GB2132637A - Process for depositing dielectric films in a plasma glow discharge - Google Patents

Abstract

In a process for forming dense and tightly adhering phosphorus or boron or phosphorus/boron-doped silicon oxide films, on the surface of conductors, semiconductors, and insulators, an inert gas, or a gaseous oxygen-containing or nitrogen containing compound is introduced into a plasma reaction chamber at a point 44 upstream of the point 36 where silane gas diluted with inert gas and the dopant gas also diluted with inert gas are introduced to the chamber, and an RF field applied to generate a plasma. Uniform deposition with negligible inclusions of foreign matter is achieved. <IMAGE>

Description

SPECIFICATION Process for depositing dielectric films in a plasma glow discharge This invention relates in general to a method of depositing dielectric films on a substrate, and more particularly it concerns a method of and apparatus for depositing dense and tightly adhering phosphorus-doped, boron-doped, and boron/phosphorus-doped dielectric films on a substrate during the manufacture of semiconductor devices.

It is well known that the controlled dissociation of polyatomic molecules and recombination of the resulting active species in a low pressure, low temperature, glow discharge plasma is a feasible method for depositing films on the surface of a variety of substrates. It has been observed, however, that such films which are formed by use of the glow discharge system and methods known to date, exhibit either a slow rate of growth or any of several undesirable film properties, such as porosity, inclusion or contaminant substances, and high interfacial stresses, particularly when overlaying highly contoured patterned structures. While these problems exist in any method employed for the deposition of dielectric films they are particularly acute in the preparation of phosphorus-doped silicon oxide films during the manufacture of semiconductor devices.

It is believed that such undesirable characteristics occur at least in part because reactant gases are either premixed or otherwise introduced together into the reaction chamber in a manner whereby both gases are subject to an exciting electric field, or are simultaneously exposed to a high thermal field. In many cases the heat transfer characteristics of the gaseous reaction environment are very good, thereby promoting the formation of powder in the gas phase in addition to film deposition onto the preheated substrate. The powder, which is the consequence of gas phase polymerizations, may be incorporated into the growing film on the heated substrate. This will result in a high degree of film porosity and a high film defect density.

This situation is further aggrevated by the mode of operation associated with such systems. In many instances these deposition systems are batch loaded, which requires venting to atmospheric pressure at the end of the deposition cycle to remove the work substrates. The repetitive cycling between vacuum and atmospheric pressure leads to turbulence within the reaction chamber, thereby dislodging loosely adhering film particulates that may be incorporated into the new film during the next deposition cycle.

It is known, for example, that during the deposition of silicon oxides (doped or undoped) using silane, complex polymerization reactions occur in the glow discharge. The main volatile products of such polymerization reactions are di- and tri-silane derivatives and hydrogen, while nonstoichiometric subhydrides separate out as precipitates. Apparently the most important initial step in the discharge reaction of silane is the breaking of the Si-H bond with the formation of either or both a sylyl or sylene intermediate. The reaction mechanism leading to sylene formation is favoured because it is exothermic.

It is worth noting that in such systems, the formation of the aforementioned undesirable byproducts is enhanced by utilization of relatively high concentrations in the gaseous mixture of silane (SiH4) and phosphorus-bearing compounds (e.g., phosphine, PH3). The situation is further aggravated by the common employment of either relatively high deposition temperatures (in excess of 4500C) or high power electric excitation fields (hundreds of watts).

If the reactant gases are premixed or otherwise introduced into the reaction chamber, together, and thereby are jointly subjected to a high powered exciting electric field or to a high temperature thermal field, adverse ion-molecule reactions may occur resulting in highly complex reaction products.

In addition to common electron impact phenomena leading to the formation of ions, there is a high probability of single collisions of, for example, fast Si atoms or ions (because of their low ionization potential) with silane molecules. Ions produced in these reactions have relatively low kinetic energies and therefore will undergo further chemical reactions.

Some of the most probable of these reactions are hydride-ion transfer reactions which have large rate constants in silane. The following reactions in the silane system can yield products containing two silicon atoms: Si+ + SiH4eSi2H2+ + H2 SiH+ + SiH4~Si2H2+ + H2 SiH3+ + SiH4~Si2H+ + 2H2 etcetera. These reactions are all exothermic and will compete with oxygen atoms for silane and its derivatives, thus suppressing silicon oxide formation. The possibility of inclusion of the various volatile by-products in the silicon oxide films is also enhanced, thereby degrading dielectric film properties.

It is therefore an object of this invention to provide an improved process to produce uniform dielectric films with negligible inclusions of foreign matter.

The present invention is a method of depositing phosphorus doped, boron doped, or boron/phosphorus doped silicon compounds on a substrate maintained at a temperature less than 3400C within a plasma reaction chamber comprising the steps of, (a) introducing into a plasma chamber silane gas diluted with a noble gas, (b) introducing into said chamber a phosphorus containing, boron containing, or phosphorus/boron containing, gaseous compound diluted with a noble gas, (c) introducing upstream of said diluted silane and upstream of said diluted phosphorus or boron gaseous compound, a gaseous oxygen containing compound or nitrogen containing compound, said gases being introduced into said reaction chamber, and (d) applying an RF electric field to said plasma reaction chamber to generate a plasma from said introduced gases.

The substrate wafers are heated to a temperature not to exceed 3400 by contact with a wafer track, which is resistance heated and maintained at ground potential.

For the purpose of deposition of pure silicon oxide in accordance with the process of this invention, a binary mixture of extremely diluted silane: 1.75% silane/balance argon, is admitted through the conductive gas dispersion device 34 (Fig. 1).

For the purpose of depositions of phosphorus-doped silane oxides a ternary gas mixture of extremely diluted silane (1.75%) and extremely diluted phosphine ( < 1%) in argon is admitted to the discharge gap through gas dispersion device 34 (Fig. 1). For depositions of boron-doped silicon oxides a ternary gas mixture of extremely diluted boron trifluoride (S1 ordiborane (41 %) and extremely diluted silane (~1.75%) in argon is admitted to the discharge gap.For depositions of boron/phosphorusdoped silicon oxides a quaternary gas mixture of extremely diluted boron trifluoride (61 %) or diborane (41 %) and diluted phosphine (S1 %) in extremely diluted silane (~1.75%) in argon is admitted to the discharge gap. The same gas dispersion device is employed.

For both undoped and doped silicon oxides, another reactant gas is dispersed upstream of the aforementioned multi-component gaseous mixtures. Commonly, nitrous oxide (N20) is employed. The described process will produce silicon nitride (doped and undoped) films if nitrous oxide is replaced by molecular nitrogen or another nitrogen-bearing compound.

In addition to employing the lowest concentrations of reactant silane (SiH4), phosphine (P H3), boron trifluoride (by3), or diborane (B2H6), in the source gas, the total RF excitation power is in the range 45 to 1 OOW. This corresponds to extremely low power densities of 0.1 1 to 0.1 9W/cm2 for 4" dia.

wafers.

Although very low concentrations of silane and dopants are employed, coupled with low deposition temperatures and low RF power levels, the observed average deposition rates are high (in excess of 2000A/min.). The corresponding dopant concentrations in the solid silicon oxide films are in the practical range of 2 to 11 atomic weight %. One reason for these results may be the relative suppression of gas phase side reactions (polymerizations) that may otherwise deplete the vital reactants. This is accomplished by a carefully balanced ratio of the reactants' partial pressures, coupled with the poor heat transfer from the heated wafer to the gas phase by Argon and nitrous oxide-gases which exhibit poor heat conductivities.The relative suppression of gas phase nucleation or polymerization reactions above the wafer's surface practically eliminates the inclusion of foreign matter in the growing films.

It is important to note that the relatively low percentage of ( < 1%) of phosphine (PH3) employed in the gas source did not cause a reduction of the average deposition rates when compared to undoped oxide deposition.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 illustrates an individual reaction chamber constructed to support the novel glow discharge deposition process disclosed herein. Such a chamber is one of five reaction chambers commonly mounted on a main evacuated vessel; Fig. 2 is a section view taken along line 2-2 of Fig. 1; and Fig. 3 illustrates, in diagrammatic form, an in-line plasma discharge deposition system constructed in accordance with the principles of the novel deposition process.

Fig. 1 illustrates an individual reaction chamber 10 made of a non-conductor (e.g., quartz), or made of metal (e.g., aluminum). In the latter case the RF coil 1 6 would be eliminated from the structure.

This reaction chamber is one of five similar reaction chambers encountered on the in-line system such as that manufactured by LFE Corporation of Clinton, Massachusetts under the designation 8500N/O.

Reaction chamber 10 is positioned atop base plate 1 2 and is sealed thereto by means of O-ring gasket 14. Chamber 10 has a generally cylindrical shape and typically has dimensions of 1 50 mm OD and is about 1 50 mm high. A multiturn coil forming inductor 16 is wound around chamber 10 and has its ends coupled to a RF power source.

Below RF coil 1 6 is located the gas dispersing device 34 made of conducting material such as aluminum. The construction of this device is described in U.S. Patent 4,066,037. Tube 32 extends axially through chamber 10 and serves as the gas inlet feed tube which is also coupled to the RF power source. The RF power source would typically have a frequency of 1 3 MHz and maximum power output of a few hundred watts.

Below and parallel to the gas dispersing device 34, which also serves as the RF electrode, there is positioned wafer 24 located concentrically about the inlet tube 32 by stop pin 5. The wafer rests on a heated and temperature-controlled vibratory track 18 in intimate contact with heater 25. The bottom base plate 1 5 which has a concentrically-located exhaust port 30 connected to a vacuum pump, is located beneath heater 25. The top and bottom base plate 12 and 1 5, along with the corresponding vertical members adjoining the former (not shown) form the main evacuated vessel which houses five reaction chambers similar to the one described herein.

For deposition of phosphosilicate giass (PSG), typically, gases containing silicon and phosphorus in a diluent inert gas are admitted to chamber 10 via inlet tube 32. Preferably these gases are silane diluted to a concentration of 1.75% and phosphine (PH3) diluted to a concentration of < 1% in argon. For deposition of borosilicate glass (BSG) or boro/phosphosilicate glass (BPSG), gases including boron trifluoride, or diborane of 61 %, or combinations of diluted phosphine ( < 1 %) with boron trifluoride (61 %), or diborane (61 %) in a large dilution of silane (~1.75%) in argon are employed. Gas dispersion device 34 is circular in shape and is positioned about 0.5" above wafer 24.Numerous holes or jets 36 in device 34 uniformly distribute the multi-component gas mixture over the surface of substrate 24.

A second gas, substantially nitrous oxide (N2O), is admitted to reaction chamber 1 via inlet tube 40 which is connected to a gas dispersion head 42 located near the top of the chamber. Gas dispersion head 42, which is also shown in the sectional view 2-2 of Fig. 2, has numerous holes or jets 44 adapted to disperse gas uniformly through chamber 10.

Nitrous oxide (N20) may be replaced by N2 or argon for the deposition of (undoped or doped) silicon nitride or silicon, respectively.

For the deposition of phosphosilicate glass (PSG), substrate 24 is admitted to the deposition zone defined by reactor 1 0 employing the vibratory wafer track in conjunction with load locks shown in Fig.

3.

Substrate 24 is dispensed from a standard wafer cassette and conveyed to load lock 70 of Fig. 3.

While isolation valve 75 is open, isolation valve 76 is closed as is isolation valve 77 on the unload lock 80.

While wafer 24 is maintained at a temperature not to exceed 3400C, a silane/phosphine/argon ternary gas mixture is admitted to each deposition zone via inlet tubes 32 located on each of the five reaction chambers 1 0. Nitrous oxide gas is admitted to each of the five reaction chambers via inlet tubes 40 located on each of the five reaction chambers 1 0.

When wafer 24 has arrived in load lock 70, valves 75 and 78 close, and locks 70 and 80 are pumped down to about 100 microns Hg. Upon attainment of this preset pressure, the inner valves 76, 77 open, and the wafer is conveyed to a first deposition zone where it is positioned by contact with stop pin 5 in Fig. 1. Inner valves 76 and 77 then close, and total RF power in the range 45-100W is activated to excite all five deposition zones simultaneously. This creates a well-confined glow discharge in the volume between dispersion device 34 and wafer 24.

The ternary gas mixture containing silane, phosphine, and argon, is dispersed immediately above the substrate, and reaction occurs between the activated oxygen (from the discharged nitrous oxide), silicon, and phosphorus (from the discharged SiH4 and PHs, respectively) to provide a uniform and contamination-free coating of phosphosilicate glass (PSG) of high quality on the surface of substrate 24; the suppression of ion-molecule reactions in the deposition zone by the low concentrations of silane and phosphorus in the gas phase results in defect-free PSG.

The correspondingly high reaction pressures (2-5 mm Hg) coupled with extremely low total RF power levels enable exceptionally good step coverage or conformal deposition profiles above highly contoured device structures, and reduce the probability of radiation damage. Boro/phosphosilicate glass (BPSG) films will enable the employment of relatively lower reflow temperatures over those encountered with PSG films having similar phosphorus content. In fact, reflow temperatures of BPSG films will be lower with relatively lower phosphorus content than PSG films with higher phosphorus content. This experimental fact will result in enhanced device reliability by reducing the probability of forming corrosive phosphoric acid when devices are exposed to moist atmospheres.This enables the potential utilization of this process and method in the fabrication of MOSFETS as well as bipolar devices.

As wafer 24 is coated in the first deposition zone, another substrate is admitted to load lock 70.

Upon termination of the deposition cycle in the first zone which is a 1/5 of the total programmed deposition cycle, substrate 24 moves from the first zone to the second zone and the next new substrate is transported from unload lock 70 to the first zone. Inner valves 76 and 77 then close and another deposition cycle commences. This sequence is repeated as long as wafers are dispensed to load lock 70. Processed wafers exit via unload lock 80 which works in parallel with load lock 70, i.e., loading and unloading occur simultaneously.

Principally, the same reactions described above for the in-line system, can occur in a batch system, whose reaction chamber was described in U.S. Patent 4,066,037 with the proper modification of the reaction gases to include N20 and PH3, etc.

It has been found that the gas reactants and operating parameters set forth in the table below, when used in conjunction with the described apparatus and methods, will produce dense and adhering films during the manufacture of semiconductor devices.

TABLE I Formation of Phosphosilicate Glass (PSG) by Glow Discharge Reactants: Pure nitrous oxide (USP 99.5%) and ternary mixture of Silane (1.77%) Phosphine (0-0.898%) in Argon by volume.

System: LFE 8500 N/O, employing reaction chambers of quartz 150mm OD x 150 mmH.

System connected to 56 cfm pump via a throttle control valve and large area paper filter. Distance RF electrode-to-wafer: 0.550".

Track temperature: 340 C.

Gaseous Gaseous N2O N2O SIH4/PH3/Ar Reactant Reactant Press. Per N2O Flow Per N2O Press. Per SiH4/PH3/Ar PH3. Concn. Ratio (O:Si) Ratio(O:P) Reactor Tot. Press. Reactor Tot. Flow Reactor Tot. Press.

Run No. %(gas phase) Per Reactor Per Reactor ( Hg) ( Hg) (SCCM) (SCCM) ( Hg) ( Hg) 1 0.0190% 73:1 6767:1 450 1590 235 1220 350 1070 2 0.0395% 72:1 3261:1 450 1575 240 1240 350 1055 3 0.0598% 71:1 2153:1 450 1600 235 1215 350 1045 4 0.115% 74:1 1117:1 450 1575 240 1230 350 1050 5 0.199% 74:1 643:1 450 1600 235 1220 350 1045 6 0.320% 73:1 402:1 450 1595 240 1230 350 1040 7 0.898% 73:1 143:1 450 1600 235 1220 350 1050 8 0.199% 74:1 646:1 720 2480 595 2465 560 1720 9 0.320% 73:1 402:1 720 2840 595 2470 560 1780 TABLE I (Continued) Total SiH4/PH3/Ar SiH4/PH3/Ar Reaction Total Ave. Index of (2) P in deposit Etch (4) flow/Reactor Tot. flow Pressure RF Power Deposition Refraction film (3) Rate Run No. (SCMM) (SCMM) ( Hg) (W) Rate ( /min)(1) (nf) (at.Wt. %) -(A/min) 1 180 870 2415 45 1195 1.479 2.0 3395 2 175 870 2400 45 1232 1.480 2.3 3990 3 170 870 2400 45 1280 1.481 3.4 4570 4 170 830 2340 45 1212 1.486 4.7 6160 5 170 820 2415 45 1253 1.485 8 6070 6 170 820 2370 45 1233 1.846 9 8560 7 170 840 8420 45 1261 1.500 11 21300 8 420 1990 4290 75 2151 1.495 6.4 8265 9 420 2050 4320 75 2210 1.518 10 9245 (1) These rates were obtained by measuring the thickness of films which were deposited for 1.0 min.

The measurements were done in 5 places per wafer and averaged. Ellipsometry (#=63428A) was employed.

(2) Measurement refer to ellipsometry &commat;#=6328 ; 5 measurements per wafer were averaged.

(3) Phosphorus atomic weight percentages inferred from Auger Electron Specrtoscopy measurements.

(4) For the assessment of film density and phosphorus - doping level, values refer to 10:1 BOE solution (40% NH4F/49% HF 10:1 by volume, KTI Inc.) &commat; 30 C.

Claims (33)

1. A method of depositing phosphorus doped boron doped, or boron/phosphorus doped silicon compounds on a substrate maintained at a temperature less than 3400C within a plasma reaction chamber comprising the steps of, a) introducing into a plasma chamber silane gas diluted with a noble gas, b) introducing into said chamber a phosphorus containing, boron containing, or phosphorus/boron containing, gaseous compound diluted with a noble gas,

c) introducing upstream of said diluted silane and upstream of said diluted phosphorus or boron gaseous compound, a gaseous oxygen containing compound or nitrogen containing compound, said gases being introduced into said reaction chamber, and d) applying an RF electric field to said plasma reaction chamber te generate a plasma from said introduced gases.

2. A method as claimed in claim 1, wherein said applied RF power is in the range of 45 to 100W.

3. A method of depositing phosphorus doped silicon or silicon containing compound onto a substrate maintained at a temperature less than 340"C within a plasma reaction chamber comprising the steps of, a) introducing into a plasma reaction chamber silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and introducing into said chamber a phosphorus gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent, b) introducing upstream from said diluted silane and upstream from said diluted phosphorus gaseous component a gas selected from the group including noble gases, or oxygen containing, or nitrogen containing compounds, said gas being introduced into the reaction chamber at a total pressure in the range of 2 mm Hg. to 5 mm Hg., and

c) applying an RF electric field to said plasma reaction chamber to generate a plasma from said introduced gases, with the RF electric field having applied RF power in the range of 45-100W.

4. A method as claimed in claim 3, wherein said noble gas is argon.

5. A method as claimed in claim 3 or claim 4, wherein the gas introduced upstream of the diluted silane and diluted phosphorus compound is argon.

6. A method as claimed in claim 3 or claim 4, wherein the gaseous compound introduced upstream of said diluted silane and diluted phosphorus compound is nitrous oxide.

7. A method as claimed in claim 4, wherein the gaseous compound introduced upstream of said diluted silane and said diluted phosphorus compound is nitrogen.

8. A method as claimed in claim 4, wherein said phosphorus gaseous compound is phosphine.

9. A method of depositing boron phosphorus doped silicon compounds on a substrate maintained at a temperature less than 340C within a plasma reaction chamber comprising the steps of: a) introducing into a plasma reaction chamber silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and introducing into said chamber a phosphorus gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1%, the boron gaseous compound also diluted in a noble gas to be equal to or less than 1%.

b) introducing upstream from said diluted silane, upstream from said diluted phosphorus gaseous compound, and upstream from said borqn gaseous compound, a gaseous compound selected from the group including noble gases, oxygen containing or nitrogen containing compounds, all of said gases being introduced into the raction chamber at a total pressure tn the range of 2 mm Hg. to 5 mm Hg., anc

c) applying an RF electric field to said plasma reaction chamber to generate a plasma from said introduced gases, with the RF electric field having applied RF power in the range of 45-1 00W.

10. A method of depositing boron doped silicon compound onto a substrate maintained at a temperature less than 340"C within a plasma reaction chamber comprising the steps of: a) introducing into a plasma reaction chamber silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent and introducing into said chamber a boron gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent.

b) introducing upstream from said diluted silane and upstream from said diluted boron gaseous component a gaseous compound selected from the group including noble gases, nitrogen containing compounds, oxygen containing compounds, all of said gases being introduced into the reaction chamber at a total pressure in the range of 2 mm Hg. to 5 mm Hg., and

c) applying an RF electric field to said plasma reaction chamber to generate a plasma from said introduced gases, with the RF electric field having applied RF power in the range of 45-1 00W.

11. A method as claimed in claim 10, wherein said boron gaseous compound is boron trifluoride.

12. A method as claimed in claim 10, wherein said boron gaseous compound is diborane.

1 3. A mixture consisting of, a) silane gas diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, b) a phosphorus-containing, boron-containing, or phosphorus/boron-containing gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent for each of the components, and

c) a gaseous oxygen-containing compound, or a gaseous nitrogen-containing compound.

14. A mixture consisting of: a) silane gas diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and b) a phosphorus-containing, boron-containing, or phosphorus/boron-containing gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent for each of the components.

1 5. A mixture consisting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, b) a phosphorus gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent, and

c) a second gaseous compound selected from the group including oxygen-containing, or nitrogencontaining compounds.

1 6. A mixture consiting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and b) a phosphorus gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent.

17. A mixture as claimed in claim 15, wherein said noble gas is argon.

1 8. A mixture as claimed in claim 15, wherein the second gaseous compound is nitrous oxide.

19. A mixture as claimed in claim 15, wherein the second gaseous compound is nitrogen.

20. A mixture as claimed in claim 15, wherein said phosphorus gaseous compound is phosphine.

21. A mixture consisting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, b) a boron gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent, and

c) a second gaseous compound selected from the group including oxygen-containing, or nitrogencontaining compounds.

22. A mixture consisting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and b) a boron gaseous compound diluted with a noble gas to a volumetric percentage equal to or less than 1 percent.

23. A mixture as claimed in claim 21, where said boron gaseous compound is diborane.

24. A mixture as claimed in claim 21, where said boron gaseous compound is boron trifluoride.

25. A mixture as claimed in claim 21, wherein the second gaseous compound is nitrous oxide.

26. A mixture as claimed in claim 25, wherein the second gaseous compound is nitrogen.

27. A mixture as claimed in claim 21, wherein said noble gas is argon.

28. A mixture consisting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, b) phosphorus/boron gaseous compounds each diluted with a noble gas to a volumetric percentage equal to or less than 1 percent, and

c) a second gaseous compound selected from the group including oxygen-containing, or nitrogencontaining compounds.

29. A mixture consisting of, a) silane diluted with a noble gas to a silane volumetric percentage between 1 and 2 percent, and b) phosphorus/boron gaseous compounds each diluted with a noble gas to a volumetric percentage equal to or less than 1 percent.

30. A mixture as claimed in claim 28, wherein the second gaseous compound is nitrous oxide.

31. A mixture as claimed in claim 30, wherein the second gaseous compound is nitrogen.

32. A mixture as claimed in claim 30, wherein said noble gas is argon.

33. A method of depositing phosphorus doped, boron doped, or boron/phosphorus doped silicon compounds on a substrate, substantially as hereinbefore described with reference to, and as shown in the accompanying drawing.