1. Field of the Invention
The present invention relates to a method of obtaining a transparent image of a resist contact hole or a feature in a resist provided on a silicon wafer through a scanning electron microscope (SEM), with an absence of deforming the feature, such as the contact hole, by a focused ion beam (FIB). In particular, the method is directed to the obtaining of a transparent image of a resist feature, such as a contact hole, by SEM without damaging the silicon wafer, and to obtain an accurate characterization of measurements of hidden features in the resist.
In the current technology which is utilized in process development and failure analysis during semiconductor manufacturing, it is of importance to be able to obtain cross-sections of critical structures which are located on a silicon wafer. Thus, the cross-section of holes or other two-dimensionally restricted features are particularly subject to problems, inasmuch as it is difficult to dice or cleave the silicon wafer precisely though the center of a given contact or feature Moreover, the loss of time involved with off-line cross sectioning of the silicon wafer, attendant by the loss of the entire wafer and the effecting of only a limited number of samplings on a given silicon wafer are all negative aspects which adversely affect the economics of such sampling or testing methods, the latter of which are essentially of a destructive nature.
In addition to conventional cross sectioning and subsequent scanning electron microscope (SEM) viewing of a feature or sample on a silicon wafer at essentially right angles thereto, the focused ion beam (FIB) system has been developed to essentially produce local milling operations which enable a considerable amount of the critical information to be gathered for various diverse situations. At this time, FIB techniques for the local milling of holes; for example such as selective carbon mill (SCM) necessitate the milling away of up to half of the hole. This is subject to considerable inaccuracies in determining the true diameter of the hole of the silicon wafer, and even more importantly, may result in the deformation of the structure by the milling process due to the direct contact of the focused ion beam with the structure of interest or feature in the silicon wafer.
2. Discussion of the Prior Art
In essence, the current technology employs the following processes:
a) Mechanical cleavage of the wafer, which consists of physically braking the wafer, and then obtaining an SEM image of the broken cross-section. This generates two problems in that, firstly, the wafer is destroyed, and secondly, the features can be deformed during cleavage. This deformation is partially addressed by depositing a metal layer (such as gold) over the surface, which adds to the cost and adversely affects the economics of the process.
b) FIB with gas assisted etching (GAE) is used to perform the milling; wherein GAE is a standard technique that includes the use of a gas, such as H2O vapor, during the milling to carry away carbon or other components to prevent re-deposition on the cross-section. This procedure is less destructive than mechanical cleavage. This technique mills away a selected portion of the resist, preferably through the center of the features (such as contact holes), leaving a cross-section which is imaged by SEM, and does not destroy the wafer substrate, so that at minimum, other portions of the wafer may be used. However, there may be encountered deformation of the features during milling of the resist. In addition, accurately locating the position of the region to be milled typically a rectangular cutout shape, where on edge of the rectangle is targeted to cut through the center of the features such as contact holes is difficult to obtain because charge accumulation tends to cause deflection of the ion beam. This milling selection process typically involves a rapid FIB scan at low beam energy to obtain an image, and then selection of a milling region (e.g. a rectangular region having an edge overlaying a line of contact holes). Because of deflection, also referred to as drift, due to charge accumulation, which can occur both during FIB imaging as well as during FIB milling the edge of the milling region is frequently selected to cross the line of contact holes at an angle, such as 3 degrees, in order to ensure that at least some of the holes are cross-sectioned across the center of the hole.
In contrast with the state-of-technology, the present invention allows the imaging of hidden features in resist without distortion of the features, avoids the problem of inaccurate milling due to charge effects (i.e., deflection or drifting of the ion beam), and does not require any mechanical cleavage of the wafer.
Accordingly, in order to clearly and unambiguously distinguish over the state-of-the art, pursuant to the present invention there is utilized a method of non-destructive testing; wherein there is obtained an accurate measurement of hidden features in resist, such as contact holes, by means of the obtaining of a transparent SEM image.
Essentially, the inventive method comprises the steps of:
Milling a section of resist, in which the edge of the milled region is offset from the features of interest by about 300 nm or less (in effect, creating a cross-section resist where there is about 300 nm of resist between the exposed cross-section plane and the edge of the feature). The offset is preferably within the range of about 100-200 nm.
Image the cross-section using an SEM between about 3-20 kev, preferably 5 kev to 17 kev.
It is the high energy SEM which provides the transparency of the image to enable a view of the hidden feature profile. The offset cross-section means that the features will be subject to little or no distortion. It is also easier to accurately target an offset milling region because the accuracy of the milling location will be less affected by drift or deflection caused by charge accumulation.
The advantages which are derived by the inventive method:
a) Saving the wafer for further production or further analysis because no mechanical cleavage of the resist wafer is necessary.
b) Using the transparency of the resist to obtain the SEM image of contact profile can eliminate the ion beam damage sustained at the edges of contact hole or resist features during regular cross-section cutting, and increases the SEM image fidelity while preserving the true profile.
c) This method has short cycle time compared with a mechanical cleaving process, and provides precise controlling over the cross-section location.
In particular, the tooling employed through the metrology of the resist provides the advantages over regular mechanical cleaving processes through the saving of the silicon wafer, reduction in cycle time and enhanced control over the positioning wherein any ion-caused damaged (due to FIB) during cutting is a concern which is solved by the present invention. In effect, the present invention uses advantages, which clearly improves over the prior art as employed in the current state of the technology.
FIB milling is used to provide side view access near the hole in the silicon wafer but not to touch the hole, thereby avoiding the issue of deforming the structure. The remaining material is less than 0.25 um, whereby thinner is better.
Electron beam is sufficiently energetic to pass through the remaining material to the actual hole, where secondary electrons are generated that can escape from the hole and be detected.
Because of the thinness of the remaining material, the primary beam is subject to only very little deflection, thereby accurately identifying the edges of the hole.
The extremes of the hole extension (diameter) are particularly well defined because these surfaces are naturally parallel to the beam and provide a maximum surface for the prime penetrating electron beam which, in turn, provides maximum production of secondary electrons.
For resist materials, SEM image starts to show transparency properties when the beam energy is higher than 3 kev. Relatively higher electron beam energy can be used to obtain smaller beam spot size, more signal, high resolution image, and deeper transparency of the features because of the above charge elimination techniques.
Because of this protective cross-sectioning technique, it can be used for contact hole or hidden feature of the most diverse sizes, for example, such as approaching 100 nm or even smaller, although not limited thereto.
Accordingly, it is an object of the present invention to provide a novel method which will obtain the image of resist contact holes or resist features on a silicon wafer by scanning electron microscope (SEM) without the deformation of the feature or features by a focused ion beam (FIB).
A more particular object of the present invention resides in the provisions of the method as described herein wherein there is employed an SEM operating at a specified beam energy, and which enables the SEM image to display transparency properties.