Weighing scales – With testing
Reexamination Certificate
1999-03-15
2001-09-04
Nguyen, Tuan H. (Department: 2813)
Weighing scales
With testing
C177S001000, C073S001010
Reexamination Certificate
active
06284986
ABSTRACT:
TECHNICAL FIELD
The present invention is directed to semiconductor manufacturing methods, and more particularly, to a method of determining the thickness of a layer deposited on a substrate. Still more particularly, the present invention is directed to a method of monitoring the thickness and uniformity of a layer deposited on a silicon substrate.
BACKGROUND OF THE INVENTION
In the process of manufacturing semiconductor wafers it is typically necessary to deposit layers of various materials onto a silicon wafer substrate to create desired electrical properties. Processes such as epitaxial deposition (EPI) or chemical vapor deposition (CVD) are commonly used to deposit materials onto substrates.
The properties of the layers deposited on the wafers often depend on the thickness and uniformity of the layers, and minute variations in layer thickness may dramatically alter the properties of the semiconductors manufactured therefrom. Semiconductor fabricators must therefore measure and control the thickness and uniformity of the layers with great accuracy. Many methods of measuring layer thickness have been devised using technologies such as ellipsometry, heat reflectivity, interferometry, and infrared reflectance. One commonly used measuring method is Fourier Transform Infrared Spectroscopy (FTIR). In this method, a silicon wafer acts as a shoulder of Michelson interforometer. The incident IR beam is split and one part of the radiation reflects from the wafer, while the other reflects from a moving mirror. These two parts of the radiation are then reflected to a detector where they produce an interference signal. Epitaxial layer thickness is calculated from the distance between interference maxima reflected from the substrate-layer interface and top silicon surface. This process may be repeated several times at different places on the wafer to ensure that the layer is uniformly deposited on the wafer.
In a semiconductor manufacturing process, it is typical to deposit a layer of material on a plurality or batch of silicon wafers at the same time. To conserve time, one wafer per batch may be tested using FTIR and the results of the test are assumed to apply to all the wafers in the batch. More regular testing, such as testing every fifth or tenth wafer, is also possible, but the more regular the testing, the slower the process becomes. Testing every wafer using FTIR may be too time-consuming for some manufacturing processes.
FTIR has other limitations. For example, the level of dopant concentration in the substrate must be three to four orders of magnitude greater than the level of dopant concentration of silicon in the deposited layer so that enough radiation can be reflected from the substrate-epitaxial layer interface. In addition, FTIR is effective when the thickness of the layer to be measured is no less than two microns. FTIR measurement of layer thickness and uniformity may not be practical for semiconductor designs that require either relatively equal levels of dopant concentration of silicon or the deposition of very thin layers. Furthermore, FTIR and the other technologies enumerated above require the purchase and maintenance of expensive and complex equipment.
SUMMARY OF THE INVENTION
The present invention provides a method of determining the thickness of a layer on a semiconductor substrate, the layer being deposited during a semiconductor manufacturing process. The substrate is weighed a first time before the layer has been deposited on it. The layer is then deposited on the substrate and the substrate, with the layer, is weighed a second time. The thickness of the layer is calculated using the difference between the second weighing and the first weighing. Specifically, the method includes determining the mass of the substrate prior to the deposition of the layer on the substrate; depositing the layer on the substrate; determining the combined mass of the substrate and the layer after the layer has been deposited on the substrate; determining the mass of the layer from the difference between the combined mass of the substrate and layer and the mass of the substrate; and calculating the thickness of the layer from the mass of the layer, the surface area of the substrate on which the layer is deposited, and the density of the layer.
The method of the present invention may be also be used to monitor thickness and uniformity of a layer deposited on a substrate during a semiconductor manufacturing process. This may be accomplished by: determining expected values for the thickness and uniformity of the layer deposited by a deposition process; correlating the expected values with an expected mass of the layer; defining a tolerance range about the expected mass of the layer, the tolerance range defining an acceptable variation from the expected mass; determining the mass of the substrate prior to the deposition of the layer; depositing the layer on the substrate using the deposition process; determining the combined mass of the substrate and layer after the layer has been deposited on the substrate; calculating the mass of the layer by subtracting the mass of the substrate from the combined mass of the substrate and layer; and further analyzing at least one of the thickness and the uniformity of the substrate if the calculated mass of the layer is outside the tolerance range.
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“Silicon Processing for the VLSI Era,” vol. 1: Process Technology, pp. 149-151, © 1986, Lattice Press, Sunset Beach, California.
Dietze Gerald R.
Kononchuk Oleg V.
Alston & Bird LLP
Nguyen Tuan H.
Schillinger Laura
SEH America Inc.
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