MXPA99005561A - Organometallics for lightwave optical circuit applications - Google Patents

Abstract

A plurality of organometallic compounds (12-18) are converted into vapors and are mixed along with combustion gases (32-38) to form a vapor stream that flows along through pipe(40). This vapor stream burns at burner (42) to form soot (50). This soot (50) deposits onto rotating substrate (60) so as to form a consolidated oxide layer.

Description

ORGANOMETALLICS FOR APPLICATIONS OF OPTICAL WAVE CIRCUIT OF LIGHT FIELD OF THE INVENTION This invention relates generally to conversion systems such as flame hydrolysis, and to the use of organometallic sources to deposit thin uniform oxide soot or precoated glass layers on flat substrates. The deposited soot is consolidated in layers of glass. The glass layers form the core and glass coatings that form the optical waveguides in integrated optical circuits.

BACKGROUND OF THE INVENTION Applicants have developed flame hydrolysis systems that deposit layers of thin oxide soot on flat surfaces for applications of light guide optical circuits (COG) and the manufacture of integrated optical waveguide devices as integrated optical circuits. These layers of soot are consolidated into glass layers that form optical waveguide cores and coatings. The patent application of E.U.A. No. 08/581, 186 of 12/29/95, Bandwidth-Adjusted Wavelength Demultiplexer, by Denis M. Trouchet, describes such optical circuits that function as a wavelength demultiplexer. Conventional methods have relied on the combustion of halide compounds which are advantageous for certain applications, but which have serious disadvantages. Halogens such as chlorine will release some of the oxides from the source material that will result in non-uniformity in the glass composition resulting from the soot layer deposited. This non-uniformity results in a degradation of the optical properties of the resultant waveguide glass layers made by these conventional halide processes. In addition to the above, the final product of the combustion reaction of halides is chlorine, which reacts with moisture in the air to form HCL, which is highly corrosive and toxic and requires equipment capable of containing it. It can therefore be seen that there is a need for a system to form layers of oxide soot on flat surfaces or substrates that provide improved uniformity in the glass composition, better optical performance of waveguide glass layers formed by the technique of flame hydrolysis, and a removal of toxic byproducts or harmful combustion.

BRIEF DESCRIPTION OF THE INVENTION The invention is directed to the use of organometallic sources for passive flat waveguide applications and to make integrated optical waveguide devices as integrated optical circuits. The advantage of using these materials is the elimination of chlorine from the system. The chlorine will dislodge some of the oxide sediments from these source materials through the reaction and present problems in obtaining the desired concentration of the desired oxide (s) in the deposited layer. Another advantage of using organometallic sources is the ease of supplying materials to a conversion site, such as an incinerator, and the added security that chlorine removal provides. A soot layer is deposited using improved flame hydrolysis deposition (FHD) techniques of the present invention. The DHF is capable of depositing uniform, thin oxide soot, or preconcrete glass layers on flat substrates. In one embodiment, the waveguide layers are made using flat substrates made of fused silica, 100 mm in diameter, and 1 mm thick. The waveguide soot glass core compositions within the GeO2-B2O3.P2O5.SiO2 system are made and coated with a glass coating layer within the ternary system B2O3.P2O5.SiO2. The glass compositions are chosen to yield objective refractive indices (for example, increasing Ge02 increases%?). The FHD system of the present invention consists of a mixture of combustible gases and organometallic vapors which are mixed and supplied within a common stream within a flame that consists of a conversion site that is directed directly to the flat white substrate of fused silica. Within the methane / oxygen flame the organometallic vapors (from at least two materials selected from octamethylcyclotetrasilix, trimethylphosphate, triethylborate, titanium isopropoxide and germanium ethoxide) are consumed to yield multicomponent particles of oxide soot. The speed of the flame, the ratios of the component gases in the flame, and the rate of steam supply control the final size of soot particles and the degree to which they are concreted. The height of the target can be changed and the target can be moved and / or rotated to control the temperature and distribution of the deposited particles. Once the soot layer is deposited to the desired thickness, the sample can be given a heat treatment in order to concretize and fully densify the glass. The concretion or consolidation depends on the composition and thickness of the glass. The thickness of the resulting waveguide layer is typically 5-9 μm thick. The consolidation temperature range is 1150-1340 ° C and the detention time in this temperature range is between 1-7 hours. To make an integrated optical waveguide device, a soot layer is deposited and concreted on a flat substrate as described above to form a core layer. A waveguide circuit is recorded inside this core layer using photolithographic and ion-reaction engraving (RIE) techniques. A coating layer is then deposited on the engraved and bonded layer.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred embodiment for practicing the invention, read in conjunction with the appended drawings, in which: Figure 1 is a schematic diagram of the flame hydrolysis system for the present invention. Figure 2 is an enlarged perspective view of an incinerator, showing the front and tail of the flame, and how to measure the height of incenter to sample. Figure 3 is an enlarged perspective view of the incinerator assembly. Figure 3a is a cross-sectional view through the center of the incinerator shown in Figure 3. Figure 4 is a side sectional view of a bubbling assembly. Figure 5 is a plan view of the vacuum fixator assembly supporting the substrate.

Figure 5a is a bottom view of the bottom of the fixator. Figure 6 is an enlarged perspective view of a second embodiment of an incinerator assembly. Figure 7 illustrates a consolidated glass roughness pattern as a function of height from incinerator to sample and the number of incinerator rows. Figure 8 illustrates a pattern of change in composition as a function of sample height to incinerator.

DETAILED DESCRIPTION OF THE INVENTION The present invention is best illustrated by FIG. 1 which is a schematic diagram of the flame hydrolysis system 10 of the present invention which is suitable for being used to produce layers of oxide soot on flat surfaces for use in optical circuit applications of light wave (LOC) and to make integrated optical waveguide devices as integrated optical circuits. The selected organometallic liquids are stored in bubblers 12, 14, 16 and 18 respectively. The organometallic vapors generated by the system are carried by a nitrogen source 20 which passes N2 through heated tubes 22 inside each bubbler. The organometallic liquid is evaporated inside the bubbler and carried by the nitrogen vapor through the heated lines 24, 26, 28 and 30, respectively. The organometallic vapors are mixed with a preselected mixture of combustible gases including air 32, nitrogen 34, oxygen 36 and methane 38, and are supplied from tube 39 through tube 40 to an incinerator assembly 42 as a single stream. The oxide soot 50 is generated by combustion of the vapors at the flame conversion site 48 (see figure 2). The soot is deposited on a substrate 53 which is held in place by a vacuum sample holder assembly 60 (Figures 5 and 5a). Optionally, the sample holder sleeve may be transversely and / or rotated by a conventional transverse planetary motion mechanism 70 well known in the art. The exhaust from combustion, which is not shown, travels through a bell 62 to a scrubber (not shown). The incinerator assembly 42 and the vacuum sample holder assembly 60 are surrounded by a filtered cover 52. As shown in Figures 3 and 3a, the conversion site of the incinerator assembly 42 consists of a housing 43 with a single row of holes. 44 of 0.76 cm in diameter. The housing contains a glass-ceramic insert 45 containing parallel rows of holes 41, and a fine mesh stainless steel screen wrap 46 that is inserted into a cylindrical multiple chamber 47. In a preferred embodiment, it provides a more even distribution of the steam mixture, the insert 84 of figure 6 replaces the screen wrap 46. The mixture of fuel-vapor organometallic gas enters the incinerator through the tube 48 which is screwed into the housing 43 and which is connected to tube 40. The manifold chamber is sealed with a threaded nut 49.

A second embodiment of an incinerator assembly that provides maintenance of flame points at equal height across the entire face of the incinerator is illustrated in Figure 6. In this embodiment, the incinerator assembly 70 comprises a housing 72 and an incinerator slot 74. The incinerator drawer consists of 2 inserts that are placed inside the cylindrical chamber of the manifold 76. A ceramic insert 80 has 2 parallel rows of holes 82. A stainless steel insert 84 it is constructed in such a way that when it rests against the ceramic insert it provides a course for the vapors to be evenly distributed, while keeping the points of the flame at equal height throughout the entire surface of the incinerator. The steam mixture enters the incinerator through the tube 86 and the chamber is sealed with a threaded nut 78. In order to ensure a layer of uniform thickness, and to avoid wefts, the length "L" of the face of the incinerator or surface The top that contains the incinerator slot or holes should be at least equal to or longer than the diameter or width of the flat substrate that is being coated. The vacuum tool holder sleeve assembly 60 which holds the substrate in place is shown more clearly in Figures 5 and 5a. The sample holder sleeve contains a vertical arrow 61, a rotating collar 62, an inner protective cover ring 63 and an outer ring 67. The lower face 64 of the sample holder sleeve (Figure 5a) shows the placement of the vacuum holes 65 which they hold sample 53 instead.

In operation, the components of the organometallic liquid such as octamethylcyclotetrasiloxane, trimethyphosphonate, triethylborate and germanium ethoxide are separately placed in bubblers 12, 14, 16 and 18 respectively. The bubblers are connected to a nitrogen-carrying gas intake valve with an aerator in its base and a steam outlet valve for each bubbler. As shown more clearly in Figure 4, which is an enlarged view of an individual bubbler as shown in Figure 1, each bubble chamber 12 is made of stainless steel and is sparsely cylindrical with a bottom and a round top. The bubbler contains ports for nitrogen intake and steam outlet 13 which flows out to incinerator 42. Nitrogen flows into the bubbler through inlet 15 at a given speed and is supplied by an aerator tube that is immersed in the liquid. and placed in the bottom of the bubbler. For temperature control, bubblers can optionally be wrapped in heat tape that is controlled by temperature controllers. The bubblers are then isolated with a suitable insulator. The output lines are also heated by heat tape wrapped at temperatures above the boiling point of the metal organ in order to ensure that the vapors remain gaseous. All the output lines lead to a single common tube 40 which acts to mix the vapors before reaching the incinerator. The temperature of the tube should be at least as high as the highest boiling point of the components.

Thermocouples can be used to monitor the temperature of organometallic liquids. The thermocouples 17 and 18 are placed inside each bubbler so that they are also immersed in the liquid. An additional thermocouple 19 is adhered to the outside of each of the bubblers, together with the inlet and outlet tubes. The bubblers also have ports for filling and draining.

A drain 21 is contained in the bottom of each bubbler which is blocked unless the bubbler is drained. The steam and gas delivery rates are governed by mass flow controllers (mfcs) 23 available under the tradename TYLAN and provide a volumetric flow rate. The organometallics in vapor form are supplied by the nitrogen stream. The methane and oxygen gases are supplied separately. The system also has the capacity for nitrogen, air and hydrogen as part of the fuel premix. For one embodiment of the invention, the scales of the mfcs are as follows: CH4 and O2 = 10 sLpm (normal liter per minute) N2 for pre-mix = 10 sLpm OMCTS N2 = 100 sccm (normal ce per minute) TMP N2 = 200 sccm TEB N2 = 50 sccm GeE = 1000 sccm The secondary regulator (which goes to mfcs): N2 = 1.054 Kg / cm2 O2 = 1.195 Kg / cm2 Only the nitrogen line is filtered to trap moisture. The lines of the mfcs are stainless steel, and they lead either to the multiple incinerator directly, or to the bubblers. The lines are preheated and temperature controlled by heat tape and commercially available temperature controllers. The temperature of each line is equal to its own bubbling temperature. Alternatively, the bubblers can be replaced with a vaporizer system that is known in the art, as shown in the US patents. Nos. 4,529,427 and JP 60-108338 and which are incorporated herein by reference. In one embodiment of the present invention, a soot core and a soot coating layer are formed as follows: The soot core glass composition No. 5 (see table 3 which is within the GeO2-B2 system? 3- P2? 5-Si? 2 (15.79, 3.86, 2.19, and 78.16% by weight) and coating No. 10 of glass layer (see Table 1) have a composition within the ternary B2? 3-P2? 5-S O2 (7.95, 3.25, and 88.8% by weight) .The compositions were analyzed by normal electron probe microanalysis (EPMA) techniques. A soot layer is deposited using normal flame hydrolysis (FHD) deposition techniques. is described for Figures 1-5 The FHD system consists basically of combustible gases and organometallic vapors that are mixed and supplied within a common current within a flame conversion site that is focused directly on a flat white substrate. of the methane / oxygen flame, the organ meta vapors (Octamethylcyclotetraciloxane, OMCTS, TMP trimethyl phosphate, TEB triethylborate and TEOG or GE germanium ethoxide) are consumed and converted into multicomponent particles of oxide soot.

The supply system uses bubblers, one for each component. The bubblers are 100-400 mL in volume and the liquid is kept at a constant level. The carrier gas is nitrogen, and the lines inside the bubblers are preheated to the same temperature as the bubbler in order to avoid liquid cooling as the N2 flows. The temperatures for the bubblers were chosen to be as low as possible while being high enough to produce adequate steam for a given N2 flow. The output lines need to exceed the boiling point in order to prevent the vapors of the liquids from condensing. The temperature controllers for the carrier gas inlet lines, bubblers, and outlet lines are preheated to the following temperatures: Temperature (° C) 81 Take the OMCTS bubbler and the bubbler itself (22 + 18) 70 Take the TMP and the bubbler TMP (22 + 16) 60 Take the TEB and the bubbler TEB (22 + 14) 51 Take the GE and the bubbler GE ( 22 + 12) > 176 OMCTS bubbler outlet line (30) > 197 TMP bubbler outlet line (28) > 117 TEB bubbler outlet line (26) > 185 GE bubbler outlet line (24) > 197 Common port below the incinerator A fused silica substrate (100 mm diameter, 1 mm thick) is cleaned and weighed before being placed on the sample holder sleeve 60 which holds the sample above the flame by vacuum. The mass flow controllers are turned on to flow the carrier gas into the bubblers to supply a given volume flow rate of steam (sccm), as well as to control the flow rate of methanol and oxygen (sLpm). The delivery rates for the materials for the core and glass liner are as follows: OMCTS TMP TEB GE CH4 O2 Core # 5 0.023 0.0002 0.009 0.006 5.85 5.6 Coating # 10 0.025 0.0040 0.007 0 5.85 5.6 The magnehelico below the incinerator monitors the back pressure, usually at 2.54 cm. H2O. The height of the substrate above the flame is set according to a predetermined distance and the substrate moves transversely at a constant speed and is rotated simultaneously by the mechanism 30 to control the temperature and distribution of the deposited particles. The thickness of the glass soot is controlled by how many times the substrate moves transversely above the flame. Typically, the thickness of the core layer is 5 to 7 microns and the thickness of the coating layer is 4 to 20 microns. Once the soot layer is deposited to the desired thickness, the sample is given a heat treatment in order to concretize and fully densify the glass. The consolidation program depends on the composition and thickness of the glass. The thickness of core layer number 5 is 5μm thick; coating layer number 10 is 4μm thick. The upper consolidation temperature for the core was 1290 ° C, and for the coating it was 1200 ° C. The holding times at these temperatures were 2 and 1 hours, respectively.

The step to form an integrated optical waveguide circuit device includes: 1. A circuit system device is recorded within the core layer using photolithographic and etching techniques by reaction of on (RIE). 2. - A coating layer is deposited and made specific, covering this circuit system device. 3. - The device is braided, packed, and connects. Tables 1 and 2 show a comparison of standard deviations for weight percentages of oxide from EPMA cross-sectional data for glasses generated by organometallic compounds of this invention and for traditionally deposited coating glasses generated by halide. (the lowest values for the glasses generated by halide (0.47 for Si? 2, 0.39 for B2O3, and 0.13 for P2O5) were used as the upper values for the degree of composition control necessary for the generated glasses of organometallic of this invention ). The glasses of this invention show significantly less variation for each oxide through the deposited glass layer.

The following tables 3 and 4 show comparisons of standard deviations of percent by weight of oxide for cross-sectional data of EPMA for core glasses generated by halide and for glasses generated by organometallics of this invention. The glasses of this invention show significantly less variation for GeO2 and S02 through the deposited core glass layer.

The following formulas illustrate the typical organometallic combustion products that can be used in the present invention.

TABLE 5 ORGANOMETALLIC COMBUSTION PRODUCTS OMCTS: Octamethylcycotetrasiloxane C8H24? 4S4 + 16 O2 = 4 SiO2 +8 CO2 + 12 H2O TMP: Trimethylphosphate 2 (CH3O) 3PO + 9 O2 = P2O5 + 6 C02 + 9 H2O TEB: Triethylborate 2B (OC2H5) 3 + 18 O2 = B2O3 + 12 CO2 + 15 H2O GeE: Germanium Ethoxide. C8H20O4Ge + 12 O2 = GeO2 + 8 CO2 + 10 H2O Titanium isopropoxide Ti (OC3H7) 4 + 18 O2 = T02 + 12 C02 + 14 H2O In a further embodiment of the present invention, the oxide soot particles can be deposited concurrently and concreted in a uniform glass layer onto the flat substrate without melting or softening the substrate. This mode provides the advantage of eliminating a separate consolidation step, prevents buckling of the sheet, and provides a glass surface that is uniform with few or no defects. The methods by which this concretion in situ can be achieved are raising the temperature of the substrate as follows: 1.- Increase the concentration of methane (hotter flame) 2.- Isolate or heat the sample holder. 3.- Decrease the height of the sample (as close as possible to the incinerator to obtain hotter and smaller soot particles) 4.- Alter the composition at a lower concretion temperature (for example, increase B2O3 / P2O5) 5.- Use a high-speed incinerator (single row versus triple row). The first method is to increase the ratio of methane to oxygen, while keeping all other conditions constant. The oxygen level is high enough to result in stoichiometric reactions while the higher methane produces a hotter flame. Samples of 10 cm in diameter are used, and the pre-specified area is circular and transparent glass. The limits of this region are measured in terms of outside diameter (cm) (see table 6). When the CH4 / O2 ratio increases, the diameter increases from 4.8 to 6.0 cm. When the ratio is almost the same but CH4 increases, the diameter increases from 6.0 to 8.8 cm.

Another method is to isolate the sample holder. This is accomplished by covering the vacuum sample holder sleeve with a thick fiberfrax layer that conforms to the shape. The preconcressioned area increases from 6.0 to 6.5 when the sample holder sleeve is kept warmer through the procedure (see table 7).

A third method is to bring the substrate holder closer to the front of the flame. The optimum height of incinerator to sample is directly above the points of the flame. A fourth method is to modify the composition of the glass.

Figure 8 shows a change in the composition in the consolidated glass with a ratio of B2O3 / P2O5 that decreases as a function of the sample height to incinerator. For higher ratios of B2O3 / P2O5, soot is more easily realized. A fifth method is to increase the speed of the flame, using a single row of incinerator holes instead of three rows of holes.

The roughness of the soot as a function of the thickness of the soot can be correlated with the number of rows of incinerator holes. The consolidated samples were measured for roughness using a profilometer, averaging three remains in three different sites over a distance of 4 mm. The average roughness as a function of sample height to incinerator is shown in Figure 7 for samples generated using a single row of incinerators and a triple row of incinerators. The glass surface is softer for a single row of incinerators and the roughness for both cases increasing the height above the incinerator. Although the present invention has been particularly shown and described with reference to the preferred embodiment as illustrated in the drawing, it will be understood by those skilled in the art that various changes can be made in detail thereto without departing from the spirit and scope of the invention. invention as defined by the claims.

Claims (24)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for forming a layer of oxide soot on a flat substrate consisting of the steps of: a) producing a halogenide-free gas stream containing a plurality of organometallic compounds in steam; b) passing said current into a conversion site to form a mixture of oxide soot particles; and c) placing a flat support near the conversion site and depositing the mixture of oxide soot particles on the support to form a coherent layer of oxide soot.

2. The method according to claim 1, further characterized in that the step of producing a halide-free gas stream containing a plurality of organometallic compounds in vapor form further includes mixing a gaseous fuel with said stream.

3. The method according to claim 1, further characterized in that the step of placing a support flat or close to the conversion site further includes placing the flat support horizontally above the conversion site.

4. The method according to claim 1, further characterized in that the organometallic compounds consist of at least two materials selected from the group consisting of octamethylcyclotetrasiloxane, trimethylphosphate, triethylborate, titanium isopropoxide and germanium ethoxide.

5. - The method according to claim 1, further characterized in that the vapors of the organometallic compounds are contained in an inert gas.

6. The method according to claim 5, further characterized in that the inert gas consists of nitrogen.

7. The method according to claim 2, further characterized in that the gaseous fuel consists of methane and oxygen.

8. A method for manufacturing an optical waveguide device comprising the steps of: a) producing a halogenide-free gas stream containing a plurality of organometallic compounds in vapor form; b) mixing the gas stream with a gaseous fuel mixture to form a common vapor stream; c) passing the vapor stream into a flame of an incinerator to form a plurality of oxide soot particles; d) depositing the oxide soot particles on a support to form a coherent layer of oxide soot; e) consolidating the oxide soot layer in a glass layer; and f) forming an optical waveguide with the glass layer.

9. A method for manufacturing an integrated optical waveguide on a flat support substrate consisting of the steps of: a) producing a halogenide-free gas stream containing a plurality of organometallic compounds in steam; b) mixing the gas stream with a gaseous fuel mixture to form a common vapor stream; c) passing the vapor stream into a flame of an incinerator to form a plurality of oxide soot particles; d) depositing the oxide soot particles on a support which is heated to an elevated temperature sufficiently to co-oxidise the oxide particles in situ to form a vitreous layer; and e) forming an optical waveguide device integrated with the vitreous layer.

10. A method for manufacturing an optical circuit consisting of the steps of: a) producing a halide-free gas stream containing a plurality of organometallic compounds in vapor form; b) mixing the gas stream with a gaseous fuel to form a common vapor stream; c) passing the vapor stream into a flame of an incinerator to form a plurality of oxide soot particles; d) depositing the oxide soot particles on a flat support which is suspended above the incinerator to form a coherent layer of oxide soot; and e) forming an optical circuit from the oxide soot layer.

11. The method according to claim 10, further characterized in that the flat support is transversely over the flame of the incinerator during the deposition process.

12. The method according to claim 11, further characterized in that the support is also rotated during the deposition process.

13. The method according to claim 10, further characterized in that the support moves on the flame of the incinerator in a planetary movement during the deposition process.

14. An apparatus for forming a layer of oxide soot on a flat substrate consisting of: a) a network of steam mixing pipe, the network of steam mixing pipe consists of a common mixing pipe, a plurality of lines of steam compound supply connected to the common mixing pipe, the steam compound supply lines for supplying a plurality of steam compounds to the common mixing pipe, a fuel gas pipe connected to the common mixing pipe, the pipe of fuel gas for supplying a fuel gas to the common mixing tube, and wherein the plurality of vapor compounds and fuel gas are mixed together within the common mixing tube to form a common stream of mixed vapor contained within the common mixing tube , the network of steam mixing tubes consisting additionally of one end of the exhaust pipe to the incinerator, the end of the exhaust pipe of the incinerator connected to the common mixing pipe, the Incinerator inlet tube oar to supply the common stream of mixed vapor contained within the common mixing tube inside an incinerator; b) an incinerator having an outer housing that includes an upper face containing an incinerator port that is connected to the end of an incinerator inlet pipe through a common internal multiple incinerator chamber, the manifold chamber within the incinerator; and c) a support for a flat substrate disposed above the incinerator, the support having means for providing horizontal movement through the face of the incinerator.

15. - The apparatus according to claim 14, further characterized in that the support also provides means for rotating a substrate above the incinerator.

16. The apparatus according to claim 14, further characterized in that the incinerator port consists of an elongated slot.

17. The apparatus according to claim 14, further characterized in that the incinerator port comprises a plurality of holes.

18. The apparatus according to claim 17, further characterized in that the holes are contained in a single row of holes equally spaced.

19. The apparatus according to claim 14, further characterized in that the incinerator contains at least one internal incinerator drawer member contained within the manifold chamber that provides a uniform flow through the length of the face of the incinerator.

20. The apparatus according to claim 19, further characterized in that at least one internal incinerator drawer member consists of two inserts that are supported against each other to provide a course for the common steam flow to be uniformly distributed to the port. incinerator.

21. - The apparatus according to claim 14, further characterized in that the length of the incinerator is equal to or longer than the diameter or width of the flat substrate.

22. A method for forming a layer of oxide soot on a substrate consisting of the steps of: a) producing a halide-free gas stream containing a plurality of organometallic compounds in vapor form; b) passing the current into a conversion site to form a mixture of oxide soot particles; and c) placing a support near the conversion site and depositing the mixture of oxide soot particles on the support to form a layer of oxide soot.

23. The method according to claim 22, further characterized in that the conversion site consists of a flame.

24. The method according to claim 23, further characterized in that the halogenide-free gas stream containing a plurality of organometallic compounds in vapor form is produced away from the flame.