DE8910733U1 - Device for evaporating compounds that are liquid at room temperature with a flow control device and an evaporator connected downstream of this - Google Patents

Description

Device for evaporating compounds that are liquid at room temperature with a flow control device and an evaporator connected downstream

The innovation concerns a device for evaporating chemical compounds that are liquid at room temperature and have a low vapor pressure, with a flow control device and an evaporator connected downstream of this, which has an evaporator chamber and a coil for heating this.

In the microstructuring of ammonium layers, fluorine or chlorine-containing compounds are usually used, which react chemically with the layers to be etched to form volatile compounds. Layers that are not to be structured are covered by a photoresist. Suitable etching chemicals for these so-called etching processes are, for example, HBr, iid or SiCi4, which are corrosive liquids under normal conditions. When these liquids are dosed quantitatively into the vacuum, a so-called boiling delay often occurs, ie a burst of explosive evaporation and thus an irregular mass flow. The semiconductor manufacturing processes take place automatically and thus the highest level of reproducibility and uniformity is required.

Plasma processes and vapor phase deposition at reduced pressure or atmospheric pressure are used to produce layers containing silicon and oxygen. Liquids such as TEOS, TMS and HMDS are used as starting monomers, which are characterized by a relatively low vapor pressure at room temperature. Here, too, boiling delay and thus irregular mass flow often occur.

From the article by B.L. Chin and E.P. van de Ven "Plasma TEOS Process for Interlayer Dielectric Applications" published in "Solid State Technology", April 1988, pages 119 to 122, it is known to pump TEOS as a liquid into the vacuum reactor/), where it evaporates and is mixed with auxiliary gases. Despite a computer-controlled liquid pump with a reproducible throughput, it is not possible to generate a sufficiently constant mass flow due to the intermittent evaporation of the liquid in the vacuum.

When considering this, it must be borne in mind that any irregularity in the mass flow will result in inhomogeneous layer properties and - in the case of a reactive device - also in deviations in the stoichiometry of the layer composition.

From the article by SP Mukherjee and PE Evans "The Deposition of thin films by the decomposition of Tetra-Ethoxy silan in a radio frequency glow discharge" published in Thin Solid Films", 1972, pages 105-118, it is known to evaporate TEOS using a temperature-controlled evaporator that enables a defined increase in vapor pressure. Here too, continuous evaporation is practically impossible to achieve. If other monomers are to be evaporated, the device must be carefully adapted to the vapor pressure curves of these monomers. Furthermore, the device in question is limited in its application, since it cannot generate an arbitrarily high vapor pressure and the easily condensable vapors are difficult to transport and dose.

U. Mackea-? and U. Merkt describe in the article "PLASMA-ENHANCED CHEMICALLY VAPOUR-DEPOSITED SILICON DIOXIDE FOR

METAL/OXTOE/SEMICONDUCTOR STRUCTURES ON InSb", published in "Thin Solid Films", 1982, pages 53-61, a very similar device -.,nut comparable \ disadvantages.

S Through the paper by R.E. Howard "Selecting Semiconductor Dopants for

t Precise Process Control, Product Quality and Yield and Safety", published in

"Microelectronic Manufacturing and Testing", December 1985, pages 20-24, it is &igr; It is also known to mix a liquid monomer contained in a quartz vessel with a

■ ■ Noble gas entrains the monomers. The disadvantage of this device is the lack of

Information about the quantities of monomers that actually enter the reaction chambers. In addition, this results in an intolerable limitation for many applications with regard to the choice of operating parameters such as pressure and gas quantity.

Finally, EP-OS O 239 664 discloses a method of the type described above, in which a needle valve is used as the flow control device, followed by an evaporator (not described in detail). Such an arrangement results in the monomer evaporating more or less uncontrollably behind the needle valve, which forms a throttle point. The evaporator then only serves to evaporate any liquid droplets that may have been entrained, thus ensuring complete evaporation. As a result, a constant mass flow cannot be achieved even with this device.

Through the patent application P3833232.9*, "Process and device for evaporating monomers that are liquid at room temperature", a process of the type described above is known, in which a flow control device is connected downstream of an evaporator chamber in which bodies with a capillary effect are arranged. The technical implementation consists in using wicks or other porous bodies that have serious adverse effects such as clogging or particle detachment. It is precisely the formation of pores that then leads to major disruptions in the layers or etching patterns produced. In addition, unstable materials are used that are exposed to corrosion by the fluorine and chlorine-containing liquids.

The innovation is therefore based on the task of specifying a device of the type described above, in which a constant, precisely adjustable

Steam flow or steam quantity can be maintained over a long period of time.

The solution to the problem is achieved in the device described at the outset in that the Störmings adjusting device is a mass flow controller and that a body with a large-area effect is arranged in the evaporator chamber, the end of which is connected to a supply line coming from the mass flow controller and which is kept at a distance from the heated walls of the evaporator chamber, but in line of sight with them.

With a corresponding process, evaporation does not take place at an uncontrollable point behind the flow control device, but exclusively and completely in the evaporator. The mass flow controller itself ensures a supply of liquid monomer that can be regulated within extremely narrow limits. In other words: there is an extremely targeted and precise conveying of liquid monomers on the one hand and very precise evaporation at a location intended and designed for this purpose, namely inside the evaporator or inside the heated body with a large surface area on the other.

Through their interaction, these two measures lead to the desired result. A carrier gas can be completely dispensed with.

The mass flow controller used after the innovation is a precision device in which a setpoint value for the throughput, i.e. the flow rate per unit of time, is specified and the actual value is adjusted to the smallest possible deviation from the setpoint value.

A mass flow regulator that is ideally suited for this purpose is offered by Bronkkorst High-Tech B. V. in Ruurlo/Netherlands under the type designation F-811-FB-33 LFM. The measuring principle for the actual value of the mass flow is based on a laminar flow in a heated line with the supply of a constant electrical power. The size of the mass flow can be determined directly from the measurement of the temperature difference between the inlet and outlet of the line. The measured value is compared with a setpoint value and the comparison value is used to control a solenoid valve that consists of a nozzle and a baffle plate in front of it, the distance of which from the nozzle is controlled by an electromagnet (module F-002D-IA-33 control valve). In this way, the mass flow can be regulated extremely precisely and also adjusted in a targeted manner. The mass flow is set between 0.1 and 30 g/hour with a linear characteristic curve with a maximum deviation of +- 0.83%.

The heating of the large-surface bodies is carried out by indirect heating from the outer shell of the evaporator. This creates a temperature gradient from the outside to the inside. In this way, the liquid monomer does not touch the highly heated

Walls of the evaporator, so that no chemical changes can occur. It is also advantageous if the body with the large-area effect is poured into a cylinder tube and this is suspended freely at its ends and if the liquid monomer is fed from one end of the body and the vaporized monomer is drawn off at the end of the cylinder and fed to the reaction zone in a vacuum chamber. It is a given advantage if the body containing the large-area effect in the cylinder creates a pressure gradient with a negative sign from the inlet side of the liquid monomer to the outlet side of the vaporous monomer. The innovation also relates to a device for carrying out the device described at the beginning with a flow control device and an evaporator connected downstream of this, which has an evaporator chamber with an inlet end and an outlet end and a heating device for heating and which is followed by a vacuum chamber for carrying out the semiconductor manufacturing processes, such as dry etching or chemical vapor deposition processes.

Of particular importance is the body that has the large-area effect. This is stainless steel material, from which defined structural shapes, in particular spheres, are formed. These fill the volume in loose bulk with an 86% space filling. This provides a defined evaporation situation, which can also be adapted to the process-specific requirements in terms of the required evaporation dynamics. This is done by varying the sphere diameter and the total number of spheres (heat capacity). The particular advantage of this solution is that no particle-forming material is used, particularly with regard to the high corrosion resistance of the electropolished stainless steel materials.

The design is such that no liquid reaches the outlet end, so that all of the liquid monomer in the area of the body is converted from the liquid phase to the vapor phase. In addition, it is ensured that any back diffusion for pressure reasons is prevented. Further advantageous designs emerge from the remaining subclaims.

An example of the innovation is explained in more detail below using Figures 1-4.

Show it:

Figure 1 shows a schematic diagram of a complete

Facility for carrying out a

dry etching process or a CVD process.

Figure 2 shows an axial section through a

inventive evaporator

Figure 1 shows a vacuum chamber 1 in which a dry etching or CVD process is carried out and which is connected to two vacuum pumps 2 and 3. In the vacuum chamber 1 there is a substrate carrier 4 which is connected to a high frequency generator 6 via a line 5. The vacuum chamber 1 is at ground potential and the line 5 is led through the chamber wall by means of an insulating feedthrough. On the substrate carrier 4 there are substrates 7 which are to be microstructured or coated by a CVD device.

The liquid monomer is located in a source container 8 and is fed to the mass flow controller 10 via the upper line 9. From this, a liquid line 11 leads to an evaporator 12, which is explained in more detail below with reference to Figures 2 and 3. A steam line 13 leads from the evaporator 12 via a shut-off valve 14 and a further line 15 to the vacuum chamber 1. An inert gas, with which a glow discharge can be maintained in the vacuum chamber 1 in the area of the substrate carrier 4, is fed into the line 15 via a line 16 and a control valve 17, while further doping and additional gases can be introduced into the vacuum chamber 1 via a further line 18 and a control valve 19. An air dryer 20 is connected upstream of the source container 8.

The entire arrangement is centrally controlled by a control unit 21, which is connected to the individual units via numerous lines shown in dashed lines. Since, with the exception of the mass flow controller 10 and the evaporator 12, the arrangement according to Figure 1 is state of the art, further discussion of the mode of operation should be superfluous.

The evaporator 12 according to Figure 2 has an evaporator chamber 22, the essential part of which is formed by a hollow cylinder 23 which is surrounded by a heating coil 24. This heating coil consists of a central heating conductor 25 and an insulating sheath 26 surrounding it; the connection ends are not shown here.

The hollow cylinder 23 is provided with ring flanges 27 and 28 at both ends. An end wall 31 is attached to the ring flange 27 on the inlet side with a sealing ring 29 and a centering ring 30 in between. This end wall 31 has a threaded connector 32 to which the liquid line 11 (Figure 1) is connected. Inside the end wall 31 there is a supply line 33 for the liquid monomer.

The steam line 13 (Figure 1) is connected to the ring flange 28 at the outlet end via a sealing ring 34 and a centering ring 35. The end wall 31 and the steam line 13 have ring flanges 36 and 37 that complement the ring flanges 27 and 28. The outsides of the ring flange pairs 27/36 and

28/37 are conical in shape so that the ring flanges can be clamped against each other by means of attached, half-shell-shaped clamping rings 38 and 39.

The heating coil 24 located between the clamping rings 38 and 39 is surrounded by an outer casing 40. The space between the outer casing and the hollow cylinder 23 is filled with a thermal insulation material 41.

In the evaporator chamber 22 there is an elongated hollow cylinder in which the body 42 has a large surface area effect, one end 42a of which is connected to the supply line 33 coming from the mass flow controller, i.e. the end is inserted into this supply line. The evaporator chamber 22 has a heated wall surface TZa, and it can be seen that the body 42 is in visual contact with this wall surface, but without touching it.

It is clear from Figure 2 that the heat of vaporization for the bodies 42 having the large-area effect is supplied almost exclusively by indirect heating from the wall surface 22a.

In the object in Figure 2, the liquid concentration decreases from left to right and the

Steam concentration increases, whereby the temperature of the surface-active bodies with a high heat capacity remains constant.

It was observed that the precisely dosed liquid evaporates absolutely smoothly and in a controlled manner and emerges at the surface 43.

Regarding the application range of the device, it should be noted that it is preferably used to produce SiO2 layers for microelectronic applications, starting from TEGS. In the case of dry etching technology, it should be mentioned that liquids such as SiC14 or, more recently, HBr are used successfully, especially for etching aluminum conductor tracks.

Claims (4)

Protection claims 1. Device for evaporating chemical compounds which are liquid at room temperature, with a flow control device and an evaporator connected downstream of the latter, which has an evaporator chamber with an inlet end and an outlet end and a heating device for heating, and downstream of which is a vacuum chamber for carrying out a chemical vapor deposition process, characterized in that the flow control device is a mass flow controller and that a body having a large-area effect is arranged in the evaporator chamber, one end of which is connected to a supply line coming from the mass flow controller and which is held at a distance from the heated wall surface of the evaporator chamber, but in line of sight with it. 2. Device according to claim 1, characterized in that the evaporator chamber is designed as a hollow cylinder surrounded by the heating device with an inlet-side end wall into which the feed line for the liquid monomer opens, and that the body having the large-area effect is arranged in the hollow cylinder in such a way that one end of it faces the feed line for the monomer and that its free surface faces the heated wall surface of the evaporator chamber. 3. Device according to claim 1 , characterized in that the bodies having the large-area effect consist of steel balls. 4. Device according to claim 1, characterized in that the bodies having the large-area effect are introduced into a cylinder-shaped body.