referring first to fig1 a number of semiconductor wafers 2 are held within a carrier 1 outside the chamber 4 . the robotic arm 3 takes each wafer in turn and moves it inside the chamber 4 through aperture 10 in the walls of chamber , which can be seen in fig2 . the arm 3 places one of the wafers 2 on the thermal transfer plate 5 within the weighing chamber 4 . the wafer 2 is left for a period of time , e . g ., between 10 and 60 seconds while it comes into equilibrium with the temperature of the equipment . the robotic arm then takes the wafer from the thermal transfer plate 5 and places it onto the weighing pan 7 of the weight instrument , e . g . a sartorius bp2iid , though any suitable accurate balance could be used . the weight readings are continually monitored until the reading has settled , usually within 15 to 45 seconds . the robotic arm 3 then removes the wafer and places it back into the carrier 1 . the equipment is mounted on a firm , level surface free from vibration and not prone to movement ( for example bowing of the floor as people or equipment move past ). the enclosure and balance can be made level by using adjusting feet located at each corner of the base of the enclosure . the wafer carrier 1 is made of static dissipative material and the thermal transfer plate is made of an electrically conducting material , so that any static electricity on the wafer is dispersed before measurement . the interior of the weighing chamber 4 is maintained at a substantially constant temperature , e . g ., within +/− 0 . 1 ° c . where the ambient conditions vary by more than this then the enclosure may be electrically heated with resistance heaters and maintained at a temperature slightly above ambient . if heated , the enclosure should be maintained within 5 ° c . of the ambient temperature , to avoid the risk of convection currents . turning now to fig2 the weighing chamber 4 is divided into two separate compartments by the divider 8 . a lid 9 is shown in position on top of the chamber in this figure . the lower compartment 16 houses the weighing balance while the upper compartment 14 houses the weighing pan 7 , and the wafer 2 when this is on the pan . all walls of the chamber , the divider and the lid are made of materials having good electrical and thermal conductivity and are in good thermal and electrical contact with each other . the chamber should be electrically conducting to prevent any electrical static charge from building up in the vicinity of the weighing instrument . the semiconductor wafers 2 are weighed in a horizontal orientation . the surfaces of the chamber above and below the wafer surface during weighing , i . e . lid 9 and divider 8 are close enough to prevent air convection currents but far enough apart that any residual electrical charge on the wafer is not attracted to the chamber surfaces . an opening 10 allows access for the robotic arm and wafer . the pressure sensor 17 , which may be e . g . a druck pmp4010ab , is mounted so that the sensing port is within the upper balance enclosure . the combined temperature & amp ; humidity sensor 15 , e . g . a pico rho2 , is also mounted within the upper balance enclosure . [ 0087 ] fig3 shows the arrangement of the thermal transfer plate . each wafer 2 is placed in turn onto the upper surface 20 of the plate 5 by the robotic arm 3 . recesses 11 in the plate allow the robot arm 3 to lower so that the wafer 2 is in contact with the surface 20 of the plate 5 . a number of grooves 12 in the plate allow trapped air to escape as the wafer is lowered and prevent it from floating out of position . the thermal transfer plate 5 is made from aluminium , although any material with good conductivity could be used . the thermal transfer plate equalises the temperature of the wafer to the temperature of the chamber to within +/− 0 . 1 ° c . this minimises the possibility of any convection currents within the balance enclosure and avoids any thermal variation of the balance that might upset its calibration . [ 0089 ] fig4 shows the control architecture . the balance and robot arm communicate to a central pc via separate rs232 data links . the combined temperature & amp ; humidity sensor , communicates via an additional rs232 link . the analogue output of the pressure sensor is converted by a 12bit a / d converter before being transmitted over an ieee1284 data link . to achieve an accurate weighing result , variations in air density due to ambient conditions are corrected for . the air density is determined and then used to calculate the buoyancy effect on the semiconductor wafer . the air density is calculated by the following formula ρ air = 0 . 3485 × p - 0 . 00132 × ( 0 . 0398 × t 2 - 0 . 1036 × t + 9 . 5366 ) × h ( 273 . 14 + t ) × 1000 where ρ air is the density of the air in g / cm 3 , p is the pressure in mbar , t is the temperature in ° c . and h is the relative humidity in %. the air density is then used to calculate the buoyancy exerted on the semiconductor wafer using the following formula : b = w w × ( ρ air ρ w - ρ air ρ c ) 1 - ρ air ρ w where b is the buoyancy effect in grams , w w is weight of the wafer in grams , ρ w is the density of the wafer in g / cm 3 and ρ c is the density of the calibration weight used to calibrate the weighing balance in g / cm 3 . the density of the wafer ρ w is well defined due to the purity and crystalline nature of the substrate . the density of single crystal silicon is 2 . 329 g / cm 3 . there is a small error due to the thickness of the thin film on the surface . a thin film of thickness 0 . 5 μm and density 1 g / cm3 on a wafer of thickness 725 μm produces a systematic error of 0 . 16 % in the density . knowing the approximate target density would allow a correction to be made , reducing the error still further . finally , the mass of the wafer m w is calculated from the weight w w and the buoyancy b using the formula preferably , enhancement would be to use a reference semiconductor wafer as the calibration weight . in order to calculate the density of a thin film either deposited or removed from the semiconductor wafer surface two separate weighing measurements are made . the wafer is pre - measured , then the wafer is subjected to the process or processes required to deposit or remove the thin film and then it is re - measured afterwards . the difference between the two mass readings provides the mass of the thin film . the thickness for insulating dielectrics may be measured using existing equipment readily available within semiconductor fabrication facilities using techniques such as ellipsometry or spectral reflectivity . typical equipment used for this are the thermawave optiprobe or the rudolph fevii . knowledge of the thickness and the mass of the thin film allows the density to be determined . alternatively , if the density of the thin film is well characterised then this technique can be used to determine the thickness of the film without the need for other equipment . in particular , this technique is applicable over wide variety of different materials including metals and insulators simultaneously and so is ideal for incorporation into statistical process control environments where often each wafer is measured after each manufacturing process . the weighing equipment can be incorporated into the material handling equipment within semiconductor fabrication facilities . for example , in wafer sort & amp ; merge stations , wip & amp ; storage stations or within cluster tool handling platforms to provide in - situ process control . this technique may also be used to determine the amount of material removed during etching or cmp processes or for timed etch applications in damascene trench applications . for a particular type of material the density of the thin film is linked to the dielectric constant ( relative permittivity ) of the material by the equation ( see also the correlation in fig5 ) where ε is the dielectric constant , ε 0 is the permittivity of free space , ρ is the density of the film and ∝ is a constant for the type of material . using this equation , the density measurement may be used to calculate the dielectric constant of the thin film . as was previously mentioned , the present invention may be used in statistical process control , in which the weight measurement is used as a statistical guide to the accuracy of the processes carried out . [ 0107 ] fig6 depicts a simplified view of part of a typical semiconductor fabrication facility . processing equipment 24 are arranged in aisles 23 and grouped according to function . measuring equipment 25 are housed in a central location . in an automated facility , cassettes 21 of semiconductor wafers are transported using an overhead track system 22 . alternatively , automated guided vehicles ( agv ) may be used . when not being processed , cassettes of wafers are kept inside stockers 26 within a wip store . a typical processing step involves a central factory computer determining which process is required next for a particular cassette . the cassette is queued within the stocker until the processing tool is available . the cassette leaves the stocker , is transported to the processing tool by the overhead track , is processed and then returned to the stocker . typically , in statistical process control environments , the thickness of the layer just deposited or removed from the wafers within the cassette will then need to be measured . in existing facilities , the central computer must determine the appropriate type of measuring equipment and the wafer is again queued waiting for its availability . when available , the cassette leaves the stocker , is transported to the measuring equipment and then returned to the stocker to wait for the next processing step . an alternative method of operation incorporating the current invention is to combine the accurate weighing apparatus of fig1 with the stocker 26 within the wip store . as described previously , the weighing apparatus can be used to determine the amount of material deposited or removed from the wafer after each processing step , by comparison of successive weighing operations . because the weighing method is applicable to a wide range of different materials this eliminates the need to visit a specific piece of measuring equipment appropriate to the last processing step . it would not be practical to incorporate all of the existing , different types of measuring equipment within the stocker 26 . incorporating the measurement as part of the stocker 26 reduces the number and variety of measuring equipment required in the facility . it also reduces the number of material movement operations required , potentially by 50 %. this reduces the capacity of the overhead track required and improves the cycle time of the facility significantly .