AFM Technique Enables Production-Worthy Divot Detection

Whenever someone talks about a "divot," most tend to think in terms of the cavity left when a piece of turf is propelled out from the ground by a not-too-well-directed club head handled by a golfer attempting to hit the ball. For device makers, however, the term has acquired an entirely different meaning. In a circuit, a divot occurs between the active area and the filled isolation trench; it is usually the result of mechanical stress along the SiO₂/Si interface that can occur in the chemical mechanical planarization (CMP) module during the shallow trench isolation (STI) process. In subsequent silicide process steps, the silicide can grow into that divot and form dendrites that provide paths for charge carriers to leak out of doped regions. This, of course, results in general device failure. Deep divots, particularly, will detrimentally affect speed and low-power performance, which in turn forces CMOS device manufacturers to keep a sharp eye on this singular topology's depth.

Various techniques have shown to have problems in monitoring and measuring this anomaly, often caused by the high level of variability from wafer-to-wafer and lot-to-lot. In the case of optical techniques such as scatterometry, for example, specialized measurement sites and model libraries become necessary to perform these measurements. This can often be time-consuming, and requires that non-yielding wafer real estate be devoted to it. With a SEM, there can be secondary electron effects and sample charging that could adversely affect the chip. Although conclusive, TEM applications are destructive and often burdened by unacceptable turnaround times.

Atomic force microscopy (AFM) is a technology that appears promising at attempting to solve this critical measurement hurdle. The advantage that AFM offers over other techniques is that it is insensitive to both materials and process variation; it is a topographical method that uses a probe. Probes that exist today are sufficiently thin for this application, which make it possible to get it into a divot and measure its depth without causing any harm to the wafer. This method is also attractive because it does not require specialized measurement sites that consume valuable wafer real estate. Because the structures are measured on the actual devices, true metrology is enabled. The problem has been, however, that some AFM procedures do not lend themselves easily to being automated, and thus ensuring that the analysis generates repeatable and accurate results — a necessity if this is to be a production-worthy application.

TEM cross-sections showing an STI divot. (Source: 2006 ICPT)

For some time, the Metrology Group of Veeco Instruments (Santa Barbara, Calif.) has been studying to solve this problem, and has arrived at what seems like a workable solution by using deep-trench-mode AFM.¹ AFM is an advanced adaptive scanning technique that considerably improves control of the probe tip/sample interaction. It will only take the necessary data points when AFM is at the predefined set point. If, for any reason, AFM does not get to a particular set point at a given sample location, it ignores it and proceeds to the following data point. One of the advantages that this kind of intelligent sampling brings is that it provides a concentration of data points for the regions of interest, which assists in efficiently eliminating data from other low-priority areas that are of little to no interest.

The initial research work was done in conjunction with a semiconductor foundry that generated two sets of controlled wafers under different process conditions. The divot was measured using AFM with the analytical method developed for this, then the wafers were sent for electrical test correlation. Bottom-line results concluded that it was possible to correlate increased divot depth to reduced sheet resistance. Implementation and AFM's unique deep-trench-mode enabled the achievement of a divot process monitoring inspection method within the STI CMP module. AFM technology now meets the requirements for a non-destructive, fast time to results, direct metrology divot monitoring solution.