GB2230607A - Temperature- controlled bodies - Google Patents

Abstract

A method of heating a body, such as a semiconductor wafer body (4), in an oven (1), the temperature control of which is effected by means of an optical pyrometer (8) directed towards a radiation emission surface of said body (4), the method comprising the step of depositing a layer (10) structure of known emissivity on said surface, such that a uniform distribution of thermal radiation across the structure will be provided for output to the pyrometer (8). The layer (10) may be of titanium or preferably, if subsequent removal should be required, of carbon.

Description

TEMPERATURE-CONTROLLED BODIES This invention relates to temperature-controlled bodies in ovens. It relates particularly to the temperature control of a body in what is called a 'cold' wall oven, that is a heating chamber in which a workpiece is immersed in thermal radiation which is not effectively identical to that which it is emitting.

One application of a 'cold' wall oven is in the treatment of semiconductor wafers where the wafer is bathed in electromagnetic radiation whilst supported in thermal isolation within a chamber to which an ambient gas can be admitted. For various reasons it is not convenient to measure the wafer temperature by a contact thermometer method so use of an optical pyrometer is preferred.

The pyrometer, therefore, is arranged to view the wafer surface through a window, in order to monitor and assist control of the wafer temperature.

Temperature measurement by pyrometry is dependent for its accuracy on exact knowledge of the effective optical emissivity of the region of the wafer being viewed. This parameter is found to be a strong function of the surface microstructure and it will be found to vary significantly with small thickness and property variations in the planar surface produced in earlier process steps. Temperature uniformity across the wafer is also affected significantly by lateral variations in such layers, through the change in radiation energy balance that occurs when the average emissivity of the surface changes.

The present invention was devised in an attempt to provide a surface structure of consistent and reproducible emissivity. This can enable the amount of temperature measurement error which is attributable to the wafer to be reduced to a low value.

According to the invention, there is provided a method of heating a body, such as a semiconductor wafer body, in an oven, the temperature control of which is effected by means of an optical pyrometer directed towards a radiation emission surface of said body, the method comprising the step of depositing a layer structure of known emissivity on said surface, such that a uniform distribution of thermal radiation across the structure will be provided for output to the pyrometer.

Preferably, the said layer structure is arranged to be opaque to the sampling wavelength of the pyrometer such that portions of the said body beneath the structure are excluded from making a contribution to the surface radiation output.

In one embodiment, the said layer structure is of titanium in a thickness range of about 0.05 to 0.10 micrometres. Alternatively, the layer structure may be of carbon in semiconductor form, such as graphite in a thickness range from 0.5 to above 1.0 micrometres.

After the heating operation, the carbon deposit may be removed by a chemical treatment, such as in an oxygen plasma or by a chemical etch.

The invention also comprises a semiconductor wafer body when treated by the aforementioned heating method.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a general view of the heating operation as carried out in a 'cold' wall oven, Figure 2 shows the basic features of a lamp source rapid thermal annealer system, Figure 3 a sequence for operating a Rapid Thermal Processor machine to enable a series of semiconductor wafers to be heat treated automatically, Figure 4, 5 and 6 are sketches illustrating deposition and removal of the layer structure.

As depicted in Figure 1, the heating apparatus comprises an oven 1 having walls 2 and means for directing an electromagnetic energy input 3 onto a workpiece 4.

The workpiece 4 is shown in cross-section and the upper side 6 is seen to be a planar body whilst beneath this are several regions 7 which have differing optical properties. On the lower side of the oven there is a transparent section forming a window 5 in the relevant wall and outside this window a pyrometer 8 is located, the pyrometer being directed through a wavelength filter 9 towards the workpiece 4. The provision of the transparent window 5 is considered to be important since most embodiments would require control of the ambient atmosphere and some would require a vacuum.

In operation of the heating apparatus, the electromagnetic energy input onto the workpiece 4 will raise the workpiece temperature and an energy loss from the workpiece will occur by thermal radiation. Some of this radiated energy will be on the lower side of the workpiece and will pass through the filter 9 and enter the pyrometer 8. An electrical output from the pyrometer would normally provide a satisfactory way of calculating the workpiece temperature, but the presence of the regions 7 with differing optical properties causes inaccuracies. This is because the regions 7 can be of dielectric and semiconductor material of varying thicknesses, thus possessing a variety of optical properties, and creating a non-uniform distribution of thermal radiation across the workpiece 4.The lack of uniformity and the fact that the effective optical emissivity of the relevant workpiece area is not known causes a serious problem in controlling the workpiece temperature.

The present invention involves the deposition of a layer 10 structure of a suitable material of known emissivity on the workpiece area directed towards the pyrometer. This material must be uniform, adherent, and stable at the working temperature. The deposited material is preferably opaque to the sampling wavelength of the pyrometer so that the effect of the different regions on the workpiece 4 will be hidden and they will thus be unable to distort the temperature control of the system.

The invention will be further described with reference to Figure 2 which shows a Rapid Thermal Annealer apparatus in vertical section. This apparatus is used in the manufacture of semiconductor devices as an ion implant damage annealer, to activate implanted dopant species, and as a method to oxidise or nitride the surface of semiconductor materials. For these processes, the process temperature needs to be well controlled in the ambient atmosphere and on the surface of the wafer.

In this apparatus, the electromagnetic energy input is provided by electric lamp sources 11 which are enclosed by reflectors 12. A silicon wafer 4 for treatment in the apparatus is supported on quartz pins (not shown) and in a condition of thermal isolation within a cell.

The cell is formed inside a space defined by upper and lower transparent windows 13 and side walls. A pyrometer 8 views the lower side of the wafer 4 through one of the windows and a filter 9 in order to monitor and control the wafer temperature.

The operation of this apparatus is as follows. A semiconductor substrate having had several dielectrics and other semiconductor materials of varying thicknesses deposited on it during its process history, is mounted face up in the chamber. Emissivity changes occurring at the wafer 4 surface are monitored by the optical pyrometer 8. If there are variations in film thickness and composition on the back of the wafer (as was described in connection with Figure 1) there will be a range of emissivities, and temperature gradients will form across the wafer 4. Therefore, the pyrometer 8 will not be able to control the temperature uniformly across the wafer area. If a material in the form of a layer 10 and opaque to the operating wavelength of the pyrometer 8 is applied to the lower surface, the deeper wafer layers will effectively be hidden from the monitoring radiation. Provided that the emissivity of this uniform layer 10 is known and that it remains in a stable condition during the processing operation, the pyrometer will be able to accurately control the temperature of the workpiece 4. This accuracy will not only apply to a single run of the apparatus but to several runs regardless of the optical properties of the initial substrate.

For the purpose of the present invention, it has been shown that a layer of titanium (having a thickness greater than 0.05 micrometres) deposited on the workpiece does meet these requirements to produce a layer 10 structure that remains stable to 1 1000C. With a workpiece formed of silicon, the heat treatment results in formation of a titanium silicide layer 10 and this enables the pyrometer 8 to control the temperature over the whole workpiece area. The emissivity of this layer over a temperature range of 800 to 1100 C is 0.4 to 0.5.A titanium layer deposited on oxide or nitride is also stable to high temperatures, so that a titanium film sputtered on to the back of a process wafer coated with any laterally or vertically varying layers in silicon, silicon oxide or silicon nitride can be rendered as an almost completely uniform emissivity surface.

The titanium layer thickness is chosen such that the subsequently formed titanium disilicide is effectively optically opaque over the relevant wavelength range. A typical thickness is Ti = 0.05 to 0.10 micrometres.

The underlying silicon layer (either specially deposited as part of the coating process, or already present at the wafer surface) is to be at least 100% thicker than is required to convert all the titanium layer to titanium disilicide (typically Si = 0.2 to 0.4 micrometres).

A two-stage heating process is used to bring the workpiece to temperature. In a first stage, at a temperature between 700 and 800 C, the titanium and silicon react rapidly to form a silicide, the outer surface of which reacts with the material of the ambient atmosphere to form silicon nitride, oxide or oxynitride according to the ambient composition. The layers formed by this reaction are adherent, resistant to chemical attack and extremely thin. It will be noted that these films are automatically created as part of the heating process at this stage, and they effectively encapsulate the titanium silicide formed so that it is stable with respect to the ambient atmosphere in subsequent stages. In addition, the titanium silicide is prevented from contaminating the heating chamber.

Titanium metal overlying ceramic layers (for example silicon oxide, silicon nitride) is also converted to titanium nitride in a nitrogen ambient at this stage, which also forms an encapsulating layer.

The second stage of the heating cycle is at the proposed working temperature. If this is greater than 9000C, the titanium silicide further reacts rapidly to form a stable disilicide and the uniform emissivity layer is established in the first few seconds of the heat treatment.

The emissivities of titanium, titanium monosilicide and titanium disilicide are very similar over the wavelength range from one to ten micrometres, so that in effect a uniform emissivity layer is present throughout the two stage heat treatment. This layer remains uniform independently of the nature of the underlying materials (for example, silicon, silicon nitride or silicon oxide) and of their relative distribution. The passivation of the titanium silicide enables heat treatment in reactive ambient atmospheres including oxygen, to be carried out at high temperatures on work wafers pretreated in the same fashion.

A further example of a suitable coating material which is capable of producing a stable layer structure and which has a constant emissivity of almost 0.9 is carbon in semiconductor form (for example, graphite in a layer thickness greater than one micrometre). This material has the advantage that it can easily be removed by a chemical treatment such as in an oxygen plasma or by a chemical etch such as sulphuric peroxide.

In the use of carbon as a layer structure material, the following features are believed to be important: (a) the deposited film must be free of hydrogen in order to be optically opaque in thicknesses within the range convenient for deposition (that is, between a half and one micrometre in thickness), and in order to prevent subsequent disruption of the film by internal stress, (b) the film must adhere well to the underlying silicon, silicon oxide and silicon nitride. Deposition by sputtering at substrate temperatures of about 2000C is required.

One novel feature which is able to rely on the chemical inertness of carbon in all ambient atmospheres except oxygen is the possibility of the incorporation of a carbon coating and a carbon removal process into the operation of the heating chamber. This can allow the coating to be applied without requiring any special action on the part of a person operating the chamber.

Figure 3 gives an example of a schdule for operating a rapid thermal processor apparatus where a carbon layer structure is used.

The wafers intended to be treated in the apparatus first enter a load lock 14 from where they are transferred to the carbon deposition stage 16. Figure 4 depicts this stage in greater detail, the carbon deposit being formed on the back of the wafer 4 by means 17 such as evaporation or sputtering. After formation of the carbon layer structure, the wafers are passed through a vacuum lock 17 and are placed in the rapid thermal processor chamber 18. The required heat treatment is carried out on the wafers and thermal control is effected by means of a pyrometer 8 (Figure 5) signal output which is determined by the carbon coating 19 emissivity. Upon completion of the heating operation, the wafers are removed from the chamber 18, pass through another vacuum lock 21 and reach a carbon strip stage 22.Figure 6 shows the stripping step in greater detail, the process being effected by means such as a direct burning in an oxygen rich atmosphere, removal at a lower temperature in an oxygen plasma or inert sputtering. It is, of course, preferable if the removal can be effected by a process which is specific for carbon removal.

After stripping is finished, the wafers are passed again through the load lock 14 stage and this completes the heat treatment operation.

It will be noted that, in some instances, use of the carbon coating layer is to be preferred over that of titanium silicide. For example, carbon is preferred where the layer needs to be subsequently removed prior to carrying out later processing stages on the wafer. Carbon is also preferred where inertness to ver3 corrosive ambients (for example, high concentrations of halogen containing ambients) is necessary.

The foregoing description of particular embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, as an alternative material for the emissivity layer, a doped silicon in a suitable thickness could be used.

Claims (8)

CLAIMS:-

1. A method of heating a body, such as a semiconductor wafer body, in an oven, the temperature control of which is effected by means of an optical pyrometer directed towards a radiation emission surface of said body, the method comprising the step of depositing a layer structure of known emissivity on said surface, such that a uniform distribution of thermal radiation across the structure will be provided for output to the pyrometer.

2. A method as claimed in Claim 1, in which the said layer structure is arranged to be opaque to the sampling wavelength of the pyrometer such that portions of the said body beneath the structure are excluded from making a contribution to the surface radiation output.

3. A method claimed in Claim 1 or 2, in which the said layer structure is of titanium in a thickness range of about 0.05 to 0.10 micrometres.

4. A method as claimed in Claim 1 or 2, in which the said layer structure is of carbon in semiconductor form, such as graphite in a thickness range from 0.5 to above 1.0 micrometres.

5. A method as claimed in Claim 4, comprising the further step, after the heating operation, of removing the carbon deposit by a chemical treatment.

6. A method as claimed in Claim 5, in which the chemical treatment is by burning in oxygen, in an oxygen plasma or by a chemical etch.

7. A method of heating a body in an oven, substantially as hereinbefore described.

8. A semiconductor wafer body, when treated by a method as claimed in any one of Claims 1 to 7.