fig1 depicts an embodiment of a via / contact test structure consisting of arrays 103 with increasing via / contact size along the row 101 direction and increasing densities or pattern factor ( exposed si area / wafer area ) along the column 102 direction . each via / contact array consists via / contact holes of a single size and density ( pattern factor ), as 104 indicates , and is etched simultaneously with the via / contacts in the functional dies into a dielectric layer of similar thickness and topography for making contact to the buried conductive layer or active regions such as source / drain . the aim of the embodiment is to have the etching process impose different etch rates , as the result of the microloading effect , and / or rie lag , over the variant holes and to get uneven etch over the structure range from under - etch to over - etch . other forms or modifications and / or embodiments may also meet this purpose and are thus intended to be covered by this disclosure . the embodiment of such test structure involves numerous implementation - specific decisions for achieving the ultimate goal , such as compliance with litho - related constraints . it is necessary to ensure that the smallest size of via / contact , usually 10 % below the design size in the functional dies , is within the exposure process window , so that no significant litho - induced size variation is transfered to the etching step . the test structures may be placed on the semiconductor wafers as “ drop - ins ,” which are located where functional dies would normally be placed . or they could be placed in wafer scribe lines , which are lines between functional dies defining diamond saw cuts that separate finished devices , the foregoing description disclosed the test structure and the anticipated etching results . further embodiments of this invention rely on voltage contrast imaging of this test structure with a sem , or the like , apparatus . according to the related patent application referenced above , under and / or over etch via / contact holes will appear differently in contrast in a sem image due to their voltage differences induced by primary electron beam irradiation . in general , primary electron irradiation will cause the surface to be positively charged or negatively charged depending on the total electron yield associated with the material as well as the immediate field conditions , such as extraction and retarding fields . the sustained positively charges tend to attract the consequent se back to the surface , thus the corresponding features appears relatively dark while the negative charged surfaces repulse the se so that the features appear relatively bright . the detected se signal is a function of the surface charging voltages , or equivalently in the contact case , the remaining sio 2 thickness . fig2 illustrates the detected se signals as a function of the thickness of under - etch ( positive thickness ) and / or over - etch ( negative thickness ). the signals are normalized with background level taken from the top surface , thus values higher than one equivalent to a bright image and those lower than one equivalent to a bright image . these curves are obtained at a charge equilibrium condition at which the overall voltage contrast is stable . curve 201 corresponds to the case where the substrate pn junction is equivalently forward - biased , as the compound effects of an external field associated with the electron optical system and local charge induced field associated with the high aspect ratio hole . for instance , for a p - type substrate with n - type source / drain under a weak enough extraction field , the contact hole acts as a faraday cup and traps se within the sidewall and bottom . the locally charged induced field predominates and imposes forward bias on the pn - junction . negative charges on the contact bottom will not sustain as they always find their way to release through the low resistant pn - junction . as the result , under - etch appears bright and over etch appears dark . however , this voltage contrast image is reversible if the external extraction field is strong enough to overwhelm the local charge induced field . once the effectively biased field for the pn - junction becomes reversed , the relatively high resistance pn - junction prevents the negative charges on the contact bottom from releasing , thus the through contact appears bright . for under - etch contacts with slight remainder , some electrons among the generated electron - hole pairs in the substrate may penetrate the thin sio 2 barrier to the bottom surface by the tunneling effect , the detected se signal decreases exponentially with the increasing sio 2 thickness . as the thickness further increases , the probability for the generated se to escape the holes increases as the aspect ratio decreases . the se signal in the reverse - biased case is depicted by curve 203 . it mat be noticed from fig2 that there is a threshold thickness beyond which via / contact appearance will transit from one contrast to its opposite . the sharp slope across the threshold point 202 implies that the vc image can be very sensitive to the actual thickness of remainders . slight variation of under - etching remainders around the threshold value may result in a significant difference in contrast or size . experiment has confirmed this predication and proper tuning of the primary electron beam energy to 200 ev and current to 75 na will shift the threshold thickness to the order of several tens of angstrom . this reduced threshold thickness provides a valuable measure for monitoring the etching process towards the just - etch or minor over - etches . the design of the test structure aims to produce such kind of etching level and covering from − 150 å over - etch to 100 å under - etch . fig3 illustrates the vc image of the test structure . via / contact holes at the lower - right corner are etched at a relatively slow etching rate and stop at under - etch with relatively more remainders , while via holes 302 at the upper - left corner 303 are of relatively larger hole size and lower density ; so that it results in excess etch ending inside the silicon substrate . as a result , the former is associated with bright holes while the latter is associated with dark ones . in between the upper - left and lower - right corners , a narrow transition region exists in which the holes turn out to be either bright or dark and merely depends on the unavoidable random thickness variation in an etching process . for a given primary beam energy and current , this transition region characterizes the etching process by its relative size and location in the vc image of the test structure as it is generally fixed for a given process . proper image processing , for instance by subtracting two similar images of the adjacent test structures , may highlight the transition portion in the resulting image , as shown in fig4 by the shadow region 401 . this resulting image is also called process an id map , or process “ fingerprint ”. shifts and / or changes of this fingerprint may happen over different parts of a wafer , or from wafer ( lot )- to - wafer ( lot ). the former implies the occurrence of non - uniform etching because the process itself mismatches the specs , or uneven dielectric thickness over the wafer ; caution must be taken if the shifting exceeds the predetermined tolerance . the latter can be attributed to long - term process parameter shifting ; manual or automatic process adjustment is necessary to keep it within the tolerance region . at a given primary beam energy and current , the vc transition happens within a narrow under - etch region with the thicknesses of remainders varying around the primary beam determined threshold thickness . in other words , the thickness of remainders of a via / contact can be determined from its corresponding transition image at known beam conditions . for instance , if the primary electron beam is of 400 ev in energy and 60 na in current , the transition region 401 represents that an equi - thickness region of about 50 å remained within via / contacts . further calibration can turn this roughly equi - thickness region into equi - thickness line 402 , as shown in fig4 , with the x - coordinate representing pattern factor , and the y - coordinate representing the via / contact sizes . other curves approximately conformal to this line exten the equi - thickness line to the nearby over - etch and under - etch regions . by doing so , a process “ fingerprint ” image is quantified with the amount of under / over etch with reference to the just - etch . for a given via / contact design rule in the functional die region , it is always possible to locate its position in this quantified “ fingerprint ” image and find its corresponding amount of over - etch thickness . with the aid of the quantified “ fingerprint ” image , manual or automatic process control ( apc ) can be made simple by controlling the percentage of over - etch time so that the amount of over - etch is tightly controlled not far away from just - etch . for example , for 0 . 18 μm via / contacts in the functional dies with a pattern factor of 2 %, one can easily locate its corresponding position 403 in the fingerprint image of fig4 . the average thickness of over - etch is found to be around 50 å , which is within the acceptable range . if wafer - to - wafer dielectric layer thickness variation or process shifting results in a change of fingerprint image , as shown in fig5 a , excessive over - etching up to 90 å is recognized , refering to 501 . apc should be acknowledged , for instance , to reduce the over - etching time by a certain percentage , or to increase the etching selectivity properly . if the fingerprint image shifts in the opposite direction as shown in fig5 b , the via / contact holes are most probably under - etched or insufficiently over - etched , refering to 502 . both cases may incur a high contact resistance thus are beyond the process window . apc should respond to this variation with longer over - etching time . the practical ways and parameters for the process adjustment in response to the process shifting and wafer - to - wafer variations depend on the device details under processing and vary from system - to - system and fab - to - fab . however , should the process parameters shift beyond the tolerance , or the dielectric layer thickness variation exceed the process window , the fingerprint image will immediately reveal this abnormality , and provide guidelines for the consequent control . this inline apc considerably reduces the final electrical failure rate and accelerates the yield ramp for new products . also , by doing so , the presently required over - etch time when etching contact holes can be reduced significantly , for instance by 10 % or higher . while the invention is susceptible to various modifications and alternative forms , specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail . it should be understood , however , that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed , but on the contrary , the intention is to cover all modifications , equivalent , and alternatives falling within the spirit and scope of the invention as defined by the appended claims .