Abstract:
Method for fabricating a structure. According to an exemplary embodiment, a structure is made by forming a layer of removable material with a first surface spaced a part from a second surface. The first surface is formed along a first region from which the material is removable. The first surface is altered by removal of material from the layer. Removed material from the first surface is monitored to detect fluctuations in a variable of composition in the layer, and removal of material from the first surface is terminated when the composition of monitored material meets a predetermined criterion. In an alternate embodiment a variable characteristic is imparted to a layer of material as a function of layer thickness and an operation is performed on the layer resulting in removal of material. Samples of removed material are monitored for variation in the characteristic and the operation is modified when a variation conforms with a criterion.

Description:
FIELD OF THE INVENTION 
     This invention relates to fabrication operations and, more specifically, to a system and a method for monitoring progress during fabrication of a part. 
     BACKGROUND 
     Fabrication of components often involves removal of material in order to create a feature or reduce a dimension. By way of example, both microelectronic machining and semiconductor device fabrication involve formation of layers of material followed by operations which then remove the material to create vias or planarize a surface. Frequently these fabrication steps are performed chemically or mechanically, and sometimes with the assistance of charged particles such as a plasma or ion beam. Common examples in semiconductor processing include etching of openings such as contact vias and isolation trenches. Also, layers may be thinned by chemical mechanical polishing (CMP) of surfaces to remove material, e.g., material deposited to fill a via, until a surface of one material composition is along the same plane as an adjoining surface of different composition. 
     Frequently, in these and many other types of fabrication steps material is removed from a surface region to a critical depth which should not be exceeded. However, the ability to directly measure progress of the activity may be difficult or cost prohibitive. For example, while it may be desirable to perform direct measurement of such progress, the fabrication activity may have to be stopped in order to obtain the measurement. Because many contemporary manufacturing operations are capital intensive it is undesirable to reduce throughput by temporary removal of a work piece from an operation in order to perform such measurement. On the other hand, when there is no alternative to removing a work piece from manufacture in order to perform measurement or other analysis, the removal is done on a limited basis (e.g., spot checking) in order to maximize throughput. 
     During manufacture, the in-line monitoring of product and analysis of process features assures that an operation is providing results within acceptable limits of variability. For example, during fabrication of semiconductor devices, when an opening is being etched through one layer of material to a second layer, it is possible to detect completion of the opening, i.e., end point, by analysis of removed material to identify material of the second layer. The usefulness of such an approach depends on the amount of over etch which can be tolerated. If the etch selectivity is low, the critical depth may be exceeded before end point is detected. 
     When a process step is well characterized and repeatable it is common to apply the process for a fixed time in order to reach an end point. As the depths of material removal become smaller the margin of error associated with timed operations remains constant such that it becomes more likely that critical depth or end point does not coincide with completion of the timed operation. 
     In this regard, endpoint detection for CMP operations has been particularly problematic. The two basic methods in use today are the timed operation and a technique of monitoring the change in frictional force associated with the polishing pad and the work piece. The timed technique may be considered largely a trial and error determination. 
     For CMP operations, changes in frictional force are detectable by measuring the responsive change in motor drive current as the motor turning the polishing pad encounters increased friction. By way of example, when polishing a silicon oxide dielectric layer in a semiconductor product, the friction detection method may incorporate an otherwise unnecessary CMP stop layer such as silicon nitride. As the CMP process initially removes only the silicon oxide, the drive force of the CMP motors remains relatively constant. When the polishing pad comes into contact with the silicon nitride the change in friction is detectable and this signals an opportunity to stop the CMP process before over polish occurs. 
     The drawback to each of these methods again relates to dimensional tolerances, or the lack of a soft landing at polish stop. Resultant over polishing or under polishing leads to non-uniformity between product wafers in the same lot as well as between wafer lots. Another undesirable effect is consequent damage to alignment marks from over polishing. If the ability to detect the alignment marks is lost, subsequent levels of deposited material cannot be properly processed, resulting in lot scrap or yield loss due to misalignment of features. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the invention, a method is provided for making a structure by forming a layer of removable material with a first surface spaced a part from a second surface. The first surface is formed along a first region from which the material is removable. The first surface is altered by removal of material from the layer. Removed material from the first surface is monitored to detect fluctuations in composition and removal of material from the first surface is terminated when the composition of monitored material meets a predetermined criterion. 
     In an alternate embodiment of the method a variable characteristic is imparted to a layer of material as a function of layer thickness and an operation is performed on the layer resulting in removal of material. Samples of removed material are monitored for variation in the characteristic and the operation is modified when a variation conforms with a criterion. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Numerous advantages of the invention will be apparent when the following detailed description is read in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates, for an example embodiment of the invention, a system for depositing a layer of material; 
     FIG. 2 illustrates a layer of material deposited according to an embodiment of the invention; 
     FIG. 3 illustrates the layer of FIG. 2 after polishing; 
     FIG. 4 illustrates an exemplary coding sequence; 
     FIG. 5 illustrates a polishing system according to one embodiment of the invention; and 
     FIG. 6 illustrates a via formed according to an alternate embodiment of the invention. 
    
    
     The drawings are not to scale, but rather, provide schematic representations of a circuit structure to emphasize relevant features relating to the invention. Accordingly, numerous incidental details are not illustrated. For example, deposited layers which may in fact be conformal are not necessarily illustrated as such. 
     Terminology 
     Surface portion means a region along the surface of a layer which may be arbitrarily identified or which may be defined based on the geometric shape of the surface or a function associated with the layer. For example, the surface of a sphere may be arbitrarily divided into two hemispheres each being a surface portion, while the surface of wafer may be divided into three surface portions consisting of first and second opposing surfaces and an edge surface connecting the opposing surfaces. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention now described may be most useful in complex semiconductor structures having relatively thin layers of material formed therein. Such structures will have manufacturing tolerances which are difficult to achieve with conventional end point detection techniques and, when fabricated with Ultra Large Scale Integration processes, e.g., geometries of 0.25 micron or less, will have circuit densities and electrical performance requirements which demand precision tolerances that are critically dependent upon accurate and timely end point detection. 
     Application of the invention is illustrated for removal of dielectric material by chemical mechanical polishing. For this example it is convenient and advantageous to vary the composition of the dielectric material with isotopic variants of the dielectric chemistry in order to determine progress of the polishing activity to end point. In other applications it may be more advantageous to modify the material being removed with other variants such as elements and chemical variants which do not adversely affect the fabrication or operation of the product, including embedded markers which are detectable by means other than the various forms of spectroscopy. 
     With reference to FIG. 1 there is shown in part a schematic representation of a chemical vapor deposition (CVD) system  10  useful for depositing a layer of material on a substrate. In the following example the system controllably reacts multiple isotopic variations of silane with oxygen to deposit a layer of silicon oxide on a semiconductor wafer  14 . The wafer  14  is positioned on a support  16  in a deposition chamber  18 . Three feedstock gas sources are shown. A supply  20  of silane formed with  28 Si is available to the chamber  18  through control valve  22 . The  28 Si of supply  20  has an isotopic purity greater than 99 percent. Such material is commercially available from Isonics corporation of Golden Colorado. A supply  24  of silane formed with naturally occurring silicon (approximately 92 percent  28 Si, 5 percent  29 Si and 3 percent  30 Si) is available to the chamber  18  through control valve  26 . A supply  28  of oxygen is available to the chamber  18  through control valve  30 . The composition of gases inserted through the valves  22 ,  26  and  30  is varied by a control circuit  34  according to a programmable input  36 . The reaction is performed under conventional process conditions. A profile of the deposited dielectric film follows the variation in isotopic composition of the inserted gases. 
     FIG. 2 illustrates in cross section a portion of a partially fabricated semiconductor device  40  of the wafer  14 . A layer  40  of dielectric material has been deposited on the device by the CVD system  10  to achieve an isotopic variation of Si concentration. For simplicity of illustration active semiconductor regions and other features common to such devices are not illustrated. Yet it is to be understood that the dielectric layer  40  may be one of many layers in a metallization system having alternate depositions of insulator and conductor metal. Such layers are configured through lithography and etch processing to provide a multiple metallization levels of an interconnect system in support of device circuit functions. 
     In FIG. 2 the layer  40  is shown to have been deposited over a metallization level comprising exemplary conductors  44  having upper surfaces  46  along a plane indicated by a reference line  48 . Other metallization levels may have been formed below the conductors  44  according to the invention; and, according to the invention, still other metallization levels may be formed over the dielectric layer  40  after the layer  40  is polished down to the plane  48 . The layer  40  has an initial surface  42  prior to polishing. The insulative layer could also be deposited directly over a semiconductor layer, in which case the polishing operation would planarize the layer after formation of trench isolation regions. FIG. 3 illustrates the layer  40  polished to the plane  48 . 
     To determine the end point for polishing the layer  40  to the plane  48  the layer  40  includes an isotope mix such as illustrated in the graph of FIG. 2 wherein the axis labeled “Layer Thickness” is relative to the lower surface  52  of the layer  40 . As illustrated in the graph, the silicon oxide dielectric layer  40  comprises a variable concentration of  28 Si. This is effected by initially injecting 100 percent  28 Si silane from supply  20  until the deposited dielectric material reaches the plane  48 , at which time silane may be introduced from the supply  24  in combination with silane from the supply  20 . In this example, the proportion of silane injected to the chamber from each of the supplies  20  and  24  is modified in a linear manner as silicon oxide is deposited so that at some point near the end of the deposition all of the silane is injected from the supply  24 . In this example, at the end of the deposition, the concentration of  28 Si has decreased from 100 percent to 92 percent, and the combined concentration, [ 29 Si]+[ 30 Si], has risen to eight percent of the total silicon concentration. 
     Generally, any variable characteristic may be imparted to the layer  40  as the layer is deposited so that a depth profile (i.e., a profile taken in a vertical direction relative to the plane of deposition) is encoded with variations which are detectable during the process of removing material. For example, a digital signal may be embedded in the depth profile by abruptly varying the injected supplies from 100 percent supply 20 to 100 percent supply  24 . The pulse trains may be of variable durations and spacings to further identify distance from end point. Such signals may be used to modify the polishing rate, e.g., by changing the rpm of the CMP machine or by altering the slurry composition. 
     See, for example, FIG. 4 which illustrates a digital encoding in the deposited layer  40  by discretely altering the composition (i.e., in a square wave fashion) from one silane supply  20  to the other silane supply  22 . Relatively short pulses of supply  24  may correspond to a digital “1” while the relatively long pulses from supply  24  may correspond to a digital “0”. Other forms of digital encryption, such as the binary code, also illustrated in FIG. 4, may be used to indicate progress to endpoint. 
     Once the layer  40  is deposited with a desired encryption, it is polished on a CMP system  80  such as schematically shown in FIG. 5. A controller  82  directs motorized movement on the CMP table  86  to remove material of the dielectric layer  40  as slurry  88  is piped to the table from a source  90 . Used slurry exits the table through a waste stream line  92 . A portion of the used slurry is diverted to an isotope detector  100  which provides a signal to the controller  82  indicative of the proportion of Si isotopes detected in the slurry during sequential analyses. Preferably the isotope detector is a mass spectrometer. 
     The slurry  88 , containing mixed isotopes, is sampled from the waste stream line  92 , e.g., in the volume of one to two cubic centimeters. The liquid suspension is volatized at high temperature and analyzed to determine the Si isotope ratio, e.g., by inductively coupled plasma mass spectrometry. 
     With the isotopes positioned as markers, the controller is programmed to respond to predetermined signals generated by the isotope detector  100  in response to the markers. For the signal shown in the graph of FIG. 2, the controller responds to detection of a maximum concentration  28 Si by terminating the polishing activity. Summarily, detection of the proportion of Si isotope markers relative to background serves as a means for linking the controlled deposition of isotope variations with the depth, e.g., relative to the plane  48 , at which the CMP system is removing material. A logic system embedded in the controller  82  is capable of modifying CMP parameters for removal rate as end point is approached. 
     The result is a well-controlled process wherein there is a measurable delay between the time isotope is incorporated into the slurry and when the isotope is analyzed. With the simple pattern shown in the graph of FIG. 2, over-polishing may result from this delay. However, a more detailed encoding, such as one which provides a signal in advance of end point in order to retard the polishing rate, will overcome such concerns. For example, according to the code of FIG. 4, isotopic variations may be modulated with the deposition system of FIG. 1 to provide pulses which vary in duration. A variety of pulse trains are created as markers, indicating, for example, percentage of polishing which remains before end point is reached. 
     Following the completion of a CMP process the slurry waste stream may be flushed to remove residue traces of the removed CVD material and to reset the normal background Si isotope ratio of the slurry mix in preparation for the next run. Any combination of flows could be used to form any concentration variation in the Si isotope forming the CVD dielectric. Linear, binary or stepped transitions are possible. If a system is developed wherein the slurry can be re-used, the slurry mixture will develop an increasing background level of the isotope selected as the marker. So long as the signal to noise ratio between selected isotope combinations is sufficient, end point markers may continue to be monitored with the same slurry. 
     Placement and monitoring of isotopic markers for end point detection may require calibration. That is, the position of markers relative to end point should be known with sufficient precision to identify end point within acceptable tolerances. This may be effected by simultaneously monitoring removal of isotopic markers with the system  80  and monitoring end point with suitable detection equipment, an ellipsometer. 
     The concepts disclosed may, more generally, be applied to removal of materials during a fabrication process. For example, with reference to FIG. 6, a layer of insulative material  120  is shown to be deposited over a metal runner  130  of an interconnect system. A via  140  is formed through the insulative layer  120  in order to provide a contact between the runner  130  and a yet-to-be-formed metal runner in a overlying level of metallization. With provision of isotopic markers in the dielectric layer  120 , end point in the etch of the via  140  may be detected with analysis of material removed from the layer  120  with a mass spectrometer. Stable isotopes of other elements, e.g., oxygen, may be used as markers in lieu of silicon isotopes. In another application, the isotopic markers may be placed in silicon semiconductor material to monitor removal of silicon during processes such as formation of trenches. It is also contemplated that in numerous applications inert chemicals may be used in lieu of isotopes as markers. This will be useful in processes which require removal of a first layer which is formed over a second layer by an etch process, wherein the etch selectivity between the first and second layer is relatively poor. Use of isotopic markers in such a process will help avoid over etching. This is useful when etching layers formed on semiconductor material that may be easily damaged or over-etched to the point of removing dopant impurities. 
     With regard to semiconductor fabrication, markers of the type suggested may be used for end point detection in a variety of removal techniques, including the varied forms of etching, e.g., isotopic and anistotropic etching, plamsa etching and ion milling. More generally, the method may be applied to a wide variety of removal processes in other industries, including abrasive removal, milling, sawing and other forms of cutting and shaping.