US3473974A

US3473974A - Utilization of trace impurities in the vapor growth of crystals

Info

Publication number: US3473974A
Application number: US616101A
Authority: US
Inventors: John W Faust Jr; Harold F John
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1967-02-14
Filing date: 1967-02-14
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21

Description

Oct. 21, 1969 w, us'r, JR ETAL 3,473,974

UTILIZATION OF TRACE IMPURITIES IN THE VAPOR GROWTH OF CRYSTALS Filed Feb. 14, 1957 ./RES|STANCE HEATERS :4 22 ,l2 .2

iv d 7i! uuvvmvn fic 8 30 I 2 A 2" 45 5o 4 44 F|G'3 48 56 4o 42 5.1 46 I24 54 R 425 .424, 3% S WWII/III] 54 44 48 n FIG.4 42 i A FiG.8 um

WITNESSES INVENTORS John W. Fousi, Jr. and w Hcrold F. John ATTORNEY ILS. Cl. 143-174 2 Claims ABSTRACT OF THE DISCLOSURE A process for growing dendritic crystals from a vapor which consists of disposing a seed, a material source and a source of a growth inhibitor or promotor in a tubetype furnace and heating the material source and growth inhibitor or promotor to an elevated temperature whereby the material and inhibitor are deposited on the seed. The

inhibitor or promotor is used to control the thickness of the crystal.

As an object of the present invention is to provide a means for controlling the thickness of a dendritic crystal grown by the vapor growth technique by the addition of trace impurities in vapor stream.

This invention relates to a crystal growth process, and is particularly directed to the growth of dendritic crystals of materials crystallizing in the diamond cubic lattice, zinc blend structure, or face centered cubic structure.

Another object of the present invention is to provide a method of controlling the thickness of a dendritic crystal while growing the dendritic crystals of a material crystallizing in the diamond cubic lattice, zinc blend, or face centered cubic structure by the deposition of a vapor upon selective portions of a seed crystal containing at least one twin plane. 1

Other objects of the present invention will, in part, be, obvious and will, in part, appear hereinafter.

For a better understanding of the nature and objects of this invention reference should be had to the following detailed description and drawings, in which:

FIGURE 1 is a schematic view in cross-section of one typet of furnace suitable for use in accordance with the teachings of this invention; and

FIGS. 2 to 10 are fragmentary views of seed crystals suitable for use in accordance with the teachings of this invention.

In accordance with the present invention and obtainment of the foregoing objects there is provided a process for growing a body of material having a crystal structure selected from the group consisting of diamond cubic lattice, zinc blend and face centered cubic comprising vaporizing a charge of the material to be grown by heating it to a temperature T which is within the vaporization temperature range of the material to be grown and which provides a rate of vaporization of from 0.01 to 0.4 microgram per square centimeter per second of the material to be grown, and vaporizing a charge consistinng of an impurity capable of affecting growth in the l1l direction by heating it to a temperature T which is within the vaporization temperature range of the impurity and which provides a vapor pressure of from 10- to 10- atmospheres passing the two vaporized materials over at least a preselected surface of a seed while maintaining the temperature of the vapor at substantially temperature T said seed being of a material having a crystal structure selected from the group consisting of diamond cubic lattice, zinc blend lattice and face centered cubic lattice, said seed containing at nited States Patent 0 ice least one twin plane, said at least one twin plane being exposed at said preselected surface, said preselected surface of the seed being at a temperature T said temperature T being no greater than the condensation temperature of the material to be grown, and T -T being at least 5, said vapors condensing on said preselected surface of the seed in a continuation of the crystal lattice structure of the seed.

It has been discovered that dendritic crystals of solid materials crystallizing in the diamond cubic lattice, zinc blend or face centered cubic structure may be prepared having a closely controllable thickness and with relatively precise flat parallel faces. These fiat dendritic crystals may be grown by depositing vapors of the material and vapors of an impurity affecting growth in the 1ll direction on a seed crystal. The thickness of the crystals may be readily controlled and surface imperfections minimized or reduced by following the teachings of this invention.

Materials crystallizing in the diamond cubic lattice structure, zinc blend structure and face centered cubic lattice structure can all be grown in the form of dendritic crystal by practicing the present invention. The materials are silicon, germanium, stoichioinetric compounds having an average of four valence electrons per atom such as compounds of elements from Group III and Group V of the Periodic Table, compounds of elements from Group II and Group VI of the Periodic Table.

Examples of compounds of elements from Groups III and V of the Periodic Table are the Group III elements aluminum, gallium, boron and indium combined with the Group V elements phosphorus, arsenic and antimony.

Examples of compounds of elements from Groups II and VI of the Periodic Table are zinc selenide (ZnSe) and zinc sulfide (ZnS).

The impurities which affect the growth of the dendrites by either increasing or decreasing the thickness of the dendrites are certain metals.

It should be understood that while the process of the present invention lends itself readily to the preparation of materials suitable for use in the preparation of semiconductor devices, the materials so produced may find other and varied uses.

It should be further understood that this process may be used to grow intrinsic dendritic crystals, crystals with a p-type conductivity, n-type conductivity and crystals with one or more p-n junctions.

For a detailed explanation of the process of this invention reference should be had to FIG. 1 which is a schematic view in cross-section of a furnace 10 suitable for use in practicing this invention.

The furnace 10 shown in FIG. 1 is the usual tube type furnace used in vapor growth. It has a tubular wall 12 and is closed at ends 14 and 16.

It should be understood that the type of furnace known to those skilled in the art as an open-end vapor furnace may also be employed in practicing the teachings of this invention.

A crucible 18 of an inert material as for example graphite is disposed on any suitable mounting means at approximately the center of the furnace 10. A charge 20 of a material crystallizing in a lattice structure selected from the group consisting of diamond cubic, zinc blend and face centered cubic and which it is desired to grow into a dendritic crystal is disposed in the crucible 20.

Suitable heating means as for example a resistance heating coil 22 is positioned around the crucible 18 and charge 20 outside of the wall 12.

A seed crystal 24 consisting of a material crystallizing in a lattice structure selected from the group consisting of diamond cubic, zinc blend and face centered cubic is disposed on a platform 26 at one end, for example end 16, of the furnace 10.

A heating means, for example, a resistance heating coil 28, is disposed around the seed crystal 24 or at least about tip 30 of the seed crystal 24 outside of the wall 12.

A second crucible 32 of an inert material as for example graphite is disposed on any suitable mounting means at the other end, end 14 of the furnace. A charge 34 of a growth affecting impurity is disposed in the crucible 32. A heating coil 27 is disposed about crucible 32 outside the wall 12.

The crucible 18 containing the charge 20 of the material grown into a dendritic crystal is disposed in the furnace between the seed 24 and the crucible 32 containing the impurity 34.

The seed crystal 24 is of a material having a crystal lattice structure selected from the group consisting of diamond cubic, zinc blend and face centered cubic. The seed will usually be of the same material as that comprising the charge 20 but it need not be as long as it has the same crystal lattice structure.

The seed crystal 24 must contain at least one twin plane and the twin plane structure must be exposed at the tip 30 of the seed crystal 24.

Seed crystals containing one twin plane can be employed in growing dendritic crystals of Group III-V compounds, Group II-VI compounds and metals. The growth facets being (111) and (100) planes.

Seed crystals containing two or more twin planes can be employed in growing silicon, germanium, silicon carbide, Group III-V compounds, Group II-VI compounds and metals. The growth facets being (111) along (111) planes.

Referring to FIG. 2 of the drawing, there is illustrated in greatly enlarged view, a section of a seed crystal 124 having three

internal twin planes

40, 42 and 44 extending entirely across tip 130. The seed crystal 124 has two relatively flat

parallel faces

46 and 48. Examination will show that the crystallographic structure of the preferred seed on both

faces

46 and 48 is that indicated by the crystallographic direction arrows at the top and bottom faces respectively, of the figure.

The spacings or lamellae between the successive

adjacent twin planes

40, 42 and 44 ordinarily are not the same. The lamellar spacings such as a between

twin planes

40 and 42 and b between

twin planes

42 and 44 are of the order of microns, that is from a fraction of a micron to to microns or possibly greater. The ratio of a to b as determined from studies of numerous grown dendritic crystals has varied in the ratio of slightly more than one to as much as 18. In all cases all the twin planes, in good dendritic seed crystals, extend entirely through the seed and extend entirely across the width of the seed or at least entirely across that tip upon which growth is to be initiated. Where the twin planes terminate internally, the seed crystal behaves as if no such twin planes are present insofar as growing dendrites from a vapor.

With reference to FIG. 2, FIG. 3, which is a view of the seed of FIG. 2 taken along the line A-A', and FIG. 4, which is a view of the seed of FIG. 2 taken along the line B-B', it can be seen that

planes

50 and 52 form a ridge designated as R in the profile of the seed at twin plane 40 on the sides and grooves designated as G in the profile of the seed at twin plane 40 at the tip. Planes 54 and 56 form a similar ridge at twin plane 44 on the sides and a groove on the tip.

Planes

52 and 56 form a groove designated as G in the profile of the seed crystal at twin plane 42 on the sides and a ridge on the tip.

The seed crystal of FIG. 2 may be comprised of silicon, germanium, or be of a stoichiometric compound having an average of 4 valence electrons per atom or a face centered cubic metal.

With reference to FIG. 5, there is illustrated a seed crystal 224 also suitable for use in the process of this invention. The seed crystal 224 contains two

twin planes

240 and 242, respectively, and has relatively

flat t'aces

246 and 248.

FIG. 6 is a view of the seed crystal 224 taken along the line A-A and FIG. 7 is a view of the seed crystal 224 taken along the line BB'. In profile the twin plane 240 formed at the intersection of

planes

260 and 262 appears as a ridge on the sides and a groove at the no and is designated in FIGS. 5, 6 and 7 by R and the twm plane 242 formed at the intersection of

planes

262 and 264 is represented at a groove designated as G on the sides and a ridge on the tip in FIGS. 5, 6 and 7. The spacing between the

twin planes

240 and 242 is substantially that set forth above in the discussion of seed crystal 124.

A seed crystal such as 224 containing two twin planes may be used in the vapor growth of silicon, germanium and stoichiometric compounds having an average of valence electrons per atom and face centered cubic metals.

With reference to FIG. 8 there is illustrated a seed crystal 324 containing a single twin plane 340 formed at the intersection of

planes

360 and 362 and which in profile appears as a groove and is designated as G. The seed crystal 324 has a first flat surface 346 and a second flat surface 348. FIG. 9 illustrates a view of the seed crystal 324 taken along the line A-A' and FIG. 10 represents a view of the seed crystal 324 taken along the line BB'. The crystallographic orientation of the various faces of the seed crystal 324 is designated directly on the surface of the crystal in FIG. 8.

A seed crystal such as the type 324 illustrated in FIG. 8 and containing only one twin plane is particularly suitable for the vapor growth of dendritic crystals of stoichiometric compounds having an average of 4 valence electrons per atom. However, such seeds can also be used to grow silicon and germanium dendrites on face centered cubic metals.

The seed of FIGS. 2 to 10, may be prepared by any suitable means known to those skilled in the art; for example, (1) the seed may be a portion of a piece of a dendrite grown from a supercooled melt, (2) a platelet grown from solution, (3) a piece of previously grown vapor dendrite, or (4) a seed cut with a suitable geometry and number of twin planes.

Impurities which decrease the thickness and enhance the width of dendrites of designated materials are set forth in Table I. Table I also sets forth the temperature LO which the impurity must be heated and the vapor pressure of the impurity necessary to affect the growth of the dendrite.

TABLE I Vapor Dendritic Temp. Pressure Material C.) Atoms 550632 l0- iO- it 300430 l0- lO- SOO975 10' lO- Ag SiGe 1,300l,500 l0-" Hg- III-V cpds. -l75 ill- 10" Z11 III-V cpds 40[)600 IO lO" Y Impurities which increase the thickness and decrease the width of dendrites of designated materials are set forth in Table II. Table II also sets forth the temperature to which the impurity must be heated and the vapor pressure of the impurity necessary to effect the growth of the dendrites.

Referring again to FIG. 1, in practicing the teachings of this invention the charge 20 of the material to be grown into a dendritic crystal is disposed in the crucible 18 and the charge 34 of the impurity to affect growth is disposed in crucible 32. The two

crucibles

18 and 32 are disposed in the furnace with crucible 32 positioned at end 14 of the furnace 10 and crucible 18 toward the center of the furnace.

The charge 20 may be comprised of the material in the pure form as for example elemental silicon, germanium, or a III-V compound such as gallium arsenide, a ILVI compound such as zinc selenide or silicon carbide. In addition, the charge may be in the form of a chemical compound such as a halogenated silane.

The charge 34 may be any of the impurities listed in Table I or Table 11 depending on the desired result and the composition of charge 20.

The seed 24 is disposed toward end 16 of the furnace on the platform 26. As explained above, the particular twin plane configuration of the seed 24 is exposed at the tip 30.

The three

resistance heaters

22, 27 and 28 are energized.

The resistance heater 22 heats the charge 20 to a temperature T which is a vaporization temperature which will give a rate of vaporization of from 0.01 to 0.4 (mgm./cm. /sec.) micrograms per square centimeter per second.

It will be understood that the vaporization temperatures of the materials having diamond cubic lattice, zinc blend lattice and face centered lattice crystal structures are not unique temperatures but vary over a relatively wide temperature range. It can be further appreciated that the rate of vaporization varies over this range. These values are set forth in various reference books as for example, Vacuum Techniques, Dushman, John Wiley and Sons, 1949.

The resistance heater 27 heats the charge 34 of impurity to a temperature sufiicient to provide a vapor pressure of 10 to 10" atmosphere within the furnace. Suitable temperatures are given in Table I and Table II.

The resistance heater 28 heats the seed crystal 24 and particularly tip 30 of the seed crystal to a temperature T which is equal to or slightly below the condensation temperature of the material it is desired to grow as a dendrite.

The AT which is equal to T T can range from 5 to 50 if the charge 20 is comprised of the element to be deposited on the tip 30 of the seed crystal 24 in the elemental form or the material in elemental form plus a doping material or carrier. The AT can range from 25 to 400 if the charge 20 is comprised of the material to be deposited in the form of a chemical compound such as a halogenated silane.

The distance a between the charge 20 and tip 30 of the seed 24 is not important as long as the vapor temperature is approximately equal to T when it reaches the tip 30 of the seed 24; T is equal to the condensation temperature of slightly less than the AT is in the required range.

The distance 2 between the seed 24 and the impurity charge 34 is not important as long as the vapor pressure of the impurity within the furnace is from 10- to 10* atmospheres.

Depending upon the length of the furnace 10, it may be necessary to dispose additional resistance heating coils along the length of the furnace to ensure the necessary temperature conditions at the seed tip.

A detailed explanation of the growth process can be had by referring to FIG. 2. As the material in the form of a vapor impinges upon the tip 130 of the seed 124 and growth is initiated at the exposed groove of the twin planes. For example, the vapor contacts the tip 130 of the seed 124 at the groove of the twin plane 40 which is formed by the intersection of planes 50 and S2 and growth is initiated in an upward direction toward flat surface 46 and in a downward direction toward the edge of twin plane 42. Growth is also initiated at the same time at the exposed groove of twin plane 44, which is formed by the intersection of

planes

54 and 56 and proceeds upward toward the exposed edge of twin plane 42 and downward toward flat surface 48.

In a similar fashion growth takes place at twin plane 42 and the grooves at the sides of the seed 124 and proceeds upward toward twin plane 40 and downward toward twin plane 44.

As shown in FIGS. 2 and 3

twin planes

40, 42 and 44 have a cross-section of a groove, a ridge and a groove at tip 130, this cross-sectional configuration is propagated during growth.

With reference to FIG. 5, it will be understood that if a seed 224 containing two internal twin planes 240 and 242 is employed, growth will be initiated from the exposed groove of twin plane 242 formed by the intersection of

planes

260 and 262 and proceed in an upward direction toward flat surface 246 and in a downward direction toward the exposed edge of twin plane 242 formed by the intersection of

planes

262 and 264. Instantaneously, of course, growth will be initiated at the exposed edge of twin plane 242 formed by the intersection of

planes

262 and 264 on the sides and extend in an upward direction towards twin plane 240 and in a downward direction towards fiat surface 248.

With reference to FIG. 8, if a seed 324 containing a single twin plane is utilized, growth will proceed from the single twin plane 340 formed by the intersection of

planes

360 and 362 and extend in an upward direction toward flat surface 346 and in a downward direction toward flat surface 348.

Seed crystals containing two or more twin planes, such as that illustrated in FIGS. 2 to 7, inclusive may be used for growing silicon, germanium, silicon carbide, and stoichiometric compounds containing an average of 4 valence electrons per atom.

Seed crystals containing a single twin plane such as that illustrated in FIG. 8 are preferably utilized in the growth of stoichiometric compounds having an average of 4 valence electrons per atom. Normally such seeds are not as satisfactory for the growth of silicon or germanium dendrites as are seed crystals containing two or more twin planes.

The manner in which the impurities of this invention affect the growth process of dendritic crystals is not known. However, the impurities do in fact affect the growth and alter the grown dendrite.

The following examples are illustrative of the teachings of this invention.

Example I A closed end tube furnace of the type shown in FIG. 1 was employed.

A graphite crucible containing 5 grams of gallium arsenide was disposed at one end of the furnace.

A gallium arsenide seed having three internal twin planes extending therethrough and exposed at one end was disposed at the other end of the furnace.

The seed rested on a graphite slab. The end of the seed at which the three twin planes were exposed extended over the side of the slab and was pointed in the direction of the crucibles.

The furnace chamber was filled with approximately 0.25 atmosphere of hydrogen.

The gallium arsenide was vaporized by heating it to 750 C. and the tip of the seed was heated to 700 C. by resistance heaters. The vapor from the gallium arsenide was deposited preferentially on the tip of the seed.

The process was carried out for five minutes after which time the seed was removed from the furnace and examined.

It was found that growth of 3 cm. in length had taken 7 place on the seed tip. The new growth had a thickness of 1 mm. and a width of 4 mm. The three twin planes of the seed had been continued through the new growth. The surfaces of the newly grown material were smooth, mirror-like in appearance and had a [111] orientation.

Example 11 The process of Example -I was repeated except that the crucible containing the gallium arsenide was disposed in the center of the furnace and a crucible containing zinc was disposed in the furnace at the opposite end from the seed.

The gallium arsenide was again heated to 750 C. and the seed to 700 C. In addition, the zinc was heated to 500 C. which resulted in a vapor pressure of zinc of 0.004 atmosphere.

It was found that growth of 3 cm. in length had taken place. The new growth had a thickness of 0.9 mm. and a width of 8 mm.

Example III The process of Example II was repeated except that tellurium was substituted for the zinc. The tellurium was heated to 600 C. with a resultant vapor pressure of 0.007 atmosphere.

The new growth Was 3 cm. long, had a thickness of 1.66 mm and a width of 0.8 mm.

Example IV A closed tube furnace of the type shown in FIG. 1 was employed.

A graphite crucible containing grams of hyper-pure silicon was disposed at one end of the furnace.

A silicon dendritic seed crystal having three internal twin planes extending therethrough and exposed at one end was disposed at the other end of the furnace.

The silicon was vaporized by heating to 1000 C. and the tip of the seed was heated to 950 C. by resistance heaters.

The vapor from the silicon was deposited preferentially on the tip of the seed.

It was found 2.5 cm. of growth had taken place on the seed tip. The dendrite had a thickness of 1.1 mm. and a width of 4.1 mm. The three twin planes of the seed had been continued through the new growth.

The surfaces of the newly grown material were smooth, mirror like in appearance and had a [111] orientation.

EXAMPLE V The procedure of Example IV was repeated except that a crucible containing thallium was disposed at the end of the furnace opposite the seed.

The thallium was heated to 925 C., resultant vapor pressure was 0.007 atmosphere.

The new growth was 2.5 cm. long, had a thickness of 1 mm. and a width of 8.3 mm.

EXAMPLE VI The procedure of Example IV was repeated except that a crucible containing gallium was disposed within the furnace at the end opposite the seed.

The gallium was heated to 1400 C., resultant vapor pressure was 0.005 atmosphere.

The new growth of 2.5 cm. long, had a thickness JI 1.82 mm. and a width of 3.9 mm.

EXAMPLE VII A closed end tube furnace of the type shown in FIG. was employed.

A graphite crucible containing 5 grams of germanium was disposed at one end of the furnace.

A germanium seed having three internal twin planes extending therethrough and exposed at one end was disposed at the other end of the furnace.

The germanium was vaporized by heating to 480 C. and the tip of the seed was heated to 450 C. by resistance heaters. The vapor from the germanium was deposited preferentially on the tip of the seed.

The process was carried out for live minutes after which time the seed was removed from the furnace and examined.

It was found that 3 cm. of growth had taken place on the seed tip. The new growth had a thickness of 1 mm. and a width of 5.3 mm. The three twin planes of the seed had been continued through the new growth.

The surfaces of the newly grown material were smooth. mirror like in appearance and had a [111] orientation.

EXAMPLE VIII The procedure of Example VII was repeated except that a crucible containing silver was disposed at :he opposite end of the furnace from the seed.

The silver was heated to 1400 C. with a vapor pressure of 0.005 atmosphere.

The new growth was 3 cm. long, had a thickness or 0.9 mm. and a width of 10.2 mm.

EXAMPLE IX The procedure of Example VII was repeated except that a crucible containing zinc was disposed at the onposite end of the furnace from the seed.

The zinc was heated to 525 C. with a resultant vapor pressure of 0.004 atmosphere.

The new growth was 3 cm. long, had a thickness at 1.7 mm. and a width of 5.1 mm.

The semiconductor materials prepared in accordance with this invention are suitable for use in making semiconductor devices.

While the invention has been described with reference to particular embodiments and examples, it will be understood, of course, that modifications, substitutions and the like may be made therein without departing from 15 scope.

We claim as our invention:

1. A process for growing a body of material having a crystal structure selected from the group consisting or diamond cubic lattice, zinc blend and face centered cubic. on a seed crystal comprising vaporizing a charge of the material to be grown by heating it to a temperature T. which is within the vaporization temperature range or the material to be grown and which provides a rate of vaporization of from 0.01 to 0.4 microgram per square centimeter per second of the material to be grown, and vaporizing a charge consisting of an impurity capable of decreasing the thickness and increasing the width of said crystal selected from the group consisting of tellurium. selenium, thallium and silver fora silicon or germanium seed crystal, or selected from the group consisting of mercury and zinc for a IIIV compound seed crystal, by heating it to a temperature T which is within the vaporization temperature range of the impurity and which provides a vapor pressure of from 10 to 10 atmospheres. passing the two vaporized materials over at least a preselected surface of said seed crystal while maintaining the temperature of the vapor at substantially temperature T said seed being of a material selected from the group consisting of silicon, germanium and III-V compounds, said seed containing at least one twin plane, said at least one twin plane being exposed at said preselected surface, said preselected surface of the seed being at a temperature T said temperature T being no greater than the condensation temperature of the material to be grown, and T -T being at least 5, said vapors condensing on said preselected surface of the seed in a continuation of the crystal lattice structure of the seed.

2. A process for growing a body of material having a crystal structure selected from the group consisting of diamond cubic lattice, zinc blend and face centered cubic, on a seed crystal comprising vaporizing a charge of the material to be grown by heating it to a temperature T which is within the vaporization temperature range of the material to be grown and which provides a rate of vaporization of from 0.01 to 0.4 microgram per square centimeter per second of the material to be grown, and vaporizing a charge consisting of an impurity capable of increasing the thickness and decreasing the width of said crystal selected from the group consisting of gallium, gold and zinc for a silicon or germanium seed crystal, or selected from the group consisting of tellurium and selenium for a III-V compound seed crystal, by heating it to a temperature T which is within the vaporization temperature range of the impurity and which provides a vapor pressure of from 10* to l() atmospheres, passing the two vaporized materials over at least a preselected surface of said seed crystal while maintaining the temperature of the vapor at substantially temperature T said seed being of a material selected from the group consisting of silicon, germanium and III-V compounds, said seed containing at least one twin plane, said at least one twin plane being exposed at said preselected surface, said preselected surface of the seed being at a temperature T said temperature T being no greater than the condensation temperature of the material to be grown, and T T being at least 5", said vapors condensing on said preselected surface of the seed in a continuation of the crystal lattice structure of the seed.

References Cited UNITED STATES PATENTS 3,152,022 10/1964 Christensen 148175 3,192,072 6/1965 Ziegler et al. 1481.6 3,341,376 9/1967 Spence et al. 117-106 XR 3,344,002 9/1967 Sirtz et al. l48l75 L. DEWAYNE RUTLEDGE, Primary Examiner P. WEINSTEIN, Assistant Examiner US. Cl. X.R.