~ Stringers ~
Stringers are unwanted residual material remaining in crevices or along edges of previous layers. Stringers can be any material. Although stringers are primarily a yield issue, they can be a reliability issue (Liston et al). |
Example #1: Residual Material In Crevices |
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| A two layer metal process with tungsten plug interconnects involves deposition of a blanket layer of tungsten. Subsequently, the tungsten is etched back removing all tungsten except that within the interconnection via openings. Metal 2 to metal 2 shorts were found in a test structure of metal 2 lines crossing over metal 1 lines. | ||
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| 1. Curve tracer testing confirmed
resistive shorts between adjacent metal 2 lines. 2. Cross sections perpendicular to metal 1 and parallel to metal 2 lines were made in the faulty test structure. Figure (1) is a field emission SEM image of the cross section. Tungsten material is bright compared to aluminum. Some tungsten can be seen at the bottom of metal 2 in crevices located between metal 1 lines. The tungsten within the metal 2 lines is not a problem. However, the metal 2 etching process is not effective on tungsten, so tungsten material will remain between adjacent metal 2 lines, providing an unwanted connection. |
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| Figure (1). This cross section shows three strips of metal 1. The two outside metal 1 strips are connected to a metal 2 line passing over at a 90º angle. The interconnection is through a tungsten filled via. (bright) Note that tungsten remains in crevices at the bottom of metal 2. Since tungsten is not etched by aluminum etch, tungsten "stringers" will connect this metal 2 strip to adjacent strips behind or in front of the cross section plane. | ||
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| Due to misprocessing, an overhang of oxide was left at the edges of first layer polysilicon strips. Subsequently, a second layer of polysilicon was deposited conformally over and under the oxide structures. When the second layer of polysilicon was patterned, material under the oxide escaped removal. The result is an electrical connection between adjacent lines of second layer polysilicon. | ||
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| 1. Test structures designed to
detect stringers measured electrically short. In this case, the test structure
included interdigitated combs of polysilicon 2 crossing first layer polysilicon at right
angles. The interdigitated combs, normally isolated from each other, were
electrically shorted. 2. A 5° angle lap of a failed test structure (Fig. 2) shows residual poly 2 along the edges of poly 1. (Poly 1 lines are vertical; poly 2 lines are horizontal.) Stringers of poly 2 appear as white lines along the edge of poly 1. 3. A cross section (Fig. 3) shows a severe geometry of poly 2 along edges of poly 1. In this process, poly 1 was wet etched, undercutting the oxide over poly 1. The oxide should have been removed by an HF dip before poly 2 deposition. |
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| Figure (2). Angle lap of failed test
structure shows "stringers" of second layer polysilicon along edges of first
layer polysilicon. (vertical)
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| Figure (3). Ninety degree cross section shows severe geometry resulting in "stringer" failure. The overhanging oxide step is an anomally. | ||
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| A one micron CMOS process uses the first polysilicon layer for transistor gates and a second layer of polysilicon for capacitors and as resistor material. Design rules do not allow poly 2 to overlap poly1, thus avoiding the possibility of stringers causing resistive shorts shown in Example #1. However, in this example, floating stringers caused abnormally high Leff. Normally, Leff tracks poly 1 critical dimension (CD). | ||
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| A cross section (Fig. 4) was made
through a failing transistor. The section was etched with Sirtl etch. The
cross section confirms the existence of poly 2 material between gate poly and the normal
phosphorus doped spacer. The triangular shaped poly 2 material blocks source/drain implants and results in longer distance between source and drain diffusions. The conductive gate material does not completely link source and drain. This gap causes an offset in Id versus Vds characteristic because the channel created by the gate does not connect source and drain. Lower Ids , higher Leff, and higher on resistance are all explained by the observed stringers. |
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| FIgure (4). Ninety degree cross section shows poly 2 material between gate and spacer. Source and drain diffusions are separated by an extra distance due to extra material. This distance defines actual gate length. (Original magnification at 20Kx. Gate length is 1.0 micron) | ||
| Works Cited |
| P Liston, K Erington, D Dreier, J Hamilton,
"Characterization of Bake Recoverable Lifetest Failures Caused by a VT Shift in a
Parasitic Thin Film Transistor", Proceedings of the 19th International Symposium for
Testing and Failure Analysis, 1993, p. 11-16.
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