Semiconductor device manufacturing: process – With measuring or testing
Reexamination Certificate
2000-06-26
2003-07-15
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
With measuring or testing
C438S783000
Reexamination Certificate
active
06593151
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to the field of semiconductor fabrication. More particularly, the invention pertains to the detection and monitoring of striations in multi-layer film stacks.
BACKGROUND OF THE INVENTION
Hundreds of processing steps, known to those skilled in the art, are typically required to fabricate integrated circuits on semiconductor substrates. The integrated circuits are created from multiple layers of various materials, semiconductors, oxides and metals, which are deposited onto thin disks of purified crystalline semiconductor substrate material, typically silicon, germanium, gallium arsenide or other materials known to those skilled in the art. These multiple layer film stacks are called wafers. After, processing, wafers are divided into separate chips.
The oxide or dielectric layers are used as insulation between conducting layers, as well as a source of dopants for diffusion, getters for impurities and passivation to protect devices against impurities, moisture and physical damage. Dielectric materials typically are used as multi-layered film stacks containing films varying in structure or composition. Commonly used dielectric materials include phosphorous-doped silicon dioxide or phosphorous silicate glass (“PSG”) and boron phosphorus silicate glass (“BPSG”).
One type of defect that is especially prevalent in phosphorous-doped oxide layers in multi-layer film stacks is a striation. Striations are localized areas with high concentrations of phosphorus which can not effectively be removed during further processing and are fatal to the product wafers. Striations are formed during deposition of the oxide layer as a result of processing problems, such as when one or more liquid phosphorus injector(s) on the production line become clogged or when condensation and vaporization occur due to cold spots in the line during chemical vapor deposition.
During processing of product wafers, oxides that are used as insulators between conducting layers are deposited and then etched to open windows for electrical connections, and oxides that are used as passivation for devices are etched to open areas for bonding. A fluoride containing solution or plasma, such as a hydrofluoric acid solution or a CHF
3
plasma, is used to etch the oxide. However, the rate of etching is phosphorous-dependent such that areas with higher phosphorous concentrations etch more rapidly than areas with lower phosphorous concentrations. An example of the deleterious effects of striations follows. A window is cut into the product wafer so that all of the layers are exposed, after which the fluoride containing etch is performed. The striations are etched at a higher rate than other areas of the product wafer, leaving a void or crack in the oxide layer. When metal is later added to the product wafer during further processing, the metal fills the hole created by the striation, negating the insulating properties of the oxide layer and breaking the current running through the product wafer, thereby effectively destroying the product wafer. Accordingly, product wafers containing striations can be detected during electrical testing of finished wafers and such product wafers are scrapped.
In large-scale production of integrated circuit devices, control wafers are used to detect defects which occur during the manufacturing process. Control wafers are generally used to check for film composition, film structure and film contaminants. Control wafers are processed separately from product wafers, typically prior to running product wafers and at regular intervals during the fabrication of product wafers. The control wafers are destructively evaluated to detect defects in the film layer stacks prior to additional production of product wafers. Using specialized control wafers as test wafers facilitates the early detection of processing problems and prevents the need to scrap entire production lines of product wafers.
Striations can only be detected in the layered oxide film stack by using destructive analytical techniques. Even if processing is interrupted to analyze the freshly deposited oxide film for striations, there is no technique available that will allow the detection of localized concentrations of phosphorous throughout the film thickness without sectioning the wafer or “burning” a hole into the wafer. For example, x-ray fluorescence spectrometry (XRF), a metrology tool which is utilized on the production line, will only detect the phosphorus levels vertically down through the entire wafer by averaging the amount of phosphorus it detects throughout the total thickness of the wafer. XRF is, therefore, incapable of reliably detecting localized areas of phosphorus. Since the localized concentrations of phosphorous may only be detected using destructive techniques, control wafers are used for the detection of striations rather than destroying product wafers.
Scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS) are alternate methods for the detection of striations in control wafers which each require the wafer to be taken off the production line for testing. For SEM analysis, the control wafer is cross-sectioned and then etched with a fluoride containing etch. Areas with higher concentrations of phosphorous etch faster than other areas, creating voids or cracks so that the striations appear as dark areas in the SEM. SIMS functions by sending an oxygen beam through the thickness of the control wafer to dislodge atoms from the material by collision with the oxygen beam, and a detector identifies what material is at each depth of the control wafer. SIMS produces a profile of the phosphorous concentration throughout the thickness of the control wafer.
Typically, control wafers are made using the same equipment and processes that are used for making the product wafers in order to simulate the product wafers. The current method of detecting striations includes a standard production recipe for a control wafer. The standard configuration for a control wafer (
5
) is depicted in FIG.
1
. There are four layers: a substrate layer (
2
), usually made of silicon, germanium, gallium arsenide or other materials known to those skilled in the art, and three additional layers that mimic the product: an undoped silicate glass (USG) layer (
4
), a PSG layer (
6
), and a BPSG layer (
8
). Layers (
4
), (
6
), and (
8
) of the control wafer (
5
) are generally found as intermediate layers in product wafers.
Preferably, the total thickness of the control wafer (
5
) is approximately the same as a finished product wafer, in order to make the control wafer (
5
) more representative of the product wafer. In the control wafer (
5
) known in the prior art, the thickness of the three silicate glass layers follows a standard recipe. The substrate layer (
2
), which may vary in thickness, is not discussed here. Typically, the USG layer (
4
) in
FIG. 1
has a thickness of about 1000 Å, the PSG layer (
6
) has a thickness of about 1500 Å, and the uppermost layer, BPSG (
8
) varies depending on the production line and the thickness of the layers on the particular product wafers, but typically has a thickness of about 6000 Å. The thickness of the PSG layer (
6
) on the control wafer (
5
) was chosen to mimic the thickness of the PSG layer on the product wafer which typically has a thickness of 1500 Å for maximum ability to getter or trap sodium impurities.
In the control wafer (
5
), the substrate layer (
2
), the USG layer (
4
) and the BPSG layer (
8
) are non-essential, that is, they have no function in the detection of striations in the control wafers. The PSG layer (
6
) is a phosphorous-doped oxide layer in the control wafer (
5
). The striations are observed in the BPSG layer. When there is a problem with the hardware on the production line during chemical vapor deposition of the PSG layer (
6
), striations may be produced in the PSG layer (
6
) and the BPSG layer (
8
) of both the control wafer (
5
) and product wafers.
The standard control wafer (
5
) configuration,
Agere Systems Inc.
Christie Parker & Hale LLP
Le Thao P
Nelms David
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