the present invention provides a surface micro sensor for monitoring the cleaning and drying processes on the surface of a dielectric film during the manufacture of ics , mems and other micro devices . the surface of the dielectric can be non - porous , in which case the surface cleaning process is rate limited by the desorption of species from the surface or by removal of the species away from the surface . the dielectric can also be porous , or have other micro features present in it . furthermore , a cell ( biologic or other ) may be placed on the surface and monitored . as shown in fig1 , an exemplary embodiment of a surface micro sensor 30 for in - situ monitoring of the process of cleaning , rinsing and drying of surfaces and the micro features in those surfaces comprises two conducting electrodes 32 and 33 ( e . g . copper or doped polysilicon with a typical thickness of 1 μm ) that lie in the same plane , embedded in the surface of a supporting dielectric 56 on a substrate 54 ( e . g . a silicon wafer or a glass slide ), and covered by a thin dielectric layer 34 ( e . g ., silicon dioxide ( sio 2 ), silicon nitride ( si 3 n 4 ) and non - porous low - k organic dielectric materials ). the covering dielectric may be as thin as a few nm , e . g . 10 nm or less , and the electrodes may be spaced as close a few microns , e . g . 2 μm or less . the “ active ” part of the electrodes lies on the surface of the supporting dielectric . a surface segment 39 of dielectric 34 is defined between the conducting electrodes at the fluid - solid interface 40 . the electrodes are adapted to receive an ac measurement signal 50 to measure the impedance of surface segment 39 when the micro sensor is immersed in a fluid 36 , being rinsed or drying . an impedance analyzer 52 measures the impedance ( ratio of voltage and current and phase difference between the voltage and current ) of the surface section 39 between the electrodes via connectors 42 and 43 ( e . g . copper or doped polysilicon ) embedded in the supporting dielectric 56 beneath the electrodes that carry the measurement signal 50 to electrodes 32 and 33 . surface segment 39 has an electrical equivalent circuit consisting of capacitors 60 and 61 formed between the electrodes and the solution surface 38 , capacitors 62 and 63 formed between electrodes and the surface double layers , the surface resistance 64 and the bulk fluid resistance 65 . at solid - solution interfaces , an interface double layer forms because charges in the solution that are mobile ( ions ) respond to the presence of fixed charges on the solid . the interface double layer is responsible for capacitance c d1 ( capacitors 62 and 63 ) between the dielectric 34 and the solution 36 , which forms an impedance z d1 = 1 / jωc d1 where ω is the measurement signal radial frequency in series with the bulk solution resistance and which shunts the surface resistance . the sensor can extract the individual components if the impedance measurement is performed over a range of measurement signal frequencies . non - linear least squares fitting of the impedance data , a well known method from the domain of impedance spectroscopy , results in the individual component values . in an alternate embodiment , the covering dielectric layer 34 is a porous dielectric material ( e . g . porous low - k organic dielectric material ). the porous dielectric material is suitably a different material than the supporting dielectric 56 . in that case , the pores fill with fluid ( i . e . the porous material is soaked ) and the sensor &# 39 ; s electrical response is indicative of the residual contamination inside the pores in the porous material because the value of the electrode - solution capacitance 60 and 61 changes . as shown in fig2 , in an alternate embodiment a cell ( biologic or other ) covers the surface segment of the covering dielectric 34 . the cell must cover at least part of each electrode . the measured impedance is now a parallel combination of the surface segment and the cell , however in a living biologic cell the conductivity of cell membrane 72 is the main contributor to the surface resistance 64 , the cell &# 39 ; s membrane capacitance is the main contributor to the double layer capacitance c d1 62 and 63 and the cell &# 39 ; s cytoplasm 74 is the main contributor to the bulk fluid resistance 65 so the individual components of the cell can be extracted . changes to the cell &# 39 ; s conductivity ( for example due to dehydration as it reacts to a toxin in its environment ) can now be observed in - situ by monitoring the surface sensor &# 39 ; s impedance . the surface sensor can also be used as a novel device and the related methodology to determine the presence of unknown toxic contaminants in water and their effects on selected model cells that represent living organ ( s ) in human body . the technology has a variety metrology and control applications and bridges the gap between the in - vitro and in - vivo testing approaches for toxicity assessment . the objective of the surface sensor &# 39 ; s use in this application is the direct probing of toxicity effects on cells in a fast and in - situ manner . the principle behind the proposed technology is a unique “ test chip ” that allows impedance micro - spectroscopy of an array of cells on special sites on the test chip . the sites are pairs of electrodes that are parts of the test chip unique microcircuit that allows detection of impedance across each pair of electrodes . the impedance of cells on each site is sensitive to the condition of the cell . if damaged or killed due to the effect of toxic compounds , or when undergoing significant metabolic change , the cells adsorbed on these sites go through a fast and significant change in electrical impedance , compared to the slow changes in a healthy cell population . comparison of the pattern and the rate of change in the impedance pattern of exposed cells with that of healthy population ( stored information ) will provide information on the presence of toxic contaminants in water . some of the key features are rapid detection ( in order of seconds to minutes ), low cost , high selectivity in both contaminants and effects on specific organs , self contained ( no external power supply or analysis devices ), ready to use ( no calibration or specialized operational procedures or expertise ), and potentially a simple “ go ” or “ no go ” output . test chips based on this technology can be used as individual units for case - by - case water testing . examples are testing water in unfamiliar surroundings , in military application by soldiers in the field , or in general by any user for environmental monitoring in remote areas with no auxiliary facilities , equipment , or power supply . the chips can be used by engineers and industrial practitioners in the operation and monitoring of water distribution and treatment facilities where a large number of sensors in multiple locations are needed . the area of the surface segment and thus the required electrode area depend on the application . the area is typically quite large ( few mm 2 ) when monitoring the micro feature in a porous dielectric layer . a large area is desirable to increase the signal to noise ratio . the area is typically quite small ( few μm 2 ) when monitoring living cells on the surface , since these cells are quite small . connectors 42 and 43 have parasitic capacitance c substrate 45 to the substrate 54 and c fluid 46 to the fluid 36 that shunt some portion of the measurement signal around the surface segment equivalent circuit 80 . if the shunt impedance is significantly lower than the surface segment equivalent circuit then the sensor &# 39 ; s impedance output as measured by impedance analyzer 52 is dominated by the parasitic capacitances and the surface segment equivalent circuit can not be effectively measured nor the individual electrical components extracted . for the sensor to be useful , the total effective parasitic capacitance must be sufficiently small to allow an electrical measurement of the surface segment equivalent circuit impedance . if the parasitic capacitance dominates the total electrical response , then the circuit will not have a good signal to noise ratio and the sensor will not be very sensitive . if the parasitic capacitance dominates , the equivalent circuit calculation predicts no discernable impedance variation between highest and lowest surface contaminant concentrations . if the electrodes are very large , then the capacitance of the electrode surface with the fluid is large enough to dominate , and the connector parasitic capacitances are relatively small . however , when the surface of the electrodes needs to be small ( bio application ), then the parasitic capacitances of the connector dominates and mitigation of the parasitic is important . to mitigate this parasitic capacitance , it can be beneficial to include guards 90 and 92 that shield connectors 42 and 43 , respectively , as shown in fig3 a . a guard is an additional conductor that divides the supporting dielectric 56 between a connector and the substrate and / or the connector and the fluid into two parts that form two new capacitors , c guard 94 and c fluid 95 ( together c fluid 46 ) or c guard 96 and c substrate 97 ( together c substrate 45 ). c guard is the capacitor between the connector and the guard . c fluid is the capacitor between the guard and the fluid . c substrate is the capacitor between the guard and the substrate . if c substrate is small so that 1 / ωc substrate & lt ;& lt ; impedance of the surface segment , ( e . g . the substrate 54 is relatively thick and made from of dielectric ), then the section of the guards between the connectors and the substrate may be omitted . if c fluid is small so that 1 / ωc fluid & lt ;& lt ; impedance of the surface segment , then the section of the guards between the connectors and the fluid may be omitted . the guards 90 and 92 are biased so that their voltages follow as closely as possible the respective electrode voltages at all times , even when the electrode voltage changes over time . the guard voltage need only track the ac component of the electrode voltage but suitably tracks the total instantaneous electrode voltage . the current required to make the guard voltage the same as the electrode voltage is supplied by buffers 98 and 99 , e . g . an operation amplifier ( opamp ), not by the measurement signal 50 . the buffer reproduces the desired voltage without significant loading it . since the voltage difference between the electrode connector and its guard is several orders of magnitude lower than the voltage difference between the electrode connector and other conductors in the neighborhood of the sensor ( substrate and fluid ), the loss or distortion of the measurement signal through the parasitic capacitance c guard will be very , very small . the measurement signal is not affected by the capacitors c fluid and c substrate because the measurement signal is buffered prior to being applied to the guard . c fluid and c substrate must be charged and discharged by the guard buffer . as shown in fig3 b , the electrodes are not required to be side by side . for example , electrodes 82 and 83 are concentric . furthermore , a larger number of electrode pairs can be collocated on the same substrate in an array and each individual site selected using two control lines 84 and 85 that connect the electrodes to contact lines 86 and 87 , respectively . in another embodiment , the electrodes could be arranged in fingered configuration , which has a small electrode spacing ( e . g . 2 um or less ) while at the same time having a very large electrode perimeter . such a configuration would allow detection of particles or cells that cross this perimeter even if there are very few such cells present and if they are very small in size . fig4 a is a plot of the surface micro sensor &# 39 ; s ideal ( i . e . when the capacitance of the electrode connector is zero ) frequency response when the parameters of the electrical equivalent circuit of the surface segment changes . the imaginary part of the impedance (− im [ z ]) is plotted versus the real part of the impedance ( re [ z ]) in what is commonly known as a nyquist diagram . different values in the equivalent circuit parameters lead to measurably different responses as the frequency changes from 1000 to 1000000 hz : 100 ( electrode - solution capacitance 60 , 61 = 5 × 10 − 8 f , surface double layer capacitance 62 , 63 = 2 × 10 − 8 f , surface resistance 64 = 5000ω , bulk fluid resistance 65 = 10000ω ), 101 ( electrode - solution capacitance 60 , 61 = 5 × 10 − 8 f , surface double layer capacitance 62 , 63 = 2 × 10 − 8 f , surface resistance 64 = 2000ω , bulk fluid resistance 65 = 10000ω ) and 102 ( electrode - solution capacitance 60 , 61 = 5 × 10 − 8 f , surface double layer capacitance 62 , 63 = 2 × 10 − 9 f , surface resistance 64 = 5000ω , bulk fluid resistance 65 = 1000ω ). fig4 b is a plot of the surface micro sensor &# 39 ; s frequency response with a guard 100 , 102 and without a guard 104 , 105 when an electrode connector parasitic capacitance of just 5 × 10 − 8 f is present . without a guard , the loss or distortion of the measurement signal through the parasitic capacitance will be very , very large . hence , surfaces with different electrical equivalent properties will not have a different sensor response . with a guard , the loss or distortion of the measurement signal through the parasitic capacitance will be very , very small . the guard makes the impedance of the shunt path ( the path the signal must take to bypass the micro feature ) that limits the operation much larger . the larger the fraction of the electrode connector that is covered by the guard , the less distortion of the measurement signal that will occur . if the electrodes are small , then the capacitance of the electrode connector capacitance becomes relatively more important . hence reducing the size of the electrode is limited if there is no guard . it is desirable to make the electrode as small as possible to reduce the sensor &# 39 ; s manufacturing cost and to allow smaller cells to be monitored . also , it is required to make the sensor electrodes small if a single cell is to fit on the electrodes , since cells have a typical diameter of about 10 μm . the signal to noise ratio of the micro sensor without the guard is determined by the geometry of the sensor and the materials choice . the signal to noise ratio of the surface micro sensor with the guard is not determined by the parasitic elements , but by the accuracy of the electronics used to measure the trench impedance . the micro sensor with guard can therefore measure the micro feature impedance more accurately in a noisier environment . for rinsing applications , the presence of ionic contaminants on the surface changes the resistivity of the surface even if very small concentrations ( parts per billion level ) are present . therefore , the impedance measured between two electrodes will depend very much on the residual contamination of the surface . even non - ionic impurities , directly and through interactions with other species present , change the dielectric properties of the surface , which in turn define the impedance . for drying applications , the removal of the water from the surface ( replacing it with air , pure nitrogen or some other gas ) will likewise result in a measurable change in impedance , since the difference between the conductivity of ultra pure water and air can easily be detected . conduction along sidewalls can be measured , so that the amount of moisture adsorbed on the sidewalls or ( slightly ) conducting residual impurities on the sidewalls will be detected . if the covering dielectric is a porous material , the surface micro sensor measures resistivity inside pores . furthermore , the surface micro sensor measures the full impedance spectrum , of which the resistivity is just a part ( impedance is a complex number quantity that is dependent on frequency while resistance is the real part of the dc value of the impedance ). this means that a large amount of other information , such as the dielectric absorption of the covering dielectric and the surface double layer and their frequency dependence is also available in the sensor &# 39 ; s output data . the surface sensor can monitor specific ionic species and or non - ionic species ( since these change the permittivity and surface adsorption ). because trace quantities of impurities can result in significant change in conductance or dielectric constant and because these electrical properties can be accurately measured , the sensitivity of the sensor is very good . an exemplary process for fabricating a particular surface micro sensor 30 of the type shown in fig1 is illustrated in fig5 a through 5 g . as shown in fig5 a , a dielectric 112 such as sio 2 is deposited with a typical thickness of about 3 μm on a substrate 113 , which can be silicon , si 3 n 4 , glass or a similar material . optionally , if the substrate itself is a dielectric such as sio 2 , this step may be omitted . a conductor 114 such as copper or doped polysilicon with typical thickness of 0 . 5 μm is deposited on dielectric 112 . as shown in fig5 b , a photoresist 116 is deposited on conductor 114 and the pattern of the bottom portion of the electrode connector ( 42 and 43 shown in fig1 ) is defined using photolithography and chemical etching . as shown in fig5 c , the photoresist is removed and a second dielectric 118 , such as sio 2 is deposited with a typical thickness of 1 μm on top of the conductor ( 114 as shown in fig5 b ). the dielectric is made planar by polishing until the conductor is just exposed . as shown in fig5 d , a third dielectric , such as sio 2 119 , is deposited with a typical thickness of 3 μm . the dielectric is patterned to open the contacts to the electrode connector using photoresist deposition , photolithography and chemical etching . the photoresist is then removed . as shown in fig5 e , a conductor 122 such as w with typical thickness of 3 μm is deposited on dielectric 119 to fill the openings . the w is etched back so that it is only left in the contact openings ( this process is called w plug formation in the microelectronics industry ). as shown in fig5 f , a conductor 124 such as copper or doped polysilicon with typical thickness of 0 . 5 μm is deposited on dielectric 119 . a photoresist 126 is deposited on conductor 124 and the pattern of the electrodes ( 32 and 33 shown in fig1 ) is defined using photolithography and chemical etching . as shown in fig5 g , the photoresist is removed and a fourth dielectric 128 , such as sio 2 is deposited with a typical thickness of 1 μm on top of the conductor 124 . the dielectric is made planar by polishing until the conductor is just exposed . the surface sensor fabrication is completed by depositing the capping dielectric layer 130 . typical dielectric films include silicon dioxide ( sio 2 ), silicon nitride ( si 3 n 4 ), high - k dielectric materials ( tio 2 ) and low - k organic materials commonly used in the microelectronics manufacturing process . the contact openings to the electrode contacts are defined by deposition of photoresist , photolithography and chemical etching . the photoresist is then removed . the use of the surface micro sensor 30 to monitor the clean / rinse / dry process is illustrated in fig6 - 8 . typically , micro sensor 30 would be placed in a cleaning solution of a known ion concentration to calibrate the sensor . once calibrated the surface micro sensor may be inserted in a cassette 150 with a number of other product wafers 152 and processed through a sequence of clean / rinse / dry baths 153 a - 153 c as shown in fig6 or mounted on a chuck 154 with a single wafer 156 and subjected to a sequence of clean / rinse / dry sprays 157 as shown in fig7 . as shown in fig8 , as the surface micro sensor passes through the clean / rinse / dry cycle the measured impedance can change and the effective surface resistance determined from this measured impedance 158 , changes fairly dramatically from a very low impedance during cleaning , to a moderate impedance during rinse and finally to a much higher impedance when the drying process is completed . by first calibrating the sensor to the allowable surface concentration , the rinse and dry process duration can be optimized . also , by first calibrating the process and then monitoring the impedance during an actual production run , the wafers can be transferred from one process to the next to ensure adequate clean / rinse / dry without wasting time or chemicals . alternatively , the sharp increases in impedance levels and subsequent leveling can be used to trigger a transfer to the next processing stage . if process calibration is both accurate and stable enough , e . g . the times to transfer , then it is possible that the micro sensor may not be needed during the actual production runs , but is merely used to periodically confirm that the process performance is still within specifications . while several illustrative embodiments of the invention have been shown and described , numerous variations and alternate embodiments will occur to those skilled in the art . such variations and alternate embodiments are contemplated , and can be made without departing from the spirit and scope of the invention as defined in the appended claims .