1500

1700+

<1000

temperature (°C)

temperature (°C)

TABLE 4.2 Important Interconnect Materials Comparison Between Traditional Aluminum Oxide and Mo/W and Glass-Ceramic/Cu

AI2O3 Glass-Ceramic and and

Property Mo/W Copper

K ceramic (dielectric constant) 10 5

Coefficient of thermal expansion (CTE) 7 x 10-6/°C 3 x 10-6/°C (Si = 3 x 10-6/°C)

Firing temperature 1600°C 1000°C

Strength 50,000 psi 30,000 psi

Metal electrical resistivity 10-6 Πcm 5.2-5.5 1.7

materials, while potentially superior to alumina, were deemed less desirable than glass-ceramic. Many had significantly higher dielectric constants. The materials that had lower dielectric constants, silica and borosilicate glass, were known to be much weaker in strength than glass-ceramics by as much as 70%. And, importantly, silica is viscous to 1500°C, making it impossible to co-fire with nonnoble metals. And, while the thermal expansion coefficient of these materials is very low, the thermal expansion coefficient of glass-ceramic can be tailored to a near match with silicon. Silica and borosilicate glass cannot. For these reasons, glass-ceramics were deemed the best choice for a new interconnect packaging material.

Within the category of glass-ceramics lie a whole host of materials. Some of those considered were cordierite (2MgO2Al2O35SiO2), beta-spodumene (Li2OAl2O34SiO2), celsian (BaO Al2O3 2SiO2), and anorthite (CaO Al2O3-2SiO2). Cordierite was chosen as the best material due to the unacceptably high thermal expansion coefficients, when sintering additives were used, of celsian and anorthite. Beta-spodumene has a very low thermal expansion coefficient, but its dielectric constant was found to be unacceptably high at approximately 9.

4.3.1.2 Cordierite Composition Optimization

Although cordierite was singled out as the best next-generation interconnect substrate material, because of its superior properties, stoichiometric cordierite by itself would not be acceptable. In order for a new interconnect substrate material to be fabricated using many of the traditional multilayer ceramic processing technologies such as green-sheet casting, punching, metal paste screening, lamination and sintering, it is required that a powder of this new material possess certain sintering characteristics. In particular, the powder of ceramic and metal must coalesce and sinter to near theoretic density. Later in this chapter, processing will be discussed in more detail. Since stoichiometric cordierite powder does not sinter to anywhere near theoretical density, sintering additives were needed. At IBM many additives were tried to improve sintering including Li2O, Na2O, B2O3, P2O5, CaO, and Fe2O3. The effects on sintering of a similar glass-ceramic, spo-dumene have been published. After making many cordierite-based compositions and studying their sintering characteristics using a dilatometer, it was determined that B2O3 and P2O5 showed the most promise. Although Li2O did significantly increase the sintered density, it also greatly decreased the sintering temperature. For reasons that will be described in more detail later in this chapter, this was not desirable. While the other additives such as Na2O and Fe2O3 were found to improve sintering, the sintered bodies still possessed significant amounts of porosity. When determining the proper sintering additives and the quantity to use, other characteristics besides density were used. It is also important to ensure that the thermal expansion coefficient be as close to Si as possible. And although a high-density is important, it is equally important that the sintering time and temperature be compatible with the metal conductor powder sintering. Since these ceramic powders will be tape cast into green sheets and eventually the green-sheet organics will need to be removed through pyrolysis, it is necessary that the sintering not occur before this process can be completed. For these reasons B2O3 and P2O5 were found to be the best candidates. In order to more fully understand the important role of these additives, compositions were prepared both with and without these additives, and the final properties of the ceramics were measured. Also, the three major components in cordierite were adjusted to achieve better sintering properties as well as a closer thermal expansion coefficient match to silicon. Stoichiometric cordierite has a thermal expansion coefficient that is much lower than Si, that is, 0.5 x 10-6 vs. 3 x 10-6/°C. By decreasing the alumina content and increasing the MgO content, the expansion coefficient increased. It was also found that by increasing the MgO content, the degree of sintering increased and more dense bodies were formed.

By adjusting the quantities of the 3 major components and by adding small quantities of B2O3 and P2O5 an optimized glass-ceramic composition was achieved.

4.3.2 Dielectric and Conductor Compatibility

Since glass-ceramic and copper powders are being fired together in the same body at the same time, it is extremely important that they are compatible. Copper also has the added requirement that it be fired in A reducing atmosphere. At the same time that the atmosphere is reducing to copper, the binder and plasticizer in the ceramic green sheet and the organics in the metal inks or pastes must be removed. If residual carbonaceous material is left behind after firing, as little as 400 ppm can significantly increase the effective dielectric constant of the substrate. In fact, as little as 800 ppm can cause the dielectric constant to increase from 5 to as much as 1000. Another challenge when co-firing glass-ceramic and copper powder simultaneously is to closely control the dimensions and integrity of the very fine lines and spaces. One very serious problem can arise if the copper powder sinters or coalesces at a much lower or higher temperature than the glass-ceramic. Most fine grained pure copper powders sinter quickly and completely by 400°C. Due to carbon removal requirements in the green sheet and metal paste, we would like the copper to sinter at temperatures as high as 800° C, which is also approaching the sintering temperature of the cordierite glass -ceramic.

These challenges can be solved by several process and material enhancements. The sintering process will be discussed in detail in the processing section. The copper chemical composition could be altered to delay the sintering to a higher temperature, but the additives must not increase the electrical resistivity of the sintered conductor. Several approaches were found to work. Copper particles could be coated with an organic barrier that prevented diffusion between the particles and thereby inhibiting the driving force for sintering. The organic barrier is removed at the desired temperature either through thermal energy or by using an atmosphere that will oxidize the organic barrier. Examples of these materials include polyvinyl alcohol, polyvinyl formate, polyvinyl butyral, acrylonitriles, epoxies, and many others.

Another approach to delaying the sintering of the copper powder until higher temperatures is to intersperse copper particles with metal or metal oxides. The particles act as grain boundary inhibitors. Some examples of these materials include chromium, molybdenum, aluminum, gold, nickel, and palladium. The resulting resistivity for copper that contained a sufficient quantity of oxide material, which sintered at a high-temperature, was measured to be less than twice the resistivity of pure copper.

4.4 LOW-TEMPERATURE MULTILAYER CERAMIC (MLC) PROCESSING

The MLC process creates multiple layers of dielectric and conductor powders prior to sintering and then uses co-firing to create a three-dimensional network of conductive wiring and dielectric. The co-fired multilayer ceramic chip carrier process lends itself to a wide array of high-density area array chip carriers. The ability of MLC chip carriers to provide high-density interconnection in each layer in addition to high-density X and Y wiring permits a wide range of chip input/output (I/O) and package I/O with various package sizes to be processed through the same tool set. Similarly, MLC lends itself to support a variety of voltages through the addition of one or more fully metallized planes. Figure 4.1 shows an overview of the process flow used for multilayer ceramics from raw materials through final test and inspection.

4.4.1 Raw Materials Preparation and Casting

The multilayer ceramic process begins with ceramic and/or glass powders with organic binders and solvents to create a ceramic slurry. The ceramic slurry is formulated to create a system capable of tape casting into thin sheets. The ceramic slurry typically is comprised of ceramic powder and glass additive or a crys-tallizable glass powder, organic binder, plasticizer, and solvents. The mixture is cast into thin ceramic green tape, which may range in thickness from approximately 0.025 to 0.750 mm depending on the tape thickness required. Figure 4.2 shows a schematic representation for a slurry being tape cast using a doctor blade. Once cast, the green or unfired tape is cut into standardized green sheets of a common X-Y size. Several sizes are currently used in production, 150 mm2 to 250 mm2 are most common although many manufacturers utilize rectangular sizes where one dimension can be 200-300 mm. Advantages of larger sizes are reduced costs; however, ability to maintain dimensional control over the entire green sheet during subsequent processes can impact yields. In either case, green sheets should have good dimensional stability and be free of defects such as voids, pin holes, impurities, contamination, and nonhomogeneities.

The thick film pastes contain metal powders dispersed in an organic vehicle plus additives to enhance shelf life and screening properties. Pastes for low-temperature co-firing ceramic (LTCC) applications generally contain silver, silver

FIGURE 4.1 Multilayer ceramic process.

FIGURE 4.2 Schematic of tape casting.

palladium, or copper powder plus organic resins and solvents. The paste constituents are weighted, mixed with a large blender, and then processed through a three-roll mill whose shearing action disperses the powder particles into a thick film paste with suitable rheological properties for screen printing X-Y patterns and filling via's in green sheets for Z connections. The paste must be compatible when co-fired with ceramic to provide the proper onset, rate, and volume shrinkage compared to the ceramic. Sintered thick-film metallizations must provide adequate surface pad adhesion characteristics to ensure mechanical integrity for package interconnection to die and board I/O pads.

4.4.2 Green Sheet Punching, Metallization, and Inspection

In the next step a series of vertical holes, termed vias, are punched into the green sheet prior to metallization with the thick film paste. A mechanical punch is normally used to create the vias with diameters of 0.05-0.25 mm (see Fig. 4.3). For via diameters of 0.075 mm or smaller, laser via formation, E-beam formation, or photo-defined vias can be utilized. Aspect ratios of green sheet thickness to via diameter of 3:1 to 1 ■ 5:1 are commonly used. Mechanical punching can provide low cost and flexibility of pattern formation, but the process must be optimized for very high via counts and densities (>80, 000 vias per layer for 150 x 150 mm green sheet size) to minimize mechanical stresses that lead to via location error due to green sheet distortion imposed by mechanical punching. Decreasing green sheet thickness and increasing punch diameter to die bushing clearance tends to reduce via positional error, because thinner sheets have less mechanically imposed stress when forming vias for a given via diameter. Green sheets with over 78,000 vias (0.01 mm via diameter) and an active area of 150 x 150 mm have been reported.

As shown in Fig. 4.4, the thick film via fill and metal printing of surface patterns ties together the punched ceramic green sheet and thick film paste in the powder (presintering) process often referred to as screen printing or green sheet personalization. In this process either vias and X-Y metallizations are achieved sequentially with paste drying after each operation or vias and X-Y pattern can be created simultaneously by extrusion printing the conductor paste. A variety of mask patterns are employed to create the circuitry for MLC chip carriers,

- Greensheet

Die Bushing

FIGURE 4.3 Schematic of punching.

including top surface through vias, X and Y signal patterns, voltage, power, and ground planes and bottom surface I/O pads. In screening thick film paste, a patterned mesh or metal mask is utilized for via hole filling and for surface pattern definition. Line widths typically ranging from 0.050 mm to over 0.250 mm can be screen printed. Solid or mesh power planes can also be screen printed as illustrated in Fig. 4.5.

Green sheet inspection is routinely conducted to ensure via fill and patterns meet specifications for complex substrates. This operation employs optical comparisons to ensure line connectivity, absence of shorts, and via fill. Inspection is most commonly used for complex packages with high layer counts, greater than 10-15 layers, or for fine line patterns and line widths of less than 70 ^m. Figure 4.6 shows a number of metallized green sheets. Inspection can also be helpful when introducing new part numbers into manufacturing to aide in yield learning. Inspections can be skipped in many volume applications of products, and is required only for reasonable yield for complex substrates.

Power i

FIGURE 4.5 Mesh and solid power planes.

Power i

FIGURE 4.5 Mesh and solid power planes.

Signal Wiring: 70 micron line width Signal Wiring: 35 micron Jine width

FIGURE 4.6 Patterned green sheets.

Signal Wiring: 70 micron line width Signal Wiring: 35 micron Jine width

FIGURE 4.6 Patterned green sheets.

4.4.3 Stacking, Lamination, and Green Sizing

A variety of green sheets sequentially stacked and laminated using pressure and temperature are required to create a laminate consisting of a three-dimensional network of conductor and dielectric materials. The ceramic powders are held together by an organic binder previously used in green sheet fabrication and conductive paste fabrication or by an added adhesive used between layers during stacking.

Alignment of each layer is critical during stacking to properly align the many Z wiring connections to create via stacks, interconnect lines between layers using vias, and avoid shorts or opens between layers. Alignment can be achieved by using registration holes in green sheets previously used for punching, screening, and inspection, by locating the holes over stacking pins. Alternatively, each layer can be optically aligned and stacked. A green laminate typically consisting of multiple chip carriers is singulated into individual carriers by green saw sizing prior to sintering. Similarly, packages are green machined to obtain corner chamfers or rounded edges.

4.4.4 Sintering

During sintering, the organic components used in "green" processing MLC laminates are removed through decomposition or pyrolysis during the initial stages of heating and up to temperatures below about 600° C. Choice of binders and plasti-cizers and the development of the proper sintering profile are critical in order to provide a minimum of residuals from this process, which can be removed subsequently in the sintering process. In the pyrolysis section of the sintering process one must avoid rapid rise in temperature, which leads to the highest decomposition of the organics so as to not delaminate or cause other defects in the multilayer component structure. Next, residues containing carbon are removed by oxidation. For LTCC, carriers containing silver or silver-palladium conductors, carbon oxidation can be performed in an oxygen atmosphere such as in air. For copper metallization, a controlled atmosphere is utilized to oxidize the carbon without oxidizing copper, using temperatures between 600-800°C. Here the rate of reaction must be adequate to complete the removal of carbon within a reasonable time period (to a low level so as to not impact the resistance of the dielectric or impede the controlled sintering of the metal or dielectric materials) but must also avoid oxidation of the metal powders in the structure. Once the organics are removed, the dielectric and conductor particles undergo densification to create an insulating dielectric chip carrier with conductive metal wiring. Densification for LTCC packages occurs below about 1000°C. Compatible shrinkage of both conductor and dielectric must be developed to ensure good dimensional control and mechanical and electrical integrity. It is important to control the onset, rate, and total volume shrinkage of the various materials in the multilayer chip carrier. Factors that can influence dimensional stability include all operations from raw materials, through "green" processing operations, up to and including sintering time, temperature, atmosphere, and sinter fixturing. The key to repeatability producing high-quality ceramic chip carriers lies in the manufacturing process, which must be controlled to consistently operate within the materials and process specifications. These controls are in addition to an engineered compatible conductor and dielectric system optimally matched for shrinkage onset, rate, and volume. Figure 4.7 shows a schematic of typical process steps for LTCC sintering.

Green Laminate & Furnace

Loading

Organic Binders Decomposition & Pyrolysis

Final Densification and Crystallization

Cooldown & Furnace Unloading

Ceramic and Metal Densification

Final Densification and Crystallization

Cooldown & Furnace Unloading

Measurement & Inspection

FIGURE 4.7 Schematic of typical LTCC sintering process.

4.4.5 Postsinter Finish Metallization

Subsequent to sintering, ceramic chip carriers can undergo precision machining to provide surfaces sufficiently flat and smooth to accommodate package applications utilizing one or more layers of advanced thin-film wiring or where an advanced sealing process requires precision tolerances such as a flange or module encapsulation. Lapping and polish techniques have been developed that can provide a surface with nominal flatness of about 1 micron per inch and a surface finish of 300 A average roughness (Rav). Thin-film materials, process, and attributes have been described elsewhere [56].

The LTCC chip carriers that do not require thin-film processing are finished with nickel and gold plating on the surface features to enhance solder and braze wetting characteristics and provide a barrier to other metallizations used in MLC fabrication. Finish metallizations can be electroplated or electroless plated. A relatively thick nickel layer (0.0005-0.004 mm) is utilized to support rework capability (i.e., multiple solder reflows). Gold thickness is dependent on application and type of interconnection. For example, thin gold (^ 0.001 mm) is used on flip-chip die attach pads whereas for wirebond and pin grid array pads, heavy gold (0.001-0.002 mm) is required. For some LTCC carriers, alternate finishing metallizations can be used, and in some cases none are required depending on surface co-fire metals used during MLC processing. For example, silver-palladium based alloys are Pb-Sn solder wettable, and for copper or copper-nickel alloys, which have been co-fired, a gold finish is often the only postfire metal deposition required.

4.4.6 Test and Inspection

Electrical test can include direct electrical shorts and opens tests. Tests can also include a functional test to support high frequency, assess simultaneous switching noise, line resistance, propagation delay, and others. Chip carriers can also be tested for defects and can be repaired with a variety of techniques reported elsewhere [21]. Chip carriers that meet electrical and customer specifications are finally inspected for surface defects prior to shipping or subsequent module processing operations.

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