WO2008037958A1

WO2008037958A1 - Effusion and cracking cell

Info

Publication number: WO2008037958A1
Application number: PCT/GB2007/003539
Authority: WO
Inventors: Peter Butcher
Original assignee: Oxford Instruments Plasma Technology Ltd
Current assignee: Oxford Instruments Plasma Technology Ltd
Priority date: 2006-09-28
Filing date: 2007-09-19
Publication date: 2008-04-03
Anticipated expiration: 2009-03-28
Also published as: GB0619163D0

Abstract

An effusion and cracking cell is provided which comprises a crucible region (20) having a first heater (24) for evaporating or sublimating a substance to produce a first gaseous species, a cracker region (50) having a second heater (51) for transforming the first gaseous species into a second gaseous species, a transfer path (40) for transferring the first gaseous species from the crucible region to the cracker region and a valve assembly located in the transfer path adapted to control the flow of the first gaseous species. A third heater (63) is provided so as to heat at least part of the transfer path between the crucible region and the cracker region for controlling the temperature of the first gaseous species therein.

Description

EFFUSION AND CRACKING CELL

This invention relates to an effusion and cracking cell for use in materials depositing systems such as molecular beam epitaxy (MBE) devices. One of the main uses of deposition devices such as MBE systems is the growth of semiconductor devices. An MBE system allows the epitaxial growth of semiconductor materials under clean and well controlled conditions using one or more effusion cells to evaporate material into a chamber, where it is deposited onto a substrate. There is a requirement for a cell that has the capacity to evaporate large quantities of material with a stable and reproducible flux. For small, experimental MBE systems, a 500 cubic cm cell capacity could be sufficient, but for larger, industrial systems, capacities of up to 10 litres may be required.

Arsenic is a key material in such processes for growing GaAs-related semiconductor devices. Arsenic normally evaporates as As₄ molecules which, due to a poor sticking coefficient, results in inefficient growth material. As such, it is preferable to pass the evaporated arsenic through a cracker where it can be changed to a predominantly As₂ species. As₂ has a much higher sticking coefficient compared to As₄.

The first effusion cells used for evaporating arsenic produced only As₄ and consisted of a simple cell for heating a crucible containing the solid arsenic. To control the flux of evaporant, either a shutter had to be placed in front of the cell or the temperature of the crucible varied. These techniques suffered from a number of problems. Arsenic evaporates at a comparatively low temperature, around 350⁰C, and at this temperature any temperature changes can take an unacceptable time to stabilise. A shutter in front of the cell's orifice, while stopping the direct line of sight to a substrate, does not control the arsenic pressure in the MBE deposition chamber, resulting in poor control over growth conditions and the possibility of saturating the vacuum pumps.

Later cells had an inline cracker fitted to them. This cracker, typically heated to around 900°C, cracks As₄ to As₂. This has the advantage of not only achieving more efficient growth but, because As₂ sticks to the MBE chamber walls, it results in a better vacuum within the chamber. A shutter in front of the cell will now be more effective at shielding flux from the substrate, because the arsenic will stick to it.

However there is still no real control over the flux rate without changing the crucible temperature. More recent arsenic cells have a crucible/valve/cracker configuration. The crucible temperature remains constant and the valve controls the flux to the substrate. In addition, when the valve is in the fully closed position, the flux is stopped completely. However, the complex valve components are particularly vulnerable to condensing evaporant. This can cause an impedance to the evaporant flow and the valve mechanism to become stiff or inoperable.

Examples of such crucible/valve/cracker configuration cells are disclosed in US- A-5,156,815 and US-A-5,080,870.

In accordance with the present invention, an effusion and cracking cell comprises a crucible region having a first heater for evaporating or sublimating a substance to produce a first gaseous species; a cracker region having a second heater for transforming the first gaseous species into a second gaseous species; a transfer path for transferring the first gaseous species from the crucible region to the cracker region; and a valve assembly located in the transfer path adapted to control the flow of the first gaseous species; wherein a third heater is provided so as to heat at least part of the transfer path between the crucible region and the cracker region for controlling the temperature of the first gaseous species therein.

The provision of a third heater arranged to heat at least a part of the transfer path between the pivotal region and the cracker region makes it possible to maintain that part of the transfer path at a sufficient temperature such that condensation of the gaseous species in this area is substantially avoided. This prevents any impedance to vapour flow and stops the valve mechanism from becoming "sticky" or jamming.

Preferably, the first, second and third heaters are controlled independently of one another. This particularly flexible arrangement makes it possible to operate each heater at different temperatures and at different times or operation cycles. However, in other embodiments, the heaters could be so linked as to operate simultaneously with one another, each heater being adapted to operate at the appropriate operating temperature for its respective region of the cell.

In a particularly preferred configuration, the valve assembly and the at least part of the transfer path are disposed in a valve region of the cell. Thus, the third heater is arranged so as to heat the region of the transfer path containing the valve assembly. This has the advantage that the valve region can be maintained at such a temperature so as to avoid condensation of the evaporant on the valve assembly components, which might otherwise interrupt vapour flow or cause the valve to jam.

Preferably, at least one of the crucible regions, the cracker region and the at least part of the transfer path are provided with means for monitoring the temperature. This makes it possible to determine the local temperature of the cell in each of those regions.

Conveniently, the at least one heater associated with the at least one of the crucible region, the cracker region and the at least part of the transfer path provided with means for monitoring temperature, is controlled under feedback control using the output of the means for monitoring temperature. That is, the means for monitoring temperature is used to establish feedback control of the heater which controls the temperature of the corresponding region in the cell.

Preferably, the first and second heaters are controlled under feedback control, means for monitoring temperature being provided in the crucible region and in the cracker region. The third heater could alternatively or in addition be controlled under feedback control, a corresponding means for monitoring temperature being provided, if so desired.

Conveniently, at least one of the first, second and third heaters is supplied with current from a constant current supply which is calibrated to maintain the at least one heater at a predetermined temperature. This straightforward heater control technique does away with the need for a means for monitoring temperature in certain regions.

In particularly preferred embodiments, the third heater is supplied with current from a constant current supply which is calibrated to maintain the third heater at a predetermined temperature. The first and/or second heaters may alternatively or in addition be controlled in this manner if so desired.

Conveniently, the means for monitoring the temperature comprises at least one thermocouple, but any other type of thermometer could be used instead.

Preferably, in use, the third heater is maintained at a sufficient temperature to substantially prevent condensation of the first gaseous species. As described above, this minimises the possibility of interrupting the vapour flow or the valve mechanism sticking. However, the heater could be used in other modes of operation as appropriate.

Preferably, the third heater is maintained at a temperature between 450 and

500⁰C, preferably between 470 and 480⁰C, more preferably approximately 475°C. In the case of arsenic, such temperatures maintain the substance in its gaseous state. Conveniently, at least a portion of the transfer path is defined by a transfer tube. This provides a convenient means of transferring the evaporant from the crucible region to the cracker region. Preferably, the transfer tube comprises a bellows for aligning the crucible region with the valve assembly. The bellows allows a degree of movement in the transfer tube which assists in aligning each end with its neighbouring component.

Preferably, the substance to be evaporated and deposited by the cell is arsenic, the first gaseous species being As₄ and the second gaseous species being As₂. However, the cell could be adapted for use with any other substance, such as phosphorous.

Preferably, the effusion and cracking cell is for use in a molecular beam epitaxy (MBE) system. However, the cell could be employed in any other deposition system which uses effusion cells.

An example of an effusion and cracking cell in accordance with the present invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a perspective view showing an effusion and cracking cell; Figure 2 is a cross-section of the effusion and cracking cell shown in Figure 1 ; Figure 3 shows an enlarged portion of the cross-section shown in Figure 2, depicting the crucible region of the cell;

Figure 4 is a schematic cross-section of a portion of the effusion and cracking cell depicting the valve region;

Figure 5 shows in more detail a portion of the valve assembly visible in Figure 4; Figure 6 is an enlarged portion of the cross-section of Figure 4, showing the valve assembly in more detail;

Figures 7A to D show a sliding rod, forming part of the valve assembly shown in Figure 6, in side view, top view, end view and perspective view respectively;

Figures 8A to D show an actuator, forming part of the valve assembly shown in Figure 6, in side view, cross-section, end view and perspective view respectively; Figures 9A to D show a valve plug, forming part of the valve assembly shown in Figure 6, in side view, cross-section, end view and perspective view respectively; Figure 10 shows a cross-section of the sliding rod, actuator and valve plug of Figures 7, 8 and 9, as assembled in use; and Figure 11 shows a cross-section through the cracker region of the cell shown in Figure 2.

Figure 1 shows an overview of an effusion and cracking cell 10. The material to be deposited, such as arsenic or phosphorous, is loaded into a crucible inside crucible region 20. The cell shown in Figure 1 is used predominantly for arsenic, but it can also be used to deposit other materials, such as phosphorous. As such, the description below will focus mainly on the use of the cell to deposit arsenic, however it will be appreciated that other materials can be deposited by the cell, and its applications are not restricted to arsenic deposition. The crucible is heated to a temperature at which the substance evaporates or sublimes producing a gaseous species. In the case of arsenic, this evaporant is typically As₄. The evaporated substance passes through a transfer region 30 into a valve region 40 through which it reaches a cracker region 50. In the cracker region 50, the flux of evaporated material passes through a heated region where the gaseous species can be cracked, for example from As₄ to As₂, if required. The substance exits the cracker region 50 into a deposition chamber (not shown).

Typically, the crucible capacity is approximately one litre or greater. For example, the version shown has a three litre capacity crucible.

Figure 2 shows a cross-section of the whole cell 10. Arsenic is placed in a crucible 22 and is heated to a temperature where it evaporates or sublimes. The evaporating arsenic flux from the crucible 22 passes through a transfer tube 31 to a valve chamber 41 in which a valve assembly determines the arsenic vapourflow. From the valve, the evaporant passes through an injector tube 51 to the cracker region 50.

The cracker can either be kept at a temperature that allows vapour flow but does not crack the molecule, or the temperature can be raised by heaters 52 so that the arsenic is converted from As₄ to As₂. From the cracker region 50, the vapour exits the cell 10, the end of which is positioned at a predetermined effusion distance from a substrate onto which the arsenic is to be deposited.

The substrate is enclosed in a deposition chamber (not shown), which is kept at a very low pressure or vacuum in order not to impede the passage of molecules from the effusion cell(s) to the substrate. The effusion cell 10 is coupled to the deposition chamber through a port in the chamber wall and sealed by a chamber mounting flange 60. In practice, more than one effusion cell 10 may be coupled to a single deposition chamber in order to allow for deposition of more than one material either sequentially or simultaneously. There may also be more than one substrate in the deposition chamber such that multiple substrates can be deposited on at any one time.

It will be noted that only the cracker region 50 of the cell 10 extends into the deposition chamber, whilst the crucible region 20 and the valve region 40 remain external. This makes it possible to easily accommodate different crucible sizes since the transfer region 30, valve region 40 and cracker region 50 can remain the same whilst the crucible 20 is varied.

The crucible region 20, transfer region 30, valve region 40, and cracker region 50 are housed within a vacuum enclosure E suitable for MBE and ultra high vacuum operation. Apertures are provided in the enclosure surrounding the cracker region so which enable fluid communication between the vacuum enclosure E and the deposition chamber such that the cell enclosure E is evacuated by the deposition chamber pumps. At various stages of operation, it is also necessary to internally evacuate the cell. For example, when the crucible is cold and during initial pump-down the valve is fully opened to allow the crucible to be pumped internally. It is also usual to have the valve open whilst raising the crucible temperature in order to release any contaminant gases. In addition, the valve must be open during venting in order to return the crucible to atmospheric pressure and prevent possible damage.

Figure 3 shows the crucible region 20 in greater detail. The crucible 22 is housed in a water-cooled vacuum enclosure 21. The entry and exit ports 21 a and 21 b for the flow of water (or other cooling fluid) are shown in Figures 2 and 3. Water cooling prevents parts of the cell which should remain cold from becoming hot and contaminating the substrate with out-gassing components during semiconductor growth. In addition, radiation shields are positioned adjacent to the heaters to reduce heat loss.

The crucible 22 consists of a first cylindrical portion 22a and a second cylindrical portion 22b, of smaller diameter. The second cylindrical portion 22b is arranged above the cylindrical portion 22a. Crucible heaters 24a and 24b are arranged around the circumference of the second cylindrical portion 22b and adjacent to its flat surface towards the top of the cell. The heaters typically comprise wire heaters which radiate heat, although any other suitable heating arrangement could alternatively be employed. Positioning the crucible heaters 24a and 24b adjacent the top half of the crucible 22 gives preferential heating to this part of the crucible 22. This is preferred in order that, after a short period of heating, the unevaporated substance migrates to the colder base of the crucible 22 (i.e. the first cylindrical region 22a) where it adheres as a solid mass. This occurs as a result of the dynamic flux between solid and gaseous states of the substance. When arsenic is heated, it creates a vapour pressure. When the valve is closed, the gas vapour continually solidifies and reevaporates to maintain a stable vapour pressure. Even when the valve is open most of the vapour carries on in this cycle. The vapour is most likely to solidify at the coldest part of the crucible (i.e. furthest from the heating elements) and so, during initial operation, there is a mass migration from the original location of the arsenic to the crucible base which is intentionally kept cooler. To maintain the evaporation rate, the crucible temperature has to be increased slightly to compensate. This has two advantages: firstly, the mass is stable and this provides a stable evaporation rate. Secondly, it allows the cell to be mounted to the MBE system in any orientation since the evaporating material will not condense or solidify in the transfer tube and valve regions. A filter 25, such as a pepper pot filter, is provided on the entrance to the transfer tube 31 in order to prevent any solid material entering the transfer tube 31 , which could otherwise happen, for example, if material were to fall into the second cylindrical region 22b when the crucible is first loaded.

The crucible 22 has two loading ports, the first loading port 23 located at the base of the crucible region 20 adjacent to the first cylindrical region 22a of the crucible

22, and the second loading port 26 located near the top of the crucible region 20 adjacent to the second cylindrical portion 22b of the crucible 22. The first loading port

23a is used for the initial loading of material into the cell when it is empty, but if reloading is required when the cell is not empty, then the second loading port 26 is used since the first loading port 22 tends to be obstructed with unevaporated material.

During typical operation, the crucible 22 is heated to between 350⁰C and 450⁰C to provide an acceptable rate of evaporation or sublimation. The temperature is measured using a thermocouple 27 positioned inside the crucible 22 to ensure an accurate reading. The temperature measurement is output to a control system which monitors the temperature and, in some examples, operates under feedback control to adjust the power supplied to the crucible heaters 24a and 24b so as to maintain the crucible 22 at a predetermined temperature.

The crucible region 20 connects to the rest of the cell via transfer region 30 which contains transfer tube 31. The vacuum enclosure E surrounding the transfer tube 31 joins the crucible region 20 to the valve region 40, sealed coupling being achieved by flanges 32 and 33. The flanges 32 and 33 are sealed using a copper gasket, as is conventional in ultra high vacuum systems. The crucible region 20 can be quickly removed and capped in the field, which is important since materials such as arsenic tend to oxidise very quickly and may become unusable if exposed to the atmosphere for more than approximately one hour. If the cell requires servicing, then the MBE system is let out to an atmosphere of dry nitrogen and then the valve in valve region 40 closed. The crucible enclosure 21 is removed by unbolting the flange that holds it to the transfer region 30 and sliding it away from the valve region to reveal the crucible 22. The transfer tube is then disconnected from the crucible and the coupling between the top heater plate and crucible released such that the crucible 22 can be removed. The crucible 22 can then be capped at the transfer tube port and back filled with nitrogen to replace the residual air.

The transfer tube 31 is shown in full in Figure 2 and partially in Figures 3 and 4. The transfer tube 31 is provided with a heater (not shown) which maintains the transfer tube 31 at a temperature above that of the crucible 22, thus avoiding any condensation or blockage in this region. The transfer tube heater is wired in series with the crucible heaters 24a and 24b, allowing them to be powered from a single supply. The transfer tube 31 is preferably made of titanium and, in the embodiment shown, is provided with a bellows 34 along a portion of its length to assist alignment between the crucible region 20 and the valve region 40. The bellows 34 are welded into the transfer tube 31 to form an integral component. The bellows 34 permit both longitudinal movement parallel to the axis of the transfer tube 31 and lateral movement in a direction perpendicular to its axis.

Details of the valve region 40 are shown in Figures 4 to 10. The valve region 40 is surrounded by a water-cooled enclosure 62 to prevent outgassing. Water (or other coolant) enters the enclosure 62 through input 62a and exits through output channel 62b. The evaporant from the transfer tube 31 enters the valve region 40 through channel D in valve block 65. A portion of the channel D is defined by a valve jet 44 through which evaporant passes from transfer tube 31 into valve chamber 41. The valve chamber 41 consists of regions A, B and C shown in Figure 4 which provides passage through the valve block 65 to the cracker region 50.

The valve mechanism essentially comprises a needle valve. The flow of evaporant through the valve jet 44 is controlled by valve plug 43 which is approximately conical in shape and is moveable between a first position, in which the plug is inserted into the jet 44, thus preventing passage of evaporant therethrough, and a second, open position in which the plug 43 is at least partially removed from jet 44 and evaporant is able to flow from the transfer tube 31 into the valve chamber 41. Using this configuration, an approximately linear relationship can be had between valve opening and evaporant flux: the further the plug 43 is inserted into the jet, the less evaporant will flow into valve region 41. When the valve plug 43 is fully inserted into the jet 44 there is a complete seal by two mating flanges, and the valve block 65 is arranged such that, when the valve is closed, the evaporant is kept a distance away from any moving parts within the valve assembly, thus minimising the chance of long term corrosion or leakage in this region.

The valve assembly has to be actuated from outside the cell and it is important that this can be achieved without evaporant leaking from the valve region 41. In the present embodiment, this is achieved by using a actuator assembly 42, 48 and 49 connected to the valve plug 43 via a sliding rod 46. The sliding rod 46 passes through a bushing 64 mounted in the wall of valve chamber 41. Preferably, the bushing extends along more than half of the length of the sliding rod 46 in order to improve the seal.

In a particularly preferred embodiment, the sliding rod 46 is made of tungsten and the bushing 64 of a ceramic such as alumina or pyrolytic boron nitride. This choice of materials is particularly effective since the materials are able to withstand the operating conditions and a good sliding fit with minimum clearance can be made between the two components. The valve is operated at around 470^cC to avoid condensation of the evaporant and at these temperatures most metals are soft so that when pressure is applied to fully close the valve, the mechanism turns to distort. In addition, similar material configurations such as an all-metal sliding seal become sticky and eventually stop sliding. It has been found that tungsten and ceramics such as alumina do not suffer from such disadvantages : alumina remains hard at the operating temperature and is ideal as a bearing material; tungsten may bend a little at the high temperature under stress, but will tend to straighten when the rod is moved. Both components lend themselves to the application since accurately ground tungsten rod is available and previously-fired alumina can be machined at high tolerances. Alumina is the preferred material because it is a relatively clean material yet is machinable. Pyrolytic boron nitride would provide even higher levels of cleanliness but it is more difficult to machine, and therefore more expensive. This configuration has been tested and there is no evidence of arsenic leaking or the mechanism sticking.

To complete the seal between the sliding rod 46 and the bushing 64, an O-ring shaped graphite seal 65 is fitted between the components and held in place by a metal collar 65A. Graphite seals are also provided at the join between the bushing 64 and the wall of the valve region 41.

Conventional cells suffer from leakage of evaporant at joints in the construction.

This can cause contamination of heaters and thermocouples which can lead to the cell failing. The present inventor has found that the use of graphite gaskets substantially eliminates leakage: after 1500 hours of operation with arsenic there was no evidence of leakage from joints with graphite gaskets. Moreover, the joint components, and flanges in particular, typically comprise titanium or tantalum. In some conventional cells, gold gaskets are used but these have been found to react with titanium at elevated temperature. No such reaction is found with graphite.

The gasket material should be soft enough to form an effective seal and also it must not react with or contaminate the joint materials. In one example, the graphite gaskets are made from 0.2mm thick flexible graphite, as defined by the supplier Goodfellow. Flexible graphite is a layered material as opposed to other forms which are generally sintered. The material is "soft" in so much that it does not deform the joint components it sits between.

In addition to the seal 65 between the sliding rod 46 and bushing 64, such gaskets are used to seal the first and second loading ports 23 and 26 in the crucible region 20, and at both ends of the transfer tube 31. There is also a graphite gasket where the valve mounting flange 66 (Figure 5) joins to the valve block 65.

These seals take aflat annular form having approximate dimensions of the joint they are used in and with holes through them to allow for bolts to pass through.

An exception is the seal 65 between the sliding rod 46 and the bushing 64 which as described above is an O-ring shape. It has a square cross-section and is approximately 4mm in diameter having a cross-section of about 1 mm².

In order for the valve assembly to seal correctly, accurate alignment between the valve plug 43 and the valve jet 44 is important. An initial alignment can be made by adjusting the position of the bushing 64 relative to the valve chamber wall, but there can be alignment difficulties if the valve jet 44 is not absolutely square to the valve plug 43. Moreover, the relative positions of the mechanism can change when the valve region is heated.

To address this, two flexible joints 45 and 47 are provided at either end of the sliding rod 46. The first flexible joint 47 connects the actuator assembly via extension

42B to the first end of the sliding rod 46, which is outside the valve region 41. The second flexible joint 45 connects the second end of the sliding rod 46 to the valve plug 43, both of which are inside the valve region 41. This configuration is shown in Figures 5, 6 and 10.

Each flexible joint 45 and 47 permits angular and/or translation movement between the components it connects. Any type of flexible joint could be used, such as a hinge or a universal joint. However, in the present embodiment, it is advantageous to keep the machining required of the sliding rod 46 to a minimum, particularly in the case where the rod is made from tungsten. Thus in the present example, the flexible joints are formed as shown in Figures 7 to 10.

The sliding rod 46 is shown in Figures 7A to D. The sliding rod 46 is provided with two slots 46a, 46b at each end of the rod, positioned a short distance away from the extremities. With tungsten, normal machining is difficult and as such these slots are preferably formed by grinding. The sliding rod 46, which is of generally circular cross-section, is inserted into flexible joint 47 at its first end and into flexible joint 45 at its second end. Flexible joint 47, and the actuator 42B with which it is integrally formed, are shown in Figure 8. Flexible joint 47 takes the form of a body having a bore 47A extending at least partially therethrough, parallel to the axis of the joint 47 and actuator 42B (marked A to A in Figure 8A). The diameter of bore 47A is larger than that of sliding rod 46 such that some relative movement between the bore and the rod is permitted. Two through holes 47B are provided which partially intercept the bore 47A perpendicular to the axis A-A. With the rod 46 in position, bolts are passed through the two through holes 47B and secured by nuts at the opposite side. As shown in Figure 10, the position of the bolts through bores 47B co-operates with slots 46A in rod 46 such that the rod is secured within bore 47A whilst a degree of movement remains. In this way, rod 46 and actuator 42B are rotatably moveable with respect to one another about axes provided by the bolts through holes 47B.

In the example shown in Figure 6, the sliding rod is rotatably moveable relative to the actuator in a horizontal plane, however, the components could readily be reorientated to permit vertical adjustment. In alternate embodiments, annular movement in more than one plane perpendicular to the axis of the actuator may be permitted. It will be seen that some translational movement is also permitted between the rod and the bore in the flexible joint. To limit this, packing material such as a thin washer (not shown) is placed between the end of the rod 46 and the closed end of the bore 47A. This also has an important function in that it takes up backlash and thereby ensures there is positive linear movement of the sliding rod 46 in both directions. By choosing the correct clearances between the sliding rod 46 and the bore 47A, X-Y and angular flexibility is achieved, whereas linear movement in the Z direction is minimised.

At the opposite end of the rod 46, the second flexible joint 45 has a similar construction to that of the first flexible joint 47. Again, angular movement between the valve plug 43 and sliding rod 46 is permitted as well as a degree of translational movement. Packing material (not shown) is disposed between the sliding rod 46 and the base of the bore 45A to take up backlash.

The flexible joint 45 is formed integrally with valve plug 43 which is typically of conical shape to allow for mating with the valve jet 44. The actuator is driven by a linear drive assembly 48 and 49 which moves the actuator 42b towards and away from the seal 64, thereby operating the valve. Any suitable linear drive could be employed, but in the present example a screw-driven linear drive is depicted, the drive screw being accommodated inside drive shaft 48, connected to stepper motor 49. The motor 49 has "on board" intelligence which allows the motor to operate without a complex controller.

In alternative embodiments, the linear drive may be manually operated, in which case the motor 49 is replaced by a manual drive arrangement. In both cases, suitable linear drive assemblies are available from VG Scienta, of Hastings, United Kingdom.

The use of two flexible joints 45 and 47 makes it possible to use a standard linear drive feedthrough with a motor to provide the movement which opens and closes the valve. No further provisions for alignment are required.

The valve region 40 is provided with a heater 63 for controlling the temperature of the evaporant in the valve chamber 41. The heater 63 may be provided in the form of a plate heater underneath the valve chamber 41 , as depicted in Figure 6, or could be provided in the form of elements surrounding the valve block 65. The valve heater is used to maintain the temperature of the evaporant to prevent it condensing in the valve region. Condensation can result in blockages in the valve mechanism, which are most evident when the cell is run in the lower temperature As₄ mode (without cracking).

However, similar problems can be encountered to a degree when the cracker is enabled.

The valve heater 63 is preferably supplied from a constant current source which maintains the valve chamber 41 at approximately 470-475⁰C, although this temperature will vary depending on the crucible and cracker temperatures. In the case of arsenic, maintaining such temperatures essentially prevents condensation. In some embodiments, it may be preferable to utilise feedback control to maintain the valve region at a particular temperature and in this case a thermometer will be included in the valve region 41.

Thus the cell has at least three heaters: the crucible heater 24, the valve heater 63 and the cracker heater 51 (described below). Typically, these three heaters are each controlled independently so that each region can be maintained at its optimum operating temperature. A controller (not shown) supplies power to the three heaters and this can be adjusted to maintain the associated region at the desired temperature. In some cases, it is preferred to use feedback control in which case a means for measuring temperature is provided in the relevant region. In the present case, the crucible and cracker heaters are controlled in this way but in other embodiments it may be advantageous to control the valve heater in the same manner. In the present example, constant current control is used for the valve heater but could alternatively or in addition be used to control the crucible and/or cracker heaters.

As described above, there may be additional heaters provided at other positions in the cell to avoid condensation. For example, the transfer tube 31 may be provided with a heater which is preferably in series with the crucible heater. Alternatively the transfer tube heater could be controlled independently.

Evaporant flux from the valve region 40 passes into the injector tube 51 (Figure 11). The injector tube 51 carries the flux from the valve region to the end of the cell where it enters the deposition chamber. Adjacent to the end of the injector tube 51 is the cracker, which is provided with heating elements 52 and a number of baffles 53 within the injector tube 51. The baffles 53 force the evaporant to have many self collisions before exiting the cell. When the cracker is heated to around 900⁰C₁ evaporated arsenic is cracked from As₄ to As₂. If the temperature of the cracker is reduced to around 600⁰C, there is very little cracking. The heater 52 is positioned around the injector tube 51 in the cracker region to heat it uniformly. The feed wires to this heater run the length of the injector tube 51 and these provide secondary heating to keep the region preceding the cracker at an elevated temperature and thus avoid condensation. A thermocouple 54 is provided to monitor the temperature of the cracker region, which may be controlled using feedback. The cracker is surrounded by a water-cooled enclosure 72 to prevent contamination by out-gassing components of the cell.

Claims

1. An effusion and cracking cell comprising: a crucible region having a first heater for evaporating or sublimating a substance to produce a first gaseous species; a cracker region having a second heater for transforming the first gaseous species into a second gaseous species; a transfer path for transferring the first gaseous species from the crucible region to the cracker region; and a valve assembly located in the transfer path adapted to control the flow of the first gaseous species; wherein a third heater is provided so as to heat at least part of the transfer path between the crucible region and the cracker region for controlling the temperature of the first gaseous species therein.

2. An effusion and cracking cell according to claim 1 wherein the first, second and third heaters are each controlled independently of one another.

3. An effusion and cracking cell according to claim 1 or claim 2, wherein the valve assembly and the at least part of the transfer path are disposed in a valve region of the cell.

4. An effusion and cracking cell according to any of the preceding claims wherein at least one of the crucible region, the cracker region and the at least part of the transfer path are provided with means for monitoring temperature.

5. An effusion and cracking cell according to claim 4 wherein the at least one heater associated with the at least one of the crucible region, the cracker region and the at least part of the transfer path provided with means for monitoring temperature, is controlled under feedback control using the output of the means for monitoring temperature.

6. An effusion and cracking cell according to claim 5 wherein the first and second heaters are controlled under feedback control, means for monitoring temperature being provided in the crucible region and in the cracker region.

7. An effusion and cracking cell according to any of the preceding claims wherein at least one of the first, second and third heaters is supplied with current from a constant current supply which is calibrated to maintain the at least one heater at a predetermined temperature.

8. An effusion and cracking cell according to claim 7 wherein the third heater is supplied with current from a constant current supply which is calibrated to maintain the third heater at a predetermined temperature.

9. An effusion and cracking cell according to any claims 4 to 8 wherein the means for monitoring temperature comprises at least one thermocouple.

10. An effusion and cracking cell according to any of the preceding claims wherein, in use, the third heater is maintained at a sufficient temperature to substantially prevent condensation of the first gaseous species.

11. An effusion and cracking cell according to claim 10 wherein the third heater is maintained at a temperature between 450 and 500 degrees C, preferably between 470 and 480 degrees C, more preferably approximately 475 degrees C.

12. An effusion and cracking cell according to any of the preceding claims wherein at least a portion of the transfer path is defined by a transfer tube.

13. An effusion and cracking cell according to claim 12 wherein the transfer tube comprises a bellows for aligning the crucible region with the valve assembly.

14. An effusion and cracking cell according to any of the preceding claims wherein the substance is arsenic, the first gaseous species is As₄ and the second gaseous species is As₂.

15. An effusion and cracking cell according to any of the preceding claims wherein the effusion and cracking cell is for use in a molecular beam epitaxy (MBE) system.