place. Other etchants, whether chemical or gaseous, which also give rise to similar V-grooves, may also be employed.A relatively steep sidewall, such as 54 44', is preferred to shallower sidewalls of, say, less then 45 , in order to conserve substrate material on the etched side of the wafer.

Of course, the rate of etching can be increased by increasing the temperature of the etchant. The etchant is selective according to the crystal orientation of the material, as described above. Thus, the orientation of the wafer should be such that a V-groove configuration is obtained, rather than a round bottom configuration.

The reason for this is that the bottom of the V-groove provides a precise location for initiation of cleaving, which in turn results in fabrication of individual diode lasers of precise length.

The etching is carried out to a depth of about 1 to 2 mils less than the total thickness of the wafer. If the etching is not deep enought, then cleavage is more difficult, since a cleavage plane is not well-defined and cleavage striations are more likely to occur across lasing facets, as with prior art techniques. If the etching is too deep, then cleaving is initiated beyond the substrate and in the region of the epitaxial layers, and will not result in a mirror facet.

Following etching, which, as shown in Figures 1a and 1 b produces a V-groove 22, the wafer is mechanically cleaved by rolling or other means so as to produce cleavages along lines 20. A knife, razor blade or other sharp instrument may be used from the n-side for cleaving, resulting in diode bars of prescribed uniform length with good cleaved surfaces. Alternatively, a convenient technique is to mount the wafer, p-side up, on a flexible adhesive tape and roll the assembly over a small radius tool, such as disclosed in U. S. Patent 3,497,948. Most preferred is to simply cleave from the p-side by pressing down over the V-grooves with a blunt instrument, such as a tweezer edge. This method is fast and accurate.The combination of etching V-grooves in the substrate to the specified depth range, followed by mechanical cleavage, results in substantially striation-free facets, as shown in Figure 2b.

While thicker substrates are desirable, the typical thicknesses of 3 to 5 mils for substrates are the result of the constraints posed by prior art techniques of cleaving wafers. Such thin wafers, however, are very fragile and often break during handling. The inventive technique discussed in further detail below is particularly useful for thicker wafers, such as on the order to 6 to 10 mils and thicker, after fabrication to form the ohmic contacts as described above.

Thus, in accordance with another aspect of the invention, diode lasers are fabricated from relatively thick wafers (e.g., 6 to 10 mils thick) by the following procedure. A channel 24 of substantially parallel sidewalls is formed in the exposed surface of the n-substrate, as shown in the cross section in Figure 4a. The channels are about 1 to 4 mils deep. If the channels are not deep enough, then the subsequent etching step, described below, results in considerable loss in surface area of the substrate. If the channels are too deep, then the V-groove formed by etching in the subsequent step will not be fully formed, and thus cleaving is initiated beyond the substrate and in the region of the epitaxial layers, and will not result in a mirror facet.

The channels are conveniently formed using a diamond circular saw blade about 1.5 to 2 mils in thickness.

While other techniques may be used, the diamond circular saw blade, which is extensively used in semiconductor processing techniques for other purposes, advantageously forms channels having substantially parallel sidewalls. Due to the increased thickness of the wafer, no dislocations are generated in the active region, which is a problem that generally accompanies use of diamond saw blades with thinner wafers.

Grooves are then etched into the bottoms of the channels with an anisotropic etchant that forms V-grooves 22a, as shown in cross section in Figure 4b. When the substrate is gallium arsenide, an example of such an etchant comprises a solution of H2SO4, H202 and H2O. The description above in reference to forming V-grooves in thinner wafers with this etchant is applicable to thicker wafers as well.

The etching is carried out to a depth sufficient to form a V-groove. If the etching is not deep enough to form the V-groove, then cleaving is more difficult, since the cleavage plane is not well-defined and cleavage striations are more likely to occur across lasing facets, as with prior art techniques. If the etching is too deep, then cleaving is initiated beyond the substrate and in the region of the epitaxial layers, and will not result in a mirror facet. The V-groove in the bottom of the channel is generally well-formed about 1 to 3 mils deeper than the initial channel, and etching may be terminated at that point.

Following etching, which, as shown in Figures 3a, 3b and 4b produces a V-groove 22a in the bottom of channel 24, the wafer is mechanically cleaved by rolling or other means so as to produce cleavages along lines 20, as described above in connection with thinner wafers.

Following cleaving into diode bars the passivation of lasing facets, individual diodes are formed by scribing the bars, as with a diamond scribe, usually on the n-side, along lines 23 (the wafer having previously been indexed by well-known means to locate stripes 17). The scribed bars are then mechanically cleaved by rolling a tool of small radius over the bars, as is customary in the art.

The foregoing methods result in good cleavage control and uniform definition of diode laser length.

Consequently, longitudinal mode distribution and threshold current exhibit little variation for devices taken from different locations in the wafer. Cleavage to prescribed lengths results in easier processability of lasing facet passivation and in improved device yields. Yields improved by at least 50% are realized using the method of the invention. Cleaving through thin GaAs (from the bottom of the V-groove to the p-side) appears to reduce cleavage striations on the lasing facet, as shown in the comparison between Figures 2a and 2b, which are photomicrographs of cleaved facets, magnified 11 00x the former produced by a prior art method as discussed above and the latter produced in accordance with the invention. An additional benefit of the invention is that gold contact pads 19 may be made thicker without affecting the quality of the cleaved surface.Such thicker contacts permit better ease of contacting to an external power source. Also, a thicker substrate may be employed than heretofore possible, thereby increasing ease of handling. The method of the invention for thicker wafers is especially efficacious for processing thicker substrates than heretobefore possible, thereby increasing ease of handling and ease of fabrication of shorter diode (cavity) lengths. Such shorter diode laser length, on the order of about 6 mils, permit lowering of the threshold current over that customarily found in the art.

Examples Example 1. Thin Wafer A A processed piece of GaAs material (average thickness about 4.0 + 0.5 mils) was divided into two pieces.

One piece was held for conventional prior art cleaving, whereas the second piece was further processed for etch cleaving in accordance with the invention. The two pieces were then cleaved at the same time by the same operator using the following method for both pieces: pressing with tweezers in the direction of the desired cleave. The experiment was then repeated in the same way with another operator.

The etch cleaving was done as follows: A pattern of parallel strips 10,am wide and 10 mils apart (center to center) were formed by exposure of photoresist through a suitable photoresist mask. The exposed photoresist portions were removed by dissolving in developer to expose portions of a top gold layer. The exposed gold portions were removed in a KI-base gold etchant in about 1 minute at 50 C to expose portions of an underlying 3% Ag/97% Sn layer. The exposed portions were removed using concentrated HCI for 10 minutes at room temperature to expose portions of underlying n-GaAs substrate. The exposed portions were etched with a V-groove etchant comprising 1 H2SO4-8H2O2-1 H2O (by volume) for 15 minutes at room temperature.The etched V-grooves were formed to an average depth of about 1.5 mils less than the thickness of the wafer.

The cleavage yields were measured in terms of the number of useful bars obtained expressed as a percentage of the total amount of material. The cleavage striation densities were measured by Nomarski optical examination of 5 mm lengths of samples representative of the two methods.

The results are shown in the Table below: Prior Art Etch Cleaving Cleaving of the Invention % Yield for Operator 1 20 100 % Yield for Operator 2 10 100 Cleavage Striation Density, mm-l 300 6 It can be seen that yields were improved by a factor of 5 to 10 and cleavage striation density was reduced by a factor of 50 employing the inventive technique.

Figures 2a and 2b are photomicrographs of the facets cleaved by the two methods, enlarged by a factor of 11 00x. The reduction of cleavage striation density afforded by the etch-cleave method of the invention is clearly visible.

Example 2. Thick Wafer Processed wafers were lapped on the n-side to 8 mils. A coating of 3% Ag/97% Sn was evaporated on the lapped side to a thickness of 1900 A. A nickel film of 2000 A lim was electroless plated on the Ag-Sn coating, followed by a gold film of 500 A. Athin layer of chromium (700 A), followed by a thin layer of gold (1000 A), was evaporated on the opposite side.

The wafer was sawed from the n-side to a depth of 2 mils using a diamond blade of 1.5 to 2.0 mil thickness.

After sawing, the sample was etched in a V-groove etchant comprising 1 H2SO4-8H202-1 H2O (by volume) for 10 min. at room temperature to a depth of 4.3 mils from the substrate surface (2.3 mils deeper than the inital channel). Cleaving was done by pressing with tweezers in the direction of the desired cleave.

The cleavage yields were measured in terms of the number of useful bars obtained expressed as a percentage of the total amount of material. The cleavage striation densities were measured by Nomarski optical examination of 5 mm lengths of samples representative of the cleaving. No comparison was made using prior art techniques with wafers about 8 mils thick, since such wafers cannot be controllably cleaved by scribing and mechanically cleaving. A comparison could be made, however, with wafers about 4 mils thick cleaved by conventional prior art techniques. The results are shown in the Table below.

Prior Art Cleaving in Accordance Cleaving with Invention % Yield 10;20 100 Cleavage Striation Density, mm-1 300 8 It can be seen that yields were improved by a factor of 5 to 10 and cleavage striation density was reduced by a factor of nearly 40 employing the inventive technique. The comparison presumably would be even greater if thick wafers could be cleaved by prior art techniques.

Claims (12)

1. A method of cleaving a semiconductor wafer comprising a substrate, at least a portion of one surface of which is metallized, and a plurality of semiconductor layers deposited on at least a portion of the opposite surface, at least one of which layers, when appropriately biased, generates coherent electromagnetic radiation; which method includes: (a) etching into the substrate with an anisotropic etchant to form V-grooves in the wafer; and (b) mechanically cleaving into bars of diodes, thereby generating substantially damage-free facets along the plane of cleaving and forming bars of diodes of substantially equal width between lasing facets.

2. A method as claimed in claim 1, in which the wafer is 3 to 5 mils thick and the V-grooves are formed to a depth of 1 to 2 mils less than the thickness of the wafer.

3. A method as claimed in claim 2, in which prior to the said etching step, the wafer is processed by: (a) forming an array of exposed lines on the metallized substrate by photolithography to define lasing ends of the devices; and (b) etching through the exposed metallized portions to expose portions of the underlying substrate.

4. A method as claimed in claim 1, in which the wafer is at least 6 mils thick.

5. A method as claimed in claim 4, in which prior to the etching step, the wafer is processed by forming channels of substantially parallel sidewalls and 1 to 4 mils deep in the substrate.

6. A method as claimed in claim 5, in which the V-grooves are etched to a depth of 1 to 3 mils deeper than the bottom of the originally parallel channels, the total depth being less than the thickness of the wafer.

7. A method as claimed in any preceding claim, which the semiconductor wafer comprises a substrate of gallium arsenide and layers of gallium arsenide and aluminum gallium arsenide.

8. A method as claimed in claim 7 in which the V-groove etchant comprises a solution of H2SO4, H202 and H20.

9. A method as claimed in claim 8 in which the etchant comprises H2SO4, H202 and H2O in a volume ratio of :8:1.

10. A method as claimed in any preceding claim in which the V-grooves have sidewalls at an angle of at least about 45 with respect to the plane of the wafer.

11. A method as claimed in claim 1 and substantially as herein described with reference to the Examples and/or accompanying drawings.

12. A semiconductor diode laser whenever produced by a method claimed in any preceding claim.