Method and apparatus for controlling the temperature of a wafer

Abstract

Method of controlling the temperature of a wafer, comprising: detecting information relating to the temperature of the wafer at a first time; detecting information relating to the temperature of the wafer at a later second time; and controlling cooling or heating of the wafer based on a comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

Description

[0001] METHOD AND APPARATUS FOR CONTROLLING THE TEMPERATURE OF A WAFER

[0002] Field of the Invention

[0003] The present invention relates to a method of controlling the temperature of a wafer and an apparatus for controlling the temperature of a wafer.

[0004] In some embodiments, the present invention relates to a semiconductor wafer mass metrology method and a semiconductor wafer mass metrology apparatus.

[0005] Background

[0006] Microelectronic devices are fabricated on semiconductor wafers using a variety of techniques, including deposition techniques and removal techniques. Semiconductor wafers may be further processed in other ways that alter their mass, e.g. by cleaning, ion implantation, lithography and the like.

[0007] A processing step performed on a wafer can be monitored by determining the change in mass of the wafer caused by the processing step, by measuring the weight of the wafer before and after the processing step using a weighing device. Processing steps carried out on wafers can cause very small changes in the mass of the wafer, which it may be desirable to determine with high accuracy. At these high levels of accuracy, errors in the measurement output of the weighing device caused by temperature variations in the wafers being measured or in the temperature of the weighing device or enclosure of the weighing device may become significant.

[0008] For example, if a wafer being weighed has a higher temperature than an enclosure of the weighing device, air currents (e.g. convection currents) may be generated in the air in the enclosure, which may affect the measurement output. In addition, the air in the enclosure may be heated, changing its density and pressure and therefore a buoyancy force exerted on the wafer by the air, which may also affect the measurement output. Such air currents and changes in the buoyancy force may also be caused by changes in the temperature of the weighing device or enclosure due to a heat load on the weighing device or enclosure.

[0009] WO2015 / 082874 describes a semiconductor wafer mass metrology method that aims to reduce such errors. In the arrangement disclosed in WO2015 / 082874, a wafer is removed from a wafer carrier and placed on an active thermal transfer plate to reduce the temperature of the wafer to a temperature close the temperature of the weighing device and enclosure. In particular, the active thermal transfer plate includes Peltier devices that are operable to actively remove and dissipate heat from the active thermal transfer plate, and therefore a wafer loaded on the active thermal transfer plate. Subsequently, the wafer is them moved from the active thermal transfer plate to a passive thermal transfer plate that is thermally coupled to the enclosure, to substantially equalise the temperature of the wafer to the temperature of the enclosure. Therefore, when the wafer is removed from the passive thermal transfer plate and placed on the weighing device in the enclosure, the temperature of the wafer is substantially equal to the temperature of the weighing device and enclosure. Furthermore, a majority of a temperature change of the wafer is performed using the active thermal transfer plate, such that a heat load on the passive thermal transfer plate and therefore the enclosure to which the passive thermal transfer plate is thermally coupled is reduced. As such, the errors discussed above caused by temperature differences or variations can be reduced.

[0010] W02020 / 064470 discloses that when using a thermal transfer plate, for example an active thermal transfer plate, the wafer is placed in contact with the thermal transfer plate for a fixed period of time deemed to be sufficient to make sure that all wafers having an initial incoming temperature within a predetermined temperature distribution are cooled to substantially the same predetermined temperature. For example, all wafers may be placed on an active thermal transfer plate for a fixed period of 40 seconds.

[0011] W02020 / 064470 further discloses that a throughput and productivity may be improved by taking into account the initial incoming temperature of the semiconductor wafer when cooling the semiconductor wafer, for example so that cooler semiconductor wafers are not cooled for as long as hotter semiconductor wafers, or for example so that a semiconductor wafer is not cooled at all if its temperature is already equal to, or within a predetermined range of, a desired temperature. In particular, W02020 / 064470 discloses detecting information relating to the temperature of the wafer, and then controlling cooling of the wafer based on the detected information. For example, wafers may be categorised as having a temperature that is either “ambient” or “above-ambient” (hot) based on whether the detected information relating to the temperature of the wafer is above or below a threshold value, and “above-ambient” wafers may be cooled for longer than “ambient” wafers.

[0012] The present inventor has realised that while this arrangement is effective in many situations, the detected information relating to the temperature of the wafer can change depending on the characteristics of the wafer. For example, the type or composition of the wafer may affect the detected information in addition to the actual temperature of the wafer. In particular, if the detected information is a detected infrared signal from the wafer, the magnitude of the infrared signal may depend on an infrared emissivity of the wafer in addition to the temperature of the wafer. Wafers having different types or compositions may have different infrared emissivities for the same temperature.

[0013] This variation in the detected information relating to the temperature of the wafer due to the type or composition of the wafer may lead to wafers being miscategorised based on the detected information, and therefore cooled for an insufficient period of time, in some cases. Alternatively, the threshold value may have to be set lower for safety, which may lead to some wafers being in contact with the thermal transfer plate for longer than is required, which will reduce the throughput.

[0014] The present invention has been devised in light of the above considerations. Summary of the Invention

[0015] As mentioned above, the present inventor has realised that the detected information relating to the temperature of the wafer may be affected by factors other than the actual temperature of the wafer, including the type or composition of the wafer. Therefore, if the detected information relating to the temperature of the wafer is compared to a threshold value in order to categorise the temperature of the wafer as being “ambient” or “above-ambient” (or hot), for example, whether or not the detected information relating to the temperature of the wafer will be greater than the threshold value may depend on both the temperature of the wafer and the type or composition of the wafer.

[0016] The present inventor has realised that a more accurate categorisation of the temperature of the wafer can be achieved based on a change of the detected information relating to the temperature of the wafer over time, rather than a single value of the detected information relating to the temperature of the wafer. For example, a wafer with a temperature that is significantly above an ambient temperature will experience a more significant change in its temperature over time when in an environment with the ambient temperature than a wafer with a temperature that is close to or equal to the ambient temperature.

[0017] According to a first aspect of the present invention there is provided a method of controlling the temperature of a wafer, comprising: detecting information relating to the temperature of the wafer at a first time; detecting information relating to the temperature of the wafer at a later second time; and controlling cooling or heating of the wafer based on a comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

[0018] Therefore, in the present invention the cooling or heating of the wafer is controlled based on the comparison of the information relating to the temperature of the wafer detected at the first time and the information relating to the temperature of the wafer detected at the second time. As such, whether or not the information relating to the temperature of the wafer changes between the first time and the second time, and if so by how much, can be taken into account when controlling the cooling or heating of the wafer.

[0019] The first aspect of the present invention may have any one, or, where compatible, any combination of the following optional features.

[0020] The wafer may be a semiconductor wafer, for example a silicon wafer.

[0021] The information relating to the temperature of the wafer may be information that is indicative of, or representative of, or proportional to, the temperature of the wafer, for example an amount of infrared (IR) radiation emitted by the wafer.

[0022] The information relating to the temperature of the wafer may be detected using a temperature sensor, for example in infra-red temperature sensor. Detecting the information relating to the temperature of the wafer typically means measuring the information relating to the temperature of the wafer, for example measuring an amount of IR radiation emitted by the wafer.

[0023] The information relating to the temperature of the wafer may be measured using a temperature sensor, for example a contact or non-contact temperature sensor.

[0024] The same type of information relating to the temperature of the wafer is detected at the first time and at the second time. For example, an amount of IR radiation emitted by the wafer may be measured at the first time and at the second time.

[0025] Controlling the cooling or heating of the wafer may comprise setting, or predetermining, or deciding, or configuring the cooling or heating of the wafer.

[0026] Comparing the detected information relating to the temperature of the wafer at the first time (a first value) and the detected information relating to the temperature of the wafer at the second time (a second value) may comprise subtracting one of the first and second values from the other, or calculating a ratio of the first and second values, for example.

[0027] The method may comprise: calculating a difference in the detected information relating to the temperature of the wafer between the first time and the second time; and controlling the cooling or heating of the wafer based on the calculated difference in the detected information relating to the temperature of the wafer.

[0028] Calculating the difference in the detected information relating to the temperature of the wafer between the first time and the second time may comprise subtracting one of the detected information from the other one of the detected information.

[0029] The difference may be zero where there is no change in the detected information relating to the temperature of the wafer between the first time and the second time. This may indicate that the temperature of the wafer has not changed between the first time and the second time.

[0030] The method may comprise: categorising the wafer into one of a plurality of categories based on the calculated difference in the detected information relating to the temperature of the wafer; and controlling the cooling or heating of the wafer based on a determined category of the wafer.

[0031] More generally, the method may comprise categorising the wafer into one of a plurality of categories based on the comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

[0032] The plurality of categories may be a plurality of temperature categories.

[0033] The wafer may be categorised into one of the plurality of categories based on a comparison between the calculated difference in the detected information relating to the temperature of the wafer and one or more threshold values.

[0034] Each of the plurality of categories may have, or be associated with, a specific configuration of the cooling or heating of the wafer, for example a specific duration of the cooling or heating of the wafer. The plurality of categories may include at least a first category for wafers having an ambient temperature or a temperature close to ambient temperature (e.g. less than ambient temperature plus a predetermined amount) and a second category for wafers having a temperature above ambient temperature (e.g. more than ambient temperature plus the predetermined amount).

[0035] The method may comprise calculating a rate of change of the detected information relating to the temperature of the wafer between the first time and the second time; and controlling the cooling or heating of the wafer based on the calculated rate of change of the detected information relating to the temperature of the wafer.

[0036] The calculated rate of change of the detected information relating to the temperature of the wafer may be an average of the rate of change between the first time and the second time.

[0037] Where the detected information has a value X at time t1 and a value Y at time t2, the rate of change may be calculated based on equation 1 below.

[0038] Rate of change = (Equation 1) t

[0039] The method may comprise: categorising the wafer into one of a plurality of categories based on the calculated rate of change of the detected information relating to the temperature of the wafer; and controlling the cooling or heating of the wafer based on a determined category of the wafer.

[0040] The plurality of categories may be a plurality of temperature categories.

[0041] The wafer may be categorised into one of the plurality of categories based on a comparison between the calculated rate of change in the detected information relating to the temperature of the wafer and one or more threshold values.

[0042] Each of the plurality of categories may have, or be associated with, a specific configuration of the cooling or heating of the wafer, for example a specific duration of the cooling or heating of the wafer.

[0043] The plurality of categories may include at least a first category for wafers having an ambient temperature or a temperature close to ambient temperature (e.g. less than ambient temperature plus a predetermined amount) and a second category for wafers having a temperature above ambient temperature (e.g. more than ambient temperature plus the predetermined amount).

[0044] The method may comprise: comparing the calculated rate of change of the detected information relating to the temperature of the wafer with a predetermined threshold value; categorising the wafer into a first category if the calculated rate of change of the detected information relating to the temperature of the wafer is less than the threshold value; and categorising the wafer into a second category if the calculated rate of change of the detected information relating to the temperature of the wafer is more than the threshold value. In particular, the calculated rate of change being more than the threshold value indicates a significant rate of change of the temperature of the wafer, which indicates that the initial temperature of the wafer was above ambient. In contrast, the calculated rate of change being less than the threshold value indicates a small rate of change of the temperature of the wafer, which indicates that the initial temperature of the wafer was close to the ambient temperature.

[0045] The method may comprise setting a first duration of cooling or heating of the wafer when the wafer is categorised into the first category; and setting a second duration of cooling or heating of the wafer when the wafer is categorised into the second category.

[0046] The temperature of the wafer may be controlled to be an ambient temperature, for example a temperature of the surrounding environment or a temperature of the air in the surrounding environment.

[0047] Controlling the cooling or heating of the wafer may comprise setting a duration of the cooling or heating of the wafer, or deciding to skip an available cooling or heating step. For example, where it is determined that the wafer has an above-ambient or hot temperature, the duration of cooling of the wafer may be extended compared to a duration of cooling for a wafer that is determined to have an ambient temperature.

[0048] Furthermore, if the first and second times are before the cooling or heating step, and it is determined that the temperature of the wafer is the ambient temperature or close to the ambient temperature, it may be decided to skip the cooling or heating step since it is unnecessary. This may further improve the throughput of the apparatus. This may be achieved by detecting the information relating to the temperature of the wafer using a temperature sensor on an end effector or other wafer transporter that is used to transport the wafer to the cooling or heating step, for example.

[0049] The method may comprise loading the wafer onto a thermal transfer plate. The information relating to the temperature of the wafer at the second time may be detected while the wafer is loaded on the thermal transfer plate.

[0050] In addition, the information relating to the temperature of the wafer at the first time may also be detected while the wafer is loaded on the thermal transfer plate. Alternatively, the information relating to the temperature of the wafer at the first time may be detected before the wafer is loaded on the thermal transfer plate, for example shortly before, for example less than 5 seconds before. For example, the information relating to the temperature of the wafer at the first time may be detected while the wafer is being lowered onto the thermal transfer plate, while the wafer is located above the thermal transfer plate.

[0051] A thermal transfer plate may be a plate or block of material for exchanging heat with the wafer.

[0052] The thermal transfer plate may be a passive thermal transfer plate or an active thermal transfer plate.

[0053] In this context, “passive” means that the cooling or heating device is neither cooled nor heated by cooling / heating elements, but rather receives its temperature by the surrounding ambient environment only. A passive thermal transfer plate is typically a plate or block of material having a high thermal mass and / or high thermal conductivity.

[0054] For example, a passive thermal transfer plate may be a plate or block of metal, such as aluminium.

[0055] Where the heating or cooling is performed using a passive thermal transfer plate, the passive thermal transfer plate may be substantially in thermal equilibrium with a processing chamber in which the semiconductor wafer will subsequently be processed, for example a measurement chamber of a metrology apparatus (thermal equilibrium may mean that the temperature difference is not more than 0.1 °C). For example, the passive thermal transfer plate may be attached to an outer surface of the processing / measurement chamber, for example using bolts having a high thermal conductivity.

[0056] Conversely, “active” means that the cooling or heating device can be heated or cooled by a powered heating or cooling device.

[0057] For example, the semiconductor wafers may be cooled using a Peltier device, or heated using a resistive heating device.

[0058] An active thermal transfer plate may comprise a plate or block of material having a high thermal mass and / or high thermal conductivity that is heated or cooled by one or more powered cooling or heating devices. For example, the plate or block of material may be a plate or block of metal, such as aluminium.

[0059] Controlling the cooling or heating of the wafer may comprise setting a duration for which the wafer will be in contact with the thermal transfer plate.

[0060] The information relating to the temperature of the wafer may be detected using a non-contact measurement method, for example using a non-contact temperature sensor.

[0061] Detecting the information relating to the temperature of the wafer may comprise detecting electromagnetic radiation emitted by the wafer, for example infrared (IR) radiation.

[0062] The information relating to the temperature of the wafer may comprise an output of an infrared temperature sensor, or may be based on an output of an infrared temperature sensor, for example calculated from the output of the infrared temperature sensor.

[0063] The second time may be a predetermined period of time after the first time. For example, the second time may be less than 10 second, or less than 5 seconds after the first time.

[0064] According to a second aspect of the present invention there is provided a wafer mass metrology method, for example a semiconductor wafer mass metrology method, that comprises: controlling the temperature of a wafer using the method of any of the preceding claims; and subsequently loading the wafer onto a measurement area of a wafer mass metrology apparatus.

[0065] The measurement area may comprise a measurement chamber of the wafer mass metrology apparatus, for example.

[0066] The measurement area may comprise a weighing device of the wafer mass metrology apparatus, for example. The measurement area may comprise, or be inside, a measurement chamber; and the temperature of the wafer may be controlled to be substantially equal to the temperature of the measurement chamber, or within ±2K, or ±1 K, or ±0.5K, or ±0.1 K, of the temperature of the measurement chamber.

[0067] According to a third aspect of the present invention there is provided an apparatus for controlling the temperature of a wafer, comprising: a sensor for detecting information relating to the temperature of the wafer at a first time and for detecting information relating to the temperature of the wafer at a later second time; a cooling or heating part for cooling or heating the wafer; and a controller configured to control cooling or heating of the wafer by the cooling or heating part based on a comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

[0068] The third aspect of the present may have any of the features of the first or second aspects of the present invention described above, where compatible.

[0069] The third aspect of the present invention may have any one, or, where compatible, any combination of the following optional features.

[0070] The temperature sensor may be embedded in the cooling or heating part. As discussed above, the temperature sensor may be an IR temperature sensor that is configured to detect IR radiation emitted by the wafer.

[0071] The controller may be configured to: calculate a difference in the detected information relating to the temperature of the wafer between the first time and the second time; and control the cooling or heating of the wafer by the cooling or heating part based on the calculated difference in the detected information relating to the temperature of the wafer.

[0072] The controller may be configured to: categorise the wafer into one of a plurality of categories based on the calculated difference in the detected information relating to the temperature of the wafer; and control the cooling or heating of the wafer by the cooling or heating part based on a determined category of the wafer.

[0073] More generally, the controller may be configured to categorise the wafer into one of a plurality of categories based on the comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

[0074] The controller may be configured to: calculate a rate of change of the detected information relating to the temperature of the wafer between the first time and the second time; and control the cooling or heating of the wafer by the cooling or heating part based on the calculated rate of change of the detected information relating to the temperature of the wafer.

[0075] The controller may be configured to: categorise the wafer into one of a plurality of categories based on the calculated rate of change of the detected information relating to the temperature of the wafer; and control the cooling or heating of the wafer by the cooling or heating part based on a determined category of the wafer. The controller may be configured to: compare the calculated rate of change of the detected information relating to the temperature of the wafer with a predetermined threshold value; categorise the wafer into a first category if the calculated rate of change of the detected information relating to the temperature of the wafer is less than the threshold value; and categorise the wafer into a second category if the calculated rate of change of the detected information relating to the temperature of the wafer is more than the threshold value.

[0076] The controller may be configured to: set a first duration of cooling or heating of the wafer when the wafer is categorised into the first category; and set a second duration of cooling or heating of the wafer when the wafer is categorised into the second category.

[0077] To control the cooling or heating of the wafer by the cooling or heating part the controller may be configured to set a duration of the cooling or heating of the wafer by the cooling or heating part, or decide to skip cooling or heating of the wafer by the cooling or heating part.

[0078] The sensor may be configured to detect the information relating to the temperature of the wafer at the first time and the information relating to the temperature of the wafer at the later second time while the wafer is loaded on the cooling or heating part.

[0079] To control the cooling or heating of the wafer by the cooling or heating part the controller may be configured to set a duration for which the wafer will be in contact with the cooling or heating part.

[0080] The sensor comprises a non-contact sensor.

[0081] The sensor may be configured to detect electromagnetic radiation emitted by the wafer.

[0082] The sensor may be configured to detect infrared radiation emitted by the wafer.

[0083] The sensor may be an infra-red temperature sensor.

[0084] The apparatus may be a wafer mass metrology apparatus that further comprises a measurement area.

[0085] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0086] Summary of the Figures

[0087] Embodiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0088] Figure 1 is a schematic illustration of a semiconductor wafer mass metrology apparatus according to an embodiment of the present invention;

[0089] Figure 2 is a schematic illustration of a thermal transfer plate that can be used in embodiments of the present invention. Figure 3 is a schematic illustration of IR temperature sensor measurement outputs obtained from two different types of wafer at different temperatures of the wafers at different clock times.

[0090] Figure 4 is a schematic illustration of idealised IR temperature sensor measurement outputs over time for two different types of wafer at two different temperatures.

[0091] Figure 5 is a schematic illustration of determining the rate of change of an IR temperature sensor measurement output for a wafer over time.

[0092] Detailed Description of the Invention

[0093] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0094] FIG. 1 shows a semiconductor wafer mass metrology apparatus according to an embodiment of the present invention. The semiconductor wafer mass metrology apparatus 1 comprises a weighing balance 3 having a weighing pan 5 for receiving a semiconductor wafer W. The weighing balance 3 is configured to provide measurement output indicative of the weight of a wafer loaded on the weighing pan 5.

[0095] The weighing balance 3 is located within a measurement chamber 7, which forms an enclosed environment around the weighing balance 3, e.g. to maintain a substantially uniform air density, air pressure and air temperature of the air around the weighing balance 3 and / or to prevent air draughts and / or to provide electromagnetic shielding. The measurement chamber 7 has an opening (not shown), e.g. a suitably sized slot in a side-wall of the measurement chamber 7, to allow a wafer to be transported into the measurement chamber 7, e.g. by a robotic arm, and positioned on the weighing pan 5. When not in use, the opening may be covered by an openable door or covering (not shown) to allow the measurement chamber 7 to be substantially closed or sealed when performing measurements using the weighing balance 3.

[0096] A passive thermal transfer plate 9 is positioned on top of the measurement chamber 7. The passive thermal transfer plate 9 comprises a plate or block of material having a good thermal conductivity. The passive thermal transfer plate also preferably has a high thermal mass, so that its temperature changes slowly and little when it is supplied with heat, and a good lateral thermal conductivity, so that it maintains a substantially uniform temperature across its upper surface. In this embodiment, the passive thermal transfer plate 9 is made from aluminium, for example a solid block of aluminium, but in other embodiments any other material with a good thermal conductivity may be used instead. The passive thermal transfer plate 9 is positioned directly on top of the measurement chamber 7, so that there is a good thermal contact between the passive thermal transfer plate 9 and the measurement chamber 7. The passive thermal transfer plate 9 is in direct physical contact with the measurement chamber 7. The passive thermal transfer plate 9 may be attached or fixed to the measurement chamber 7, for example using one or more bolts (not shown) and / or a thermally conductive bonding layer (not shown).

[0097] As a result of the good thermal contact between the passive thermal transfer plate 9 and the measurement chamber 7, the passive thermal transfer plate 9 may be substantially in thermal equilibrium with the measurement chamber 7 and therefore may have substantially the same temperature as the measurement chamber 7. The weighing balance 3 may also be in thermal equilibrium with the measurement chamber 7 and therefore may also have substantially the same temperature as the measurement chamber 7. As such, the passive thermal transfer plate 9 may be substantially in thermal equilibrium with the weighing balance 3 and therefore may have substantially the same temperature as the weighing balance 3.

[0098] The measurement chamber 7 and therefore the weighing balance 3 and passive thermal transfer plate 9 may have an ambient temperature, in other words an average temperature of the surrounding environment or of the air in the surrounding environment.

[0099] The weighing balance 3 and the weighing pan 5 may be considered as comprising a measurement area of the semiconductor wafer mass metrology apparatus 1. Alternatively, the measurement chamber 7 may be considered as comprising a measurement area of the semiconductor wafer mass metrology apparatus 1.

[0100] The semiconductor wafer mass metrology apparatus 1 of FIG. 1 further comprises an active thermal transfer plate 11 , an infrared (IR) temperature sensor 13 embedded in the active thermal transfer plate 11 and arranged to detect IR radiation from a wafer loaded on the active thermal transfer plate 11 , and a controller 15, which are discussed in more detail below.

[0101] The I R temperature sensor 13 is embedded in the active thermal transfer plate 11 and directed upwards with an unobstructed line of sight between the IR temperature sensor and a lower surface of a wafer situated above the active thermal transfer plate 11 (and therefore above the IR temperature sensor 13).

[0102] A plurality of Peltier devices 17 are attached to a bottom side of the active thermal transfer plate 11 . Each Peltier device 11 has a heat sink 19 attached to the bottom side thereof. An air flow 21 can be provided in a region 23 beneath the bottom side of the thermal transfer plate 9 in order to remove heat from the Peltier devices 17 and from the heat sinks 19. Of course, the configuration of the air flow 21 may be different to that shown in FIG. 1 , for example, air may be blown out of the bottom of the region 23 by a fan, or an alternative cooling mechanism may be provided instead of the air flow 21 , such as a liquid cooling mechanism.

[0103] In FIG. 1 the active thermal transfer plate 11 is shown as being positioned to the right-hand side of the measurement chamber 7. However, in other embodiments the active thermal transfer plate 11 can be positioned differently, for example to a different side, above or below the measurement chamber 7, or closer or further away from the measurement chamber 7 than illustrated in FIG. 1 . In other embodiments, the active thermal transfer plate 11 may be attached or connected, directly or indirectly, to the passive thermal transfer plate 9. In that case, the active thermal transfer plate 11 may be thermally insulated or isolated from the passive thermal transfer plate 9, and / or there may be thermal insulation or a thermal break between them.

[0104] In use, a wafer transporter, for example an end effector of a robotic arm, is used to remove a wafer from a wafer carrier such as a Front Opening Unified Pod (FOUP) (not shown), or alternatively from another processing apparatus (not shown), and to transport the wafer to the active thermal transfer plate 11 and position the wafer on the active thermal transfer plate 11 . When the wafer is removed from the wafer carrier (or the other processing apparatus) it may have a temperature of approximately 70°C, for example, depending on the type of processing that was performed on the wafer and the length of time since the processing was performed. For example, the wafer may have been processed at a processing station of a semiconductor device production line, which may have heated the wafer to a temperature of 400 to 500°C, before the wafer was loaded into the wafer carrier.

[0105] When the wafer is positioned on the active thermal transfer plate 11 , heat is conducted from the wafer to the active thermal transfer plate 11 so that the temperature of the wafer is decreased. Typically, the wafer is positioned on the active thermal transfer plate 11 for a sufficiently long period of time for the wafer and the active thermal transfer plate 11 to achieve thermal equilibrium (e.g. so that they have substantially the same temperature). As discussed below, in this embodiment the cooling of the wafer by the active thermal transfer plate 11 is controlled by the controller 15 based on measurement output of the IR temperature sensor 13.

[0106] The active thermal transfer plate 11 may be controlled to have an ambient temperature, and / or a same temperature as a temperature of the measurement chamber 7.

[0107] Transfer of heat from the wafer to the active thermal transfer plate 11 would act to increase the temperature of the active thermal transfer plate 11. In that case, the thermal equilibrium temperature of the wafer and the active thermal transfer plate 11 may be different to a desired temperature of the wafer (for example the ambient temperature). In order to prevent the temperature of the active thermal transfer plate 11 from increasing due to the heat load from the wafer, the active thermal transfer plate 11 is operable to actively dissipate the heat load removed from the wafer. In particular, the Peltier devices 17 are operated to actively remove heat from the active thermal transfer plate 11. In other words, electrical power is supplied to the Peltier devices 17 to cause them to act as active heat pumps that transfer heat from their upper surfaces in contact with the active thermal transfer plate 11 to their lower surfaces to which the heat-sinks 19 are attached.

[0108] An air flow 21 is provided in the region 23 beneath the active thermal transfer plate 11 in which the Peltier devices 17 and the heat sinks 19 are positioned in order to remove heat from the Peltier devices 17 and the heat sinks 19. The heat removed from the wafer using the active thermal transfer plate 11 is therefore transported and dissipated away from the measurement chamber 7 of the semiconductor wafer mass metrology apparatus 1 by the air flow 21 , so that this heat has no effect on the temperature of the semiconductor wafer mass metrology apparatus 1 . The air flow 21 may be generated by one or more fans, for example posited in, or at the edges of, the region 23. In other words, heat is actively dissipated from the active thermal transfer plate 11 .

[0109] As mentioned above, actively dissipating heat from the active thermal transfer plate 11 will prevent heat from building up in the active thermal transfer plate 11 , which would cause an increase in the temperature of the active thermal transfer plate 11. In this embodiment, the heat removed from the wafer is effectively / efficiently disposed of by being dissipated by the active thermal transfer plate 11 . This may enable the active thermal transfer plate 11 to remain at the ambient temperature despite the heat load from the wafers.

[0110] The active thermal transfer plate 11 is operated to remove a bulk of a heat load from the wafer, so that the temperature of the wafer is reduced to close to the desired temperature of the wafer when it is positioned on the weighing pan 5. The active thermal transfer plate 11 may remove over 90%, or over 95%, of the heat that needs to be removed to reduce the temperature of the wafer to the desired temperature. Put another way, the active thermal transfer plate 11 may cause over 90%, or over 95%, of the temperature change required to decrease the temperature of the wafer from its initial temperature to the desired temperature when it is positioned on the weighing pan 5.

[0111] In this embodiment, it is desired to substantially match the temperature of the wafer to the temperature of the measurement chamber 7, so that there is substantially no temperature difference between the wafer and the measurement chamber 7 (and therefore substantially no temperature difference between the wafer and the weighing balance 3) when the wafer is loaded on the weighing pan 5. In this embodiment, the active thermal transfer plate 11 may cool the semiconductor wafer to within ±1 °C of the temperature of measurement chamber 7. For example, where the measurement chamber has a temperature of 20°C, the active thermal transfer plate 11 may cool the semiconductor wafer to a temperature of (20±1)°C. As mentioned above, the measurement chamber may have an ambient temperature, i.e. a temperature equal to a temperature of the surrounding environment or surrounding air. However, in other embodiments the amount of cooling provided by the active thermal transfer plate 11 may be different to this, provided that as a minimum the active thermal transfer plate 11 provides over 50% of the required temperature change of the semiconductor wafer, and preferably over 80%.

[0112] Once the wafer has been cooled to a temperature close to the desired temperature using the active thermal transfer plate 11 , it is transported to the passive thermal transfer plate 9 using a wafer transporter, such as an end effector of a robot arm.

[0113] As discussed above, when the wafer is positioned on the passive thermal transfer plate 9 there is good thermal contact between the wafer and the passive thermal transfer plate 9. Therefore, the wafer is cooled by heat being conducted from the wafer to the passive thermal transfer plate 9. Typically the wafer is positioned on the passive thermal transfer plate 9 for a sufficient period of time for the passive thermal transfer plate 9 and the wafer to become substantially in thermal equilibrium, so that they have substantially the same temperature (i.e. the temperature of the wafer is matched or equalised to the temperature of the passive thermal transfer plate 9 and therefore to the temperature of the measurement chamber 7). For example, the wafer may be positioned on the thermal transfer plate 7 for a period of up to 60 seconds.

[0114] The wafer has already had the bulk of its heat load removed by the active thermal transfer plate 11 before it is positioned on the passive thermal transfer plate 9. Therefore, the thermal load on the passive thermal transfer plate 9 during the temperature equalisation is low, and the temperature of the passive thermal transfer plate 9 and the measurement chamber 7 (which have a high thermal mass) therefore remains substantially constant during the temperature equalisation. In addition, relatively little heat has to be exchanged to bring the wafer into thermal equilibrium with the passive thermal transfer plate 9.

[0115] Once the temperature of the wafer is substantially equalised to the temperature of the measurement chamber 7 (e.g. when the wafer has been on the thermal transfer plate 7 for a predetermined period of time) the wafer is transported by a wafer transporter from the thermal transfer plate 7 to the weighing pan 5. The weighing balance 3 is then used to provide measurement output indicative of the weight of the wafer. Because the temperature of the wafer has been substantially matched to the temperature of the measurement chamber 7, and without significantly changing the temperature of the measurement chamber 7 (because the heat load on the measurement chamber 7 is small), any errors in the measurement output caused by temperature variations may be substantially zero. For example, there may be no significant convection currents generated in the measurement chamber 7, no significant changes in the buoyancy force on the wafer (which would be caused by heating of the air in the measurement chamber 7), and no significant temperature changes (e.g. temperature increase or temperature non-uniformity) in the weighing balance 3 due to the presence of the wafer on the weighing pan 5.

[0116] Of course, in other embodiments it is not essential to provide both the active thermal transfer plate 11 and the passive thermal transfer plate 9. Instead, only one of the active thermal transfer plate 11 and the passive thermal transfer plate 9 may be provided in some embodiments, and the temperature of the wafer may be controlled before performing a weight measurement using only the active thermal transfer plate 11 or only the passive thermal transfer plate 9.

[0117] Furthermore, in other embodiments the active thermal transfer plate 11 and the passive thermal transfer plate 9 may be configured to heat a wafer that has a temperature lower than the desired temperature. In that case, the Peltier devices 11 , heat sinks 19 and air flow 21 may be replaced with one or more heating devices for heating the active thermal transfer plate 11 .

[0118] As mentioned above, the semiconductor wafer mass metrology apparatus 1 includes an infrared (IR) temperature sensor 13 embedded in the active thermal transfer plate 11 . The IR temperature sensor 13 is arranged to detect infrared radiation from a wafer loaded onto the active thermal transfer plate 11. In particular, the IR temperature sensor is arranged to detect IR radiation from a wafer positioned on a top surface of the active thermal transfer plate 11 . The configuration of the active thermal transfer plate 11 and the IR temperature sensor 13 are illustrated schematically in FIG. 2, for example, which illustrates the IR temperature sensor 13 being embedded in a top surface of the active thermal transfer plate 11 and arranged to detect IR radiation from a wafer loaded on the top surface of the active thermal transfer plate 11 .

[0119] As mentioned above, the present inventor has realised that the measurement output of the IR temperature sensor may be different for different types or compositions of wafer having the same temperature. In particular, the magnitude of the measurement output of the IR temperature sensor will be greater for a wafer having a high (or higher) IR emissivity compared to a wafer having a low (or lower) IR emissivity. Therefore, the present inventor has realised that the magnitude of the measurement output of the IR temperature sensor may be an unreliable indicator of the actual temperature of the wafer when dealing with different types or compositions of wafers.

[0120] The variation in IR emissivity between different types or compositions of wafer are caused by variations between the wafers. For example, different wafers may have different dopant elements added to a silicon material of the wafer to change its electrical and / or optical properties, and / or different wafers may have been processed in different ways, for example through deposition of layers with different physical or chemical properties onto the front or back side of the wafer, etching (or partial etching) through layers of the wafer, patterning, heating cycles, polishing, etc., all of which may affect the IR emissivity of the wafer. In addition, wafers may be made of materials other than silicon, for example glass.

[0121] This issue is illustrated in FIG. 3, for example. FIG. 3 illustrates IR temperature sensor measurement outputs for two different types of wafer for different temperatures of the wafers at different clock times. The temperature of the wafer is measured using the I R temperature sensor 13 at the time of loading the wafer onto the active thermal transfer plate 11 . The vertical lines labelled with A in FIG. 3 correspond to a first type of wafer that has a higher IR emissivity and the vertical lines labelled with B in FIG. 3 correspond to a second type of wafer that has a lower IR emissivity. The horizontal axis in FIG. 3 is a clock time (in 24 hour format) that the wafer was loaded onto the active thermal transfer plate 11 and the temperature ofthe wafer was measured using the IR temperature sensor 13, and the vertical axis in FIG. 3 is the temperature of the wafer determined by the IR temperature sensor 13.

[0122] The dashed line in FIG. 3 indicates an estimated actual wafer temperature, which is determined independently of the output of the IR temperature sensor 13, for example using a different type of temperature sensor or based on a calculation. The estimated actual wafer temperature decreases with the clock time due to cooling of the wafers in the wafer carrier before they are loaded onto the active thermal transfer plate 11 .

[0123] As illustrated in FIG. 3, the temperature of the wafer measured using the IR temperature sensor 13 is close to the estimated actual wafer temperature for the first type of wafer (A) with a higher IR emissivity. The output of the IR temperature sensor 13 is therefore accurate for the first type of wafer (A). However, as illustrated in FIG. 3, the temperature of the wafer measured using the IR temperature sensor 13 is significantly less than the estimated actual wafer temperature for the second type of wafer (B) that has a lower IR emissivity. For example, for the first vertical line 25 in FIG. 3 for the second type of wafer (B), the IR temperature sensor 13 detected the temperature of the wafer as being approximately 28°C even though the estimated actual temperature of the wafer was >50°C. The output of the IR temperature sensor 13 is therefore not accurate for the second type of wafer (B).

[0124] If a threshold for deciding whether or not the temperature of the wafer is ambient or above-ambient was set to 30°C, for example, then based on the first vertical line 25 in FIG. 3 for the second type of wafer (B), the wafer would have been categorised as being ambient, even though the temperature of the wafer was >50°C. As a result, the wafer may then not have been placed in contact with the active thermal transfer plate 11 for a sufficiently long period of time to achieve a desired temperature of the wafer.

[0125] Alternatively, if the threshold for deciding whether or not the temperature of the wafer is ambient or above-ambient was set lower in order to avoid this problem (a “safe” level), for example at 25°C, although this would lead to the wafer corresponding to the first vertical line 25 in FIG. 3 for the second type of wafer (B) being correctly categorised as above-ambient, it would also lead to some of the first type of wafer (A) that have a temperature close to ambient temperature being identified as being above-ambient temperature. As a result, such wafers may then be placed in contact with the active thermal transfer plate 11 for longer than is required to achieve the desired temperature. This would reduce the throughput of the apparatus 1 .

[0126] Therefore, in the present embodiment, rather than categorising wafers as being ambient or aboveambient temperature based on a single measurement output of the IR temperature sensor 13, wafers are instead categorised as being ambient or above-ambient temperature based on a change in the measurement output of the IR temperature sensor 13 due to the wafer being in contact with the active thermal transfer plate 11 for a period of time.

[0127] As mentioned above, the active thermal transfer plate 11 is controlled to have an ambient temperature. Therefore, if a wafer that has a temperature that is significantly above ambient is placed on the active thermal transfer plate 11 , the temperature of the wafer will change significantly over time, and as a consequence the measurement output of the IR temperature sensor will also change significantly over time, regardless of the IR emissivity of the wafer or the magnitude of the IR temperature sensor measurement output. On the other hand, if a wafer having a temperature close to or equal to ambient is placed on the active thermal transfer plate 11 , the temperature of the wafer will not change significantly over time, regardless of the IR emissivity of the wafer or the magnitude of the IR temperature sensor measurement output.

[0128] Therefore, a wafer can be categorised as having an ambient temperature or an above-ambient temperature by detecting whether or not there is a significant change in the measurement output of the IR temperature sensor when the wafer has been placed on the active thermal transfer plate 11 for a predetermined period of time, regardless of the IR emissivity of the wafer or the magnitude of the IR temperature sensor measurement output.

[0129] FIG. 4A shows the idealised I R temperature sensor measurement output over time for a wafer placed on the active thermal transfer plate 11 for a higher IR emissivity wafer (left in FIG. 4A, 27) and a lower IR emissivity wafer (right in FIG. 4A, 29), where the wafer has an initial temperature that is significantly above the ambient temperature.

[0130] As can be seen in FIG. 4A, the magnitude of the IR temperature sensor is less for the lower IR emissivity wafer than for the higher IR emissivity wafer. However, for both the higher IR emissivity wafer and the lower IR emissivity wafer the IR temperature sensor measurement output decays towards a constant value as the temperature of the wafer approaches ambient temperature on the active thermal transfer plate 11 . Therefore, for both the higher IR emissivity wafer and the lower IR emissivity wafer the fact that the initial temperature of the wafer was above-ambient can be detected by detecting a significant change in the IR temperature sensor measurement output caused by the wafer being in contact with the active thermal transfer plate 11 .

[0131] FIG. 4B shows the idealised I R temperature sensor measurement output over time for a wafer placed on the active thermal transfer plate 11 for a higher IR emissivity wafer (left in FIG. 4B, 31) and a lower IR emissivity wafer (right in FIG. 4B, 33), where the wafer has an initial temperature that is equal to the ambient temperature.

[0132] As can be seen in FIG. 4B, the magnitude of the IR temperature sensor is less for the lower IR emissivity wafer than for the higher IR emissivity wafer. However, for both the higher IR emissivity wafer and the lower IR emissivity wafer the IR temperature sensor measurement output remains constant over time, since the wafer has the same temperature as the active thermal transfer plate 11 . Therefore, for both the higher IR emissivity wafer and the lower IR emissivity wafer the fact that the initial temperature of the wafer was ambient can be detected by detecting that there is no change in the IR temperature sensor measurement output caused by the wafer being in contact with the active thermal transfer plate 11 .

[0133] Therefore, an ambient temperature wafer can be reliably differentiated from an above-ambient temperature wafer by detecting a significant change in the IR temperature sensor measurement output due to the wafer being placed on the active thermal transfer plate 11. If a significant change is detected, this indicates that the initial temperature of the wafer was different to the temperature of the active thermal transfer plate 11 , and that the wafer should therefore be categorised as above-ambient temperature. In contrast, if no significant change is detected, this indicates that the initial temperature of the wafer was the same as the temperature of the active thermal transfer plate, and that the wafer should therefore be categorised as ambient temperature.

[0134] In order to determine whether or not there is a significant change in the measurement output of the IR temperature sensor caused by the wafer being in contact with the active thermal transfer plate 11 , first and second measurement outputs of the IR temperature sensor for the wafer are obtained at different times. At least the second measurement output is obtained after the wafer has been in contact with the active thermal transfer plate 11 for a predetermined period of time. The first measurement output may be obtained before the wafer is in contact with the active thermal transfer plate 11 , or while the wafer is in contact with the active thermal transfer plate 11 . In this embodiment, a first measurement output of the IR temperature sensor is obtained immediately after the wafer has been placed in contact with the active thermal transfer plate 11 . Taking the first measurement at this point in time may maximise the amplitude of the first measurement output, which may lead to a better signal to noise ratio.

[0135] However, it is not essential for the wafer to be in contact with the active thermal transfer plate 11 when the first measurement output of the IR temperature sensor is obtained. In particular, since the IR temperature sensor is located within the body of the active thermal transfer plate 11 , directed upward so that it has an unobstructed line of sight to the lower face of the wafer situated above it, the first measurement can be taken at any point at which the wafer is positioned above the IR sensor, which includes where the wafer is positioned above the top surface of the active thermal transfer plate 11 but is not in contact with the top surface.

[0136] In general, taking the first measurement when the output from the IR temperature sensor in near to its maximum, prior to any significant change in temperature resulting from contact with the active thermal transfer plate 11 , will maximise the signal to noise ratio. This can be achieved by taking the first measurement immediately prior to contact of the wafer with the active thermal transfer plate 11 , at the instant of contact with the active thermal transfer plate 11 , or immediately after contact. Of course, the first measurement may be performed before or after these times, but the signal to noise ratio may be poorer (but still acceptable).

[0137] In addition, a second measurement output of the IR temperature sensor is obtained after the wafer has been in contact with the active thermal transfer plate 11 for a predetermined period of time, which in this case is 4 seconds.

[0138] Based on the first and second measurement outputs, the temperature of the wafer is categorised as being either ambient or above-ambient. A duration for which the wafer is in kept in contact with the active thermal transfer plate 11 is then set based on the categorisation. In particular, if the wafer is categorised as having an ambient temperature, a duration for which the wafer is kept in contact with the active thermal transfer plate 11 before being removed may be set to a first predetermined value.

[0139] In contrast, if the wafer is categorised as having an above-ambient temperature, a duration for which the wafer is kept in contact with the active thermal transfer plate 11 is set to a second predetermined value. The second predetermined value is longer than the first predetermined value, so that above-ambient temperature wafers are kept in contact with the active thermal transfer plate 11 for longer than ambient temperature wafers.

[0140] Therefore, a duration for which the wafer is kept in contact with the active thermal transfer plate 11 may be set based on the categorisation of the temperature of the wafer, and therefore based on the first and second measurement outputs of the IR temperature sensor for the wafer.

[0141] These steps are performed by the controller 15 described above.

[0142] An example of how the categorisation of the temperature of the wafer is achieved is illustrated in FIG. 5.

[0143] As illustrated in FIG. 5, a first measurement output X is obtained from the IR temperature sensor at a first time t1 immediately after the wafer has been loaded onto the active thermal transfer plate 11 . Subsequently, a second measurement output Y is obtained from the IR temperature sensor at a second time t2, while the wafer is in contact with the active thermal transfer plate 11 and has been in contact with the active thermal transfer plate 11 for a predetermined period of time. For example, the time t2 may be 4 seconds after the time t1 .

[0144] Based on the measurement outputs X and Y and the times t1 and t2 an average rate of change of the measurement output of the IR temperature sensor between the times t1 and t2 can be calculated. In particular, the average rate of change of the measurement output is calculated based on equation 1 below.

[0145] Rate of change = (Equation 1) t

[0146] The average rate of change is then compared to a predetermined threshold value for the rate of change and the wafer is categorised as having either an ambient temperature or an above-ambient temperature based on a result of the comparison. In particular, if the calculated average rate of change is greater than the predetermined threshold, the wafer is categorised as having an above-ambient temperature. In contrast, if the calculated average rate of change is less than the predetermined threshold, the wafer is categorised as having an ambient temperature. As mentioned above, based on a result of the categorisation, a duration of time that an ambient temperature wafer is in contact with the active thermal transfer plate 11 is controlled to be less than a duration of time than an above-ambient temperature wafer is in contact with the active thermal transfer plate 11 . This increases the throughput of the apparatus 1 .

[0147] Of course, in other embodiments wafers may be categorised into more than two categories. For example, wafers having above-ambient temperatures could be categorised into more than one category depending on how much their temperature is above the ambient temperature. Each of the different categories could then be assigned a corresponding time for which the wafers in that category should be in contact with the active thermal transfer plate 11 , for example so that very hot wafers are kept in contact with the active thermal transfer plate 11 for longer than hot wafers. This may further help to increase the throughput of the apparatus 1 while ensuring that wafers are still cooled to the required temperature.

[0148] As an alternative approach, a wafer could be kept in contact with the active thermal transfer plate 11 until the output of the IR temperature sensor becomes constant, or substantially constant (i.e. does not change over time). This indicates that the temperature of the wafer is constant or substantially constant over time, and the wafer can therefore be removed from the active thermal transfer plate 11 . The method may therefore comprise obtaining a first measurement output of the IR temperature sensor at a first time after the wafer has been placed on the active thermal transfer plate 11 and then obtaining a second measurement output at a second time after the wafer has been placed on the active thermal transfer plate 11 , and determining if the first measurement output and the second measurement output are the same or substantially the same. The method may further comprise removing the wafer from the active thermal transfer plate 11 when the second measurement output and the first measurement output are the same or substantially the same. This may correspond to the wafer having the same temperature as the active thermal transfer plate 11 , or substantially the same temperature.

[0149] However, with this alternative approach both the first measurement output and the second measurement output are a combination of ambient signal + noise, and a result of a comparison between the two measurement outputs may be dominated by sensor noise, which may result in an unreliable detection of when the wafer has the same temperature as the active thermal transfer plate 11 .

[0150] The above description has been in the context of setting a duration for which the wafer is in contact with the active thermal transfer plate 11 . In addition, a duration for which the wafer is in contact with the passive thermal transfer plate 9 may also be set based on the first and second measurement outputs obtained by the IR temperature sensor 13 of the active thermal transfer plate 11 . For example, if a wafer is categorised as having an above-ambient temperature based on the first and second measurement outputs, the wafer may also be kept in contact with the passive thermal transfer plate 9 for a longer period of time than a wafer that is categorised as having an ambient temperature based on the first and second measurement outputs. In other words, durations of cooling by both the active thermal transfer plate 11 and the passive thermal transfer plate 9 may be determined based on the categorisation of the temperature of the wafer determined based on the first and second measurement outputs obtained by the IR temperature sensor 13 of the active thermal transfer plate 11 .

[0151] In alternative embodiments, the passive thermal transfer plate can be provided with the IR temperature sensor and the same steps described above can be carried out for the passive thermal transfer plate. Therefore, more generally embodiments of the present invention may comprise detecting a change in the measurement output of an IR sensor for a wafer caused by the wafer being in contact with a thermalisation station, such as an active thermal transfer plate or a passive thermal transfer plate.

[0152] Of course, the present invention may be applied to both the active thermal transfer plate 11 and the passive thermal transfer plate 9, such that both are provided with a respective IR temperature sensor.

[0153] In the above description the information relating to the temperature of the wafer is detected using an IR temperature sensor embedded in the active thermal transfer plate 11 . However, it is not essential for the IR temperature sensor to be embedded in the active thermal transfer plate 11 and instead the IR temperature sensor may be provided separately to the active thermal transfer plate 11 , for example above or to the side of the active thermal transfer plate 11 (similarly with the passive thermal transfer plate 9).

[0154] In addition, a different type of temperature sensor can be used instead of an IR temperature sensor, for example a contact temperature sensor or a non-contact temperature sensor. In some embodiments an infrared (IR) camera may be used as the temperature sensor. For example, an IR camera may be positioned above the active thermal transfer plate 11 or the passive thermal transfer plate 9. The above description has been in the context of detecting a change in the measurement output of an IR temperature sensor for the wafer caused by the wafer having been in contact with the active thermal transfer plate 11 . However, it is not essential for the change in the measurement output to be caused by the wafer having been in contact with the active thermal transfer plate 11 .

[0155] Instead, a change in measurement output of an IR temperature sensor, or a different type of temperature sensor, can be detected before the wafer is placed on the active thermal transfer plate 1 1 . For example, a temperature sensor may be embedded in an end effector (or other wafer transporter) that is used to transport the wafer from a wafer carrier such as a FOUP or a wafer processing apparatus to the active thermal transfer plate 11 . The temperature sensor may be a non-contact temperature sensor such as an IR temperature sensor or a contact temperature sensor such as a thermocouple, for example. A change in the measurement output of the temperature sensor may then be detected while the wafer is being transported by the end effector. In particular, first and second measurement outputs of the temperature sensor may be obtained while the wafer is in contact with the end effector. In the same manner as discussed above, the wafer can then be categorised as having an ambient temperature or above-ambient temperature based on the first and second measurement outputs.

[0156] An advantage of detecting the change in the measurement output before the wafer is placed in contact with the active thermal transfer plate 11 is that it may be decided to skip placing the wafer on the active thermal transfer plate 11 if the temperature of the wafer is determined to be ambient temperature. For example, the wafer may be conveyed directly to the passive thermal transfer plate 9 without first being placed on the active thermal transfer plate 11 if the temperature of the wafer is deemed to be ambient temperature.

[0157] However, a better signal to noise ratio may be achieved by detecting the change in the measurement output while the wafer is in contact with the a thermal transfer plate, since this may lead to a more significant change in the temperature of a non-ambient wafer over time.

[0158] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0159] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0160] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventor does not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0161] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0162] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

23Claims:1 . A method of controlling the temperature of a wafer, comprising: detecting information relating to the temperature of the wafer at a first time; detecting information relating to the temperature of the wafer at a later second time; and controlling cooling or heating of the wafer based on a comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

2. The method according to claim 1 , wherein the method comprises: calculating a difference in the detected information relating to the temperature of the wafer between the first time and the second time; and controlling the cooling or heating of the wafer based on the calculated difference in the detected information relating to the temperature of the wafer.

3. The method according to claim 2, wherein the method comprises: categorising the wafer into one of a plurality of categories based on the calculated difference in the detected information relating to the temperature of the wafer; and controlling the cooling or heating of the wafer based on a determined category of the wafer.

4. The method according to any of the previous claims, wherein the method comprises: calculating a rate of change of the detected information relating to the temperature of the wafer between the first time and the second time; and controlling the cooling or heating of the wafer based on the calculated rate of change of the detected information relating to the temperature of the wafer.

5. The method according to claim 4, wherein the method comprises: categorising the wafer into one of a plurality of categories based on the calculated rate of change of the detected information relating to the temperature of the wafer; and controlling the cooling or heating of the wafer based on a determined category of the wafer.

6. The method according to claim 4 or claim 5, wherein the method comprises: comparing the calculated rate of change of the detected information relating to the temperature of the wafer with a predetermined threshold value; categorising the wafer into a first category if the calculated rate of change of the detected information relating to the temperature of the wafer is less than the threshold value; and categorising the wafer into a second category if the calculated rate of change of the detected information relating to the temperature of the wafer is more than the threshold value.

7. The method according to claim 6, wherein the method comprises: setting a first duration of cooling or heating of the wafer when the wafer is categorised into the first category; andsetting a second duration of cooling or heating of the wafer when the wafer is categorised into the second category.

8. The method according to any of the preceding claims, wherein controlling the cooling or heating of the wafer comprises setting a duration of the cooling or heating of the wafer, or deciding to skip an available cooling or heating step.

9. The method according to any of the preceding claims, wherein: the method comprises loading the wafer onto a thermal transfer plate; and the information relating to the temperature of the wafer at the second time is detected while the wafer is loaded on the thermal transfer plate.

10. The method according to claim 9, wherein: the information relating to the temperature of the wafer at the first time is detected while the wafer is loaded on the thermal transfer plate.

11. The method according to claim 9 or 10, wherein controlling the cooling or heating of the wafer comprises setting a duration for which the wafer will be in contact with the thermal transfer plate.

12. The method according to any of the preceding claims, wherein the information relating to the temperature of the wafer is detected using a non-contact measurement method.

13. The method according to any of the preceding claims, wherein detecting the information relating to the temperature of the wafer comprises detecting electromagnetic radiation emitted by the wafer.

14. The method according to any of the preceding claims, wherein the information relating to the temperature of the wafer comprises an output of an infrared temperature sensor, or is based on an output of an infrared temperature sensor.

15. A wafer mass metrology method comprising: controlling the temperature of a wafer using the method of any of the preceding claims; and subsequently loading the wafer onto a measurement area of a wafer mass metrology apparatus.

16. The wafer mass metrology method according to claim 15, wherein: the measurement area is inside a measurement chamber; and the temperature of the wafer is controlled to be substantially equal to the temperature of the measurement chamber, or within ±2K, or ±1 K, or ±0.5K, or ±0.1 K, of the temperature of the measurement chamber.

17. An apparatus for controlling the temperature of a wafer, comprising:a sensor for detecting information relating to the temperature of the wafer at a first time and for detecting information relating to the temperature of the wafer at a later second time; a cooling or heating part for cooling or heating the wafer; and a controller configured to control cooling or heating of the wafer by the cooling or heating part based on a comparison of the detected information relating to the temperature of the wafer at the first time and the detected information relating to the temperature of the wafer at the second time.

18. The apparatus according to claim 17, wherein the controller is configured to: calculate a difference in the detected information relating to the temperature of the wafer between the first time and the second time; and control the cooling or heating of the wafer by the cooling or heating part based on the calculated difference in the detected information relating to the temperature of the wafer.

19. The apparatus according to claim 18, wherein the controller is configured to: categorise the wafer into one of a plurality of categories based on the calculated difference in the detected information relating to the temperature of the wafer; and control the cooling or heating of the wafer by the cooling or heating part based on a determined category of the wafer.

20. The apparatus according to any of claims 17 to 19, wherein the controller is configured to: calculate a rate of change of the detected information relating to the temperature of the wafer between the first time and the second time; and control the cooling or heating of the wafer by the cooling or heating part based on the calculated rate of change of the detected information relating to the temperature of the wafer.

21. The apparatus according to claim 20, wherein the controller is configured to: categorise the wafer into one of a plurality of categories based on the calculated rate of change of the detected information relating to the temperature of the wafer; and control the cooling or heating of the wafer by the cooling or heating part based on a determined category of the wafer.

22. The apparatus according to claim 20 or claim 21 , wherein the controller is configured to: compare the calculated rate of change of the detected information relating to the temperature of the wafer with a predetermined threshold value; categorise the wafer into a first category if the calculated rate of change of the detected information relating to the temperature of the wafer is less than the threshold value; and categorise the wafer into a second category if the calculated rate of change of the detected information relating to the temperature of the wafer is more than the threshold value.

23. The apparatus according to claim 22, wherein the controller is configured to:26 set a first duration of cooling or heating of the wafer when the wafer is categorised into the first category; and set a second duration of cooling or heating of the wafer when the wafer is categorised into the second category.

24. The apparatus according to any of claims 17 to 23, wherein to control the cooling or heating of the wafer by the cooling or heating part the controller is configured to set a duration of the cooling or heating of the wafer by the cooling or heating part, or decide to skip cooling or heating of the wafer by the cooling or heating part.

25. The apparatus according to any of claims 17 to 24, wherein: the sensor is configured to detect the information relating to the temperature of the wafer at the later second time while the wafer is loaded on the cooling or heating part.

26. The apparatus according to claim 25, wherein: the sensor is configured to detect the information relating to the temperature of the wafer at the first time while the wafer is loaded on the cooling or heating part.

27. The apparatus according to claim 25 or 26, wherein to control the cooling or heating of the wafer by the cooling or heating part the controller is configured to set a duration for which the wafer will be in contact with the cooling or heating part.

28. The apparatus according to any of claims 17 to 27, wherein the sensor comprises a non-contact sensor.

29. The apparatus according to any of claims 17 to 28, wherein the sensor is configured to detect electromagnetic radiation emitted by the wafer.

30. The apparatus according to any of claims 17 to 29, wherein the sensor is configured to detect infrared radiation emitted by the wafer.

31. The apparatus according to any of claims 17 to 30, wherein the apparatus is a wafer mass metrology apparatus that further comprises a measurement area.

Images