Книги+1 / 2013 [Chandan_Kumar_Sarkar]_Technology_CAD

consists of the final silica glass films. BPSG can be fabricated by several methods like CVD (chemical vapor deposition), sol-gel, and FHD (flame hydrolysis deposition). Usually the CVD procedure is used to form BPSG films [3, 21, 23]. BPSG provides void free fill of 0.2 to 0.8 μm wide spaces between succeeding higher metals or conducting layers. BPSG basically works as an insulating layer for inter-metal layers.

deposition oxide thickness=0.7 concentr

8.22  BPSG Anneal

Deposited borophosphosilicate glass needs annealing, as shown in Figure 8.14, which will be performed by the following steps:

DIFFUSION TEMPERAT=800 TIME=20 F.N2=10.0

DIFFUSION TEMPERAT=800 TIME=15 F.O2=9.5

The BPSG layer deposited on it is not smooth due to uneven device structure. It needs chemical-mechanical polishing (CMP) [3, 24, 25]. A nitride layer of thickness 0.15 µm is deposited on it to determine the minimum y coordinate and

FIGURE 8.14 (See color insert)

Structure of the device after application of annealing step for BPSG.

FIGURE 8.15 (See color insert)

Polishing of the device top surface by removal of excess oxide.

is named CMP1. During polishing, the oxide above that minimum y coordinate is etched out to make the surface smooth. Thus four points are defined using the minimum y coordinate for two points, and the uneven portion is removed by etch operations. After the removal of this oxide a smooth surface can be achieved, and this can be seen from Figure 8.15. Statements below will perform this CMP task.

DEPOSITION NITRIDE THICKNES=0.15 CONCENTR

extract nitride/oxide y.extract minimum name=CMP1 etch nitride all

ETCH OXIDE START X=0.0 Y=-14.6 ETCH CONTINUE X=5 Y=-14.6

ETCH CONTINUE X=5.0 Y=@CMP1 ETCH DONE X=0.0 Y=@CMP1

8.23  Contact Mask Formation

For metal contacts of different terminals like bulk, source, gate, and drain, metallization is required. Aluminum is used most of the time for metallization in integrated circuits, because aluminum and its alloys have low

FIGURE 8.16 (See color insert)

Selective etching has been performed to deposit aluminum through it for the first layer of metal contacts.

resistivity (2.7 μΩ for aluminum). Aluminum adheres well to silicon dioxide, though use of aluminum in shallow junctions may create problems like spiking and electromigration. A mask named contact has been assigned in the mask file having three lengths assigned to it: one to create source-bulk contact, one for gate, and one for drain contact. Three lengths assigned to the mask contacts are 300 to 1100, 2550 to 2850, and 4400 to 4850, or effectively 0.3 to 1.1 μm, 2.55 to 2.85, and 4.4 to 4.85 μm. The first mask is chosen in such a way that both bulk and source region contact formation are possible, as for an n-MOSFET bulk and source normally remain in the same potential most of the time. The second region from 2.5 to 2.85 μm is chosen for gate contact, and the last region from 4.4 to 4.85 μm is for drain contact. Figure 8.16 shows the structure where selective etching has been performed, after which aluminum is deposited through those regions for the contacts.

DEPOSIT PHOTORESIST NEGATIVE THICKNESS=1 EXPOSE MASK=contact

DEVELOP etch oxide

etch PHOTORESIST

8.24  First Layer of Metal (metal-1) Deposition

The statement below will deposit aluminum of thickness 0.3 µm on the entire material.

deposition aluminum thickness=0.3 concentr

8.25  Metal-1 Mask

As the metal will be deposited on the entire wafer, the undesired part of the metal must be removed from the entire material. So another mask is assigned in the mask file and called here. The mask name is metal1, which has three regions assigned in it. The regions are 150 to 1200, 2400 to 3000, and 4300 to 4950, meaning 0.15 to 1.2 μm, 2.4 to 3.0 μm, and 4.3 to 4.95 μm. Only on these regions will the aluminum remain on the wafer, as the contact material and rest of the aluminum will be removed by an etch statement. Figure 8.17 shows the final structure formed after deposition on metal.

DEPOSIT PHOTORESIST POSITIVE THICKNESS=1

EXPOSE MASK=metal1

DEVELOP

FIGURE 8.17 (See color insert)

First layer of metal deposition through the opening of BPSG for the direct contacts from the device.

etch aluminum etch PHOTORESIST

8.26  Inter-Metal Dielectric (IMD) Deposition

For final outer world connection to the device another layer of metallization is required [3]. Another layer of oxide which is called inter-metal dielectric (IMD) is deposited here by the statement below of thickness 0.5 μm which is shown in Figure 8.18. From the figure it is obvious that the top surface requires chemical-mechanical polishing again.

deposition oxide thickness=0.5 concentr

To polish the top surface, a layer of nitride of thickness 0.15 µm is deposited on it to determine minimum y coordinates at which nitride and oxide meet. Until that point, oxide is etched to get a smooth oxide on the top. Figure 8.19 shows the structure in which the top surface is smoothened by CMP operation.

DEPOSITION NITRIDE THICKNES=0.15 CONCENTR

extract nitride/oxide y.extract minimum name=CMP2 etch nitride all

FIGURE 8.18 (See color insert)

Deposition of IMD after formation of the first layer of metal contacts.

FIGURE 8.19 (See color insert)

Structure achieved by smoothening the top surface by the polishing operation again.

ETCH OXIDE START X=0.0 Y=-15.2

ETCH CONTINUE X=5 Y=-15.2

ETCH CONTINUE X=5.0 Y=@CMP2

ETCH DONE X=0.0 Y=@CMP2

8.27  Second Layer of Metal (metal-2) Mask

For the final layer of metallization, another mask is required and through its opening another layer of metal is deposited on it. The mask assigned here is named metal-2 and three regions assigned here are 200 to 1100, 2000 to 3100, and 4200 to 4900 to connect this layer to the previous layer of metal. Figure 8.20 shows the opening regions through which the next layer of metal will be deposited.

DEPOSIT PHOTORESIST NEGATIVE THICKNESS=1 EXPOSE MASK=metal2

DEVELOP etch oxide

etch PHOTORESIST

FIGURE 8.20 (See color insert)

Opening region has been created by etching through which the second layer of metal will be deposited.

8.28  Second Layer of Metal (metal-2) Deposition

Now the aluminum of thickness 0.2 μm will be deposited on it, as shown in Figure 8.21 for the next layer of metallization.

deposition aluminum thickness=0.2 concentr etch PHOTORESIST

8.29  Metal-2 Final Mask

Figure 8.22 shows a layer of aluminum is deposited on the top, in which a part of metal is essential and the remaining part has to be removed from the top portion. The following statements will remove the undesired metal from it, lengths assigned in the mask file named metal3 are 200 to 1100, 2200 to 3100, and 4200 to 4900, or effectively 0.2 to 1.1 μm, 2.2 to 3.1 μm, and 4.2 to 4.9 μm. Figure 8.23 shows the final device structure.

DEPOSIT PHOTORESIST positive THICKNESS=1 EXPOSE MASK=metal3

FIGURE 8.21 (See color insert)

Second layer of metal is being deposited and connected to the first layer of the metal through the openings.

FIGURE 8.22 (See color insert)

Final device structure, y coordinate is taken up to –12.0 µm.

FIGURE 8.23 (See color insert)

Final simulated structure of the 5 µm MOSFET.

DEVELOP

etch aluminum etch PHOTORESIST

SELECT Z=LOG10(BORON)

FOREACH X (14 TO 21 STEP 1)

COLOR MIN.V=X MAX.V=(X + 1) COLOR=(X - 1) END

SELECT Z=LOG10(phosphor)

FOREACH X (14 TO 21 STEP 1)

COLOR MIN.V=X MAX.V=(X + 1) COLOR=(X - 3) END

SELECT Z=LOG10(arsenic)

FOREACH X (19 TO 21 STEP 1)

COLOR MIN.V=X MAX.V=(X + 1) COLOR=(X - 5) END

SELECT Z=LOG10(antimony)