Ion milling method

An ion milling method is disclosed that provides a manufacturing technique for mass producing microscopic surface features using a wide variety of media that includes semiconductors, metals, and glasses. In the preferred embodiment, vertical and 45 degree mirrors are formed simultaneously in semiconductor laser diodes in order to produce monolithic two dimensional arrays of surface emitting lasers. Standard double heterostructure semiconductor laser diodes are first grown on a wafer using metalorganic chemical vapor deposition techniques. An ion milling gun is oriented at a particular angle from the longitudinal axis of the active layer of the laser and emits a stream of atomic particles toward the lasers producing a generally two sided cut or notch that extends downward from the top surface of the semiconductor laser and traverses the active layer. The two sides of the cut consist of a vertical face that is perpendicular to the active layer and an inclined mirror surface that connects to the bottom of the vertical face and the slopes back upward to the top of the laser. Although the preferred utilization of this invention is the production of high power semiconductor laser arrays and subsequent wafer scale integration, the ion milling technique may be employed to construct a wide variety of micro-miniature radiation interfaces, reflectors, transmitters, or absorbers. Virtually any surface that requires a specifically determined configuration of uniform topography of atomic proportions may be produced.

BACKGROUND OF THE INVENTION 
The present invention relates to methods for forming and shaping minute 
surfaces with great precision. In particular, this invention may be 
employed to fabricate a wide variety of complex devices having intricate 
geometric features. The ion milling method was developed in order to 
manufacture surface emitting semiconductor lasers, but the technique may 
be utilized to efficiently and accurately mass produce a virtually 
infinite number of different surface features of nearly any medium on a 
microscopic scale. 
The technical background of the present invention generally pertains to 
recent efforts to design and manufacture extremely small lasers from 
semiconductor materials. Semiconductor lasers are typically multilayered 
structures having dimensions measured in millionths of a meter and 
including different kinds of semiconductor material. One of the chief 
advantages of using semiconductor lasers to generate output radiation is 
their extraordinarily high efficiency. The various layers comprising these 
minute lasers are composed of chemically doped semiconductor elements or 
compounds. Before the doping process, semiconductor material generally 
contains an equal number of negative and positive particles. The doping 
process alters the relative number of negatively charged electrons or 
positively charged holes by introducing additional numbers of charged 
particles into the originally neutral semiconductor matrial. Regions of 
the laser that have been doped with extra electrons are called n-type, 
while those populated by a majority of holes are referred to as p-type. 
The basic structure of a semiconductor laser is that of a diode, an 
electrical device that conducts current in only in one direction. A simple 
cube-shaped structure that illustrates the most fundamental semiconductor 
laser design is shown in FIG. 1. A diode can be formed by joining a region 
of n-type material with a region of p-type material. In a semiconductor 
laser, a relatively thin zone of material that is capable of lasing is 
sandwiched between the n- and p-type regions. This central zone is called 
the active layer. When an electrical potential is imposed across the n and 
p regions through metal contacts attached to the exterior faces of the 
laser, the electrons and holes respond to the mutually attractive 
electrical field that this biasing voltage creates. The particles migrate 
across the boundaries of the central junction into the active layer and 
combine with their opposites. This combination process is accompanied by 
the emission of laser light. The strata above and below the narrowly 
confined active layer have a lower index of refraction than the active 
layer, which means that the laser light is repeatedly reflected between 
the n and p regions within the active layer. The only places that are 
available as exits for the laser output are the peripheral edges of the 
active layer along the outer wall of the semiconductor laser. 
Since the laser output can only radiate from a narrow stripe that extends 
around the mid-section of the entire structure, it is exceedingly 
difficult to control and use the energy produced by this very simple 
laser. In this embodiment, the output fans out from the cube in every 
direction from the plane of the active layer. The energy that is generated 
is weak and diluted, since the stream of light cannot be gathered into a 
concentrated beam that can be pointed and controlled to accomplish some 
task. because these laser cubes are so small, one obvious way to bolster 
the total energy output would be to combine them together in an array. 
Although an assembly of many individual cubes deployed together in a 
planar arrangement is an attractive alternative, the simple cube structure 
depicted in FIG. 1 cannot fulfill this design because most of the energy 
emitted by each individual laser would be directed at a neighboring cube 
in the two dimensional array. At best, this laser architecture may be 
employed to form a long row of individual cubes that would emit a wide but 
still relatively weak stream of laser radiation. 
Over the past decade, this very simple device has been vastly improved and 
refined. The current generation of semiconductor laser diodes includes 
structures having many complex layers that are formed with a multitube of 
exotic techniques. Recent efforts have produced complex architectures 
called double and buried heterojunction designs that are fabricated using 
an assortment of laboratory processes. Perhaps the single most important 
objective of recent research in this field has been the quest to produce a 
two dimensional array of semiconductor lasers that emit laser output in a 
direction that is perpendicular to the plane of the active layer. 
Organizing many individual lasers that emit light from their top surfaces 
together in a matrix would provide a means of constructing highly powerful 
radiation sources. Recent experimentation has yielded semiconductor lasers 
that incorporate tiny mirrors oriented 45 degrees from the plane of the 
active layer that are capable of directing some of all of the lasers 
emission through apertures above the mirrors. Most of these advances 
utilize cleaving, wet-chemical etching, dicing, second-order grating, or 
mass-transport procedures that are generally difficult to perform, 
unreliable, and unsuitable for high volume manufacture. 
The electronics industry has devoted enormous efforts in the past several 
years to find a solution to the long-felt need for a method of fabricating 
surface emitting semiconductor laser diodes. Such a method would enable 
not only laser manufacturers but also designers of integrated circuits to 
control the size and shape of sub-micron features with unprecedented 
accuracy. Such an advance in the technology would be a fundamental 
construction technique for optical computer circuits, in which photons 
would replace electrons as the carriers of information within complex 
light pathways. The ideal solution to this problem would provide a 
practical and efficient means for growing thousands, millions, and, 
perhaps, billions of lasers simultaneously layer by layer on a single 
wafer. This method would be equally effective in fashioning sub-micron or 
atomic scale features in a diverse range of media. Although a chief use 
would certainly include the production of lasers from semiconductor 
materials, the technique would be invaluable in constructing any sort of 
micro-miniature radiation interface, reflector, transmitter, or absorber. 
Virtually any surface that requires a specifically determined 
configuration of uniform topography could be achieved using such an 
invention, irrespective of whether the original medium was a 
semiconductor, a metal, or an active or passive optical material. The Ion 
Milling Method claimed in this patent application addresses these 
objectives and provides a solution to this long-felt need. 
SUMMARY OF THE INVENTION 
The present invention provides a method for mass producing microscopic 
surface features using a wide variety of media that includes 
semiconductors, metals, and glasses. In the preferred embodiment, vertical 
and 45 degree mirrors are formed simultaneously in semiconductor laser 
diodes in order to produce monolithic two dimensional arrays of surface 
emitting lasers. Standard double heterostructure semiconductor laser 
diodes are first grown on a wafer using metalorganic chemical vapor 
deposition techniques that are well known to persons ordinarily skilled in 
the art. After the wafer is protected with a mask that shields areas that 
are not to be milled, the wafers are loaded in a chamber, and the chamber 
is evacuated. An inert gas such as argon is then introduced into the ion 
milling chamber in order to maintain a predetermined vacuum pressure. An 
ion milling gun is then aimed at the wafer workpiece and is oriented at a 
particular angle from the longitudinal axis of the active layer of the 
laser. The ion milling gun is then activated, sending a stream of atomic 
particles towards the lasers on the wafer. As the milling process 
proceeds, the rate of removing materials from the workpiece may be 
controlled and monitored by adjusting the intensity of the ion gun and by 
sampling the pressure in the chamber. The result of the ion milling 
process is a generally two sided cut or notch that extends downward from 
the top surface of the semiconductor laser and traverses the active layer. 
The two sides of the cut comprise a vertical face that is perpendicular to 
the active layer and an inclined mirror surface that connects to the 
bottom of the vertical face and then slopes back upward to the top of the 
laser. Depending on the medium, a small horizontal shelf region may be 
formed between the vertical cut and the inclined mirror, which gives the 
resulting milled notch a three sided configuration. The notches may then 
be finished by applying various stabilizing, dielectric, or metal coatings 
to enhance the laser performance or maximize reflectivity. 
An appreciation of other aims and objects of the present invention and a 
more complete and comprehensive understanding of this invention may be 
achieved by studying the following description of a preferred embodiment 
and by referring to the accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Appendix I of this patent application consists of a publication entitled 
"Surface-Emitting GaAlAs/GaAs Laser With Etched Mirrors" by J. J. Yang, M. 
Jansen, and M. Sergant. This article was published in Electronics Letters, 
Volume 22, Number 8, on pages 438-439, dated Apr. 10, 1986. This article 
briefly explains the results obtained through the practice of the 
invention claimed in this application and is hereby incorporated by 
reference. 
The present invention provides a method for mass producing microscopic 
surface features using a wide variety of media that includes 
semiconductors, metals, and glasses. In the preferred embodiment, vertical 
and 45 degree mirrors are formed simultaneously in semiconductor laser 
diodes in order to produce monolithic two dimensional arrays of surface 
emitting lasers. Standard double heterostructure semiconductor laser 
diodes are first grown on a wafer using metalorganic chemical vapor 
deposition techniques that are well known to persons ordinarily skilled in 
the art. Stripes having widths of four microns may be defined on the wafer 
using conventional 1350J-SF photoresist material. The wafer is protected 
with a mask of material that is highly resistant to ion mlling. This step 
shields areas that are not to be milled so that they are retained in their 
original condition. Most metals are relatively resistant to ion milling, 
as compared to semiconductor material. The wafers are then loaded in a 
chamber, and the chamber is evacuated to about 10.sup.-6 torr. An inert 
gas such as argon is then introduced into the ion milling chamber in order 
to maintain a vacuum pressure of about 8.times.10.sup.-5 torr. 
An ion milling gun is then aimed at the wafer workpiece. The device used by 
the applicants in actual tests of the method of the invention claimed 
below was a Microetch machine manufactured by Veeco. The gun is oriented 
at a particular beam angle from the longitudinal axis of the active layer 
of the laser. The beam angle is determined by referring to the empirical 
plot shown in FIG. 12, which is discussed in greater detail below. For 
example, FIG. 12 indicates that the proper beam angle (measured along the 
x-axis) for forming a 45 degree surface at the same time as a 90 degree 
surface (measured along the y-axis) is about 30 degrees or about 60 
degrees measured along the y-axis. 
Once the proper spatial adjustments are completed, the ion milling gun is 
then activated. In this procedure, the Microetch System, which is a 
conventional laboratory ion gun, is typically operated at magnet and 
accelerator power levels of 35 volts, 0.8 amberes and 500 volts, 500 
milliamperes, respectively. Once the gun is energized, a stream of ions is 
sent towards the lasers on the wafer. Any particles, whether atomic, 
ionic, molecular, charged, or uncharged, will perform the ion milling 
task. The momentum of the impinging particles cuts away portions of the 
workpiece in the beam's path. The typical milling rate encountered in 
actual tests by the inventors was 3 millionths of a meter of depth milled 
per hour. As the milling process proceeds, the rates of removing materials 
from the workpiece may be controlled and monitored by adjusting the 
intensity of the ion gun and by sampling the pressure in the chamber. In 
order to prevent overheating, the chamber is typically cooled to provide a 
roughly constant temperature of 20 degrees Centigrade. 
The result of the ion milling process is a generally two sided but or notch 
that extends downward from the top surface of the semiconductor laser and 
traverses the active layer The two sides of the notch or cut consist of a 
vertical face or wall that is perpendicular to the active layer and an 
inclined mirror surface that connects to the bottom of the vertical face 
and then slopes back upward to the top of the laser. Depending on the 
medium, a slight horizontal shelf region may be formed between the 
vertical cut and the inclined mirror, which gives the resulting milled 
notch a three sided configuration. The notch may be milled to a smooth 
finish by reducing the intensity of the ion beam for a short period of 
time after the geometry of the milled features are substantially 
completed. In actual tests, the inventors found that is was useful to 
reduce the accelerator voltage to 250 volts and the accelerator current to 
200 mA for about 30 minutes at the end of the entire milling procedure. 
The notches may then be finished by applying various stabilizing, 
dielectric, or metal coatings to enhance the laser performance or maximize 
reflectivity. Various reactive chemicals may also be injected into the 
chamber at the conclusion of the milling process in order to remove slag 
that sometimes forms at the base of the notch. 
While the method recited above calls for a fixed ion beam angle throughout 
the entire milling process, persons possessing ordinary skill in the art 
will readily understand that it would be possible to vary the orientation 
of the ion beam according to a time dependent function during the milling 
operation. This would enable the production of a nearly infinite variety 
of any desired planar, non-planar, curved, or irregular surface to be 
formed on a given workpiece. Similarly, the workpiece itself need not be 
limited to a planar surface. 
The ion mlling method is not limited to workpieces composed of 
semiconductor materials. Any substance that can be placed in an evacuated 
chamber that is susceptible to milling by atomic bombardment may be 
utilized. Because of the broad range of semiconductors, metals, and 
glasses that may be shaped using this technique, the ion milling method 
may be employed ot produce any radiation interface, absorber, transmitter, 
or reflector on a microscopic scale. 
The devices produced using the method of the present invention may be more 
clearly understood by referring to the twelve drawing figures. FIG. 1 is a 
perspective view of one of the early versions of a cube shaped 
semiconductor laser diode 10 that is well known to persons ordinarily 
skilled in the art. The laser 10 emits radiation from a narrow region 
coincident with the lateral edges of a junction that encloses an active 
layer 12 at the mid-section of its side walls 14. Laser emission is 
stimulated by applying a forward bias voltage through metal contacts 18 
and leads 20 and 22 that causes excess holes from the p layer and excess 
electrons from the n layer to migrate to the active layer 12 and 
recombine. One great disadvantage of this design is that the laser 
radiation diverges in a weak, fan-shaped beam 16. Another problem with 
this version of the laser diode is that the largest useful array that can 
be assembled using this configuration is a long strip or bar one or two 
diodes wide. 
FIG. 2 is a cross-sectional view of a semiconductor laser 24 produced in 
accordance with the present invention. Two cleaved end surfaces 26 and 28 
provide the lateral boundaries for six horizontal strata: a GaAs:Si 
substrate 30, a GaAs:Se buffer layer 32, a lower GaAlAs:Se cladding layer 
34, ana ctive layer 36 composed of a laser gain medium, an upper cladding 
layer 38 composed of GaAlAs:Zn, and a GaAs:Zn contact layer 40. A 
generally two-sided notch or cut 42 formed by the methods claimed below 
includes a substantially vertical wall 44 and an inclined mirror 46, which 
is shown extending down into the laser 24, traversing the active layer 36 
and terminating at a notch intersection 48 in the vicinity of the lower 
cladding layer 34. Actual tests performed by the inventors have produced 
notches measuring only a few millionths of a meter in depth. When the 
laser is stimulated by applying a bias voltage, a population inversion is 
created in the active layer 36 and laser light propagates back and forth 
between the cleaved end surfaces 26 and 28. Although the method of the ion 
milling invention allows for the fabrication of a surface feature such as 
mirror 46 at virtually any angle with respect to the longitudinal axis of 
the workpiece, the 45 degree configuration shown for illustrative purposes 
in FIG. 2 is one of the most useful embodiments of the present invention. 
The advantage of the 45 degree mirror is that the output of the laser is 
directed up and out of the notch 42 instead of the more usual emission in 
the plane of the active region 36. The ion-milled mirror 46 provides an 
inexpensive, reliable, and effective means of producing huge arrays of 
surface emitting lasers that each contribute powerful beams of laser 
radiation 50 propagating in a direction orthogonal to the plane of the 
array. 
Although the ideal shape of notch 42 is two-sided, various physical 
phenomena sometime contribute to create a third segment referred to as a 
shelf region 52 between vertical wall 44 and the inclined rror 46. In 
addition to depicting the elaborate surface stripe geometry 53 of another 
of the embodiments of the present invention, FIG. 3 provides a perspective 
view of a generally three-sided notch 54 comprising a vertical wall 44 and 
an inclined mirror 46 connected by a curved shelf region 52. FIG. 4 
includes a cross-sectional view of an ion milled semiconductor laser with 
a three sided notch 54. FIGS. 13a and 13b are reproductions of photographs 
of actual results of the ion milling process. 
FIGS. 5 and 6 present schematic 60 and perspective illustrations of the ion 
gun apparatus 60 that is employed in the ion milling method. The device 
used by the inventors to perform actual tests reported in FIGS. 7 through 
12 was a Microetch System manufactured and sold by Veeco. This device is 
readily commercially available laboratory equipment that is well known to 
those skilled in the art. 
FIG. 12 is an empirical plot collected during actual tests of semiconductor 
lasers fabricated using the ion milling method. The solid curve 64 and the 
dashed curve 66 show that in order to fabricate a surface feature such as 
a 90 degree surface at the same time as an inclined mirror at an angle of 
45 degrees (y-axis), the beam angle between the ion gun and the wafer must 
be approximately 60 degrees (y-axis), or 30 degrees (x-axis). If a single 
surface away from the beam (curve 64) is desired the slope of curve 64 is 
1.5 and it therefore follows that the beam angle (y-axis) must generally 
be 2/3 the value of the surface feature that is to be ion milled on a 
workpiece. 
Although the present invention has been described in detail with reference 
to a particular preferred embodiment, persons possessing ordinary skill in 
the art to which this invention pertains will appreciate that various 
modifications and enhancements may be made without departing from the 
spirit and scope of the claims that follow.