Etch process monitors for buried heterostructures
The U.S. Government has certain rights pursuant to the terms of a contract MDA904-83-C-0454. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The subject matter is generally directed to the field of the etching of hourglass shaped mesas in crystalline material. This is a standard process in the fabrication of buried heterostructure laser diodes and amplifiers. It is important to the device performance that the mesa width be precisely controlled. The invention is specifically directed to a visual in-process monitor or aid to the progress of the etching. This process might also be applied to the fabrication of other integrated optical components such as waveguides and couplers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial cross-sectional view of a multilayer epitaxial structure on a semiconductor substrate (e.g. gallium arsenide (GaAs) or indium phosphide (InP)) in which long mesas are to be formed by etching. The photoresist defines the starting width on the etched mesa. 
     FIG. 2 is a diagrammatic end view of an array of integrated buried heterostructure lasers or optical amplifiers. 
     FIG. 3 is a top view of the photoresist pattern on the substrate prior to etching the widths of the laser/amplifier stripes and the etch monitors indicated in microns. 
     FIG. 4 is a pictorial view of the etched mesas on the buried heterostructure wafer. The etch indicators (-1 through -3) are used to deduce how wide the laser/amplifier mesa waist is by noting how many mesas have fallen off during etching. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1, there is generally shown a multilayer epitaxially grown GaAs-AlGaAs or InP/InGaAsP structure for use in constructing a buried heterostructure laser. For these lasers the n-type substrate (e.g. GaAs) 10 has grown thereover by MOCVD (metal organic chemical vapor deposition) or MBE (molecular beam epitaxy) a number of layers. The epitaxial layers grown from the substrate 10 may include a GaAs buffer layer 11, a cladding layer 12 of Al x  Ga 1-x  As where x=0.3≲x≲0.4, an active region 13, a cladding layer 14, and a GaAs layer 15. Over these epitaxial layers is shown a silicon nitride layer 16. The structure so far described is to be divided into one or many isolated lasers. For the buried heterostructure shown in the following Figures, the isolation is accomplished by etching. 
     The unwanted epitaxial material between the individual buried heterostructure lasers (that is, in FIG. 2, between lasers 1, 2 and 3) is etched out leaving the lasers standing like mesas, and then the etched regions are refilled by growing cladding material 20 to fill the voids and thereby to provide isolation between the individual buried heterostructure lasers. The etching of the hourglass shaped mesas for buried heterostructure lasers and amplifiers is extremely critical, but is not well controlled because wet chemical etches are used. It is important to the device performance that the mesa width be tightly controlled. Up to now, only the length of time of etching has been used to determine the mesa width. This invention provides a visual aid to the progress of etching. 
     In the fabrication of buried heterostructures (BH) or buried optical guide (BOG) injection lasers or amplifiers, one of the most critical processing steps is the etching of the mesa stripe. Because of the crystalline structure of the semiconductor material and the action of the etching solution, a properly oriented etching mask (in the [110] direction on a (100) surface) will produce a mesa having an hourglass shaped cross-section (see FIGS. 2 and 3). The narrowest (i.e. waist) part of the mesa (mesa width) should be at the active layer 13. The mesa width determines many of the important device characteristics: lasing threshold current, single-mode operation, and maximum optical power. Therefore, an accurate knowledge of the mesa width is essential. The mesa widths are designed to be 1-3 microns, depending on the device application. The mesas are etched using a wet chemical solution. In general, wet chemical etches are not very controllable and some variation in etch rates will occur depending on solution strength, temperature, material composition, and doping. Thus, there can be large sample-to-sample variations in mesa width due to the etching. 
     The etching process monitors or etch indicators described in this invention provide accurate in-process information on the progress of the etching. Referring to FIG. 3, a top view of a mask, the etch monitors are a series of short stripes 30 which are placed on the photolithographic mask along with the stripes 31 to form the laser mesas. The width of the monitor stripes get progressively narrower in increments of 0.5 microns for example. For one embodiment of the etch mask the laser/amplifier stripe is 15 microns wide. The 5 monitor stripes in FIGS. 3 and 4 are 14L (-1), 13.5(-1.5), 13(-2), 12.5(-2.5), and 12(-3) microns wide. The negative numbers in parenthesis indicate how much narrower the monitor stripe is than the laser stripe. 
     The purpose of these lines is to monitor the progress of the mesa formation. During etching, the waist of the mesa becomes narrower. The final width of the mesa waist of the amplifier is designed to be 1.2 microns. If this were to be attempted using a single etch step, control would be very difficult. The wet etching for the mesas is not consistant enough to allow control by time and achieve a 1.2 micron wide mesa. The etch indicators give a close estimate of how wide the mesa width of the amplifier is. As the sample is etched, the waist of the narrowest mesas will go to zero and the top of the mesas fall off, starting with the 12 micron wide (-3) stripe. This can be seen by looking at the wafer from the top. When the 12 micron wide stripe falls off, it is known that the amplifier mesa width is between 2.5 and 3 microns. When the 12.5 micron wide (-2.5) stripe falls off, the amplifier mesas are less than 2.5 microns wide. The sample is allowed to etch until the indicator falls off which signifies the desired laser/amplifier mesa width and the etching is stopped. The sample is then cleaved through the mesa stripes and the mesa width is accurately measured in the scanning electron microscope (SEM). The final mesa etching occurs during the cleaning before regrowth when less than 0.5 microns of material is removed from each side of the mesa to obtain a 1.2 micron wide waist.