Polyimide passivation

Imide passivated linear devices was determined from I-V characteristics of statistically significant numbers of devices following severe PTHB test (i.e., 15 psi, 120°C, 100% relative hvimidity, and 30 V bias). Two coat (3 p) polyimide passivation provided almost twice the mean time to failure of 1 p thick PSG passivation. Polyimide protection against high humidity (13,14) and Na" " diffusion (15) has been reported previously. 

The polyimide passivation,by device design should be compatible with a variety of interfaces,in the present case,the Ni/Fe active elements,A1 conductorsjSiC interlevel insulation and the terminal metallurgy using Cu and Cr. 

The devices passivated by the polyimide process were compared for performance with similar devices using SiC passivations. In all the cases,it was found that an 0.8p cured film of polyimide derived from PMDA/0DA performed equally well as the SiO passivated devices. Bias margins (indicative of magnetic device performance) were within the specified limits. The polyimide passivated devices were compatible with standard terminal metallurgies and also chip mounting metallurgies. Device performance... 

Polyimides, both photodefinable and nonphotodefinable, are coming iato iacreased use. AppHcatioas iaclude planarizing iatedayer dielectrics oa iategrated circuits and for interconnects, passivation layers, thermal and mechanical stress buffers ia packagiag, alpha particle barriers oa memory devices, and ion implantation (qv) and dry etching masks. 

Since the end of the 1970s, the polyimides have been introduced for the production of electronic components mainly for the passivation. But more and more they are interesting for the integrated circuits and multichip modulus fabrications. Processability and dielectric and thermomechanical properties are the most attractive features of these materials for the electronic31 and electro-optical applications.32... 

The present work is a report of the properties of polyimide which define functionality as an interlevel dielectric/passivant. Thus, the planarizing and patterning characteristics and electrical characteristics of current vs voltage, dissipation, breakdown field strength, dielectric constant, charge and crossover isolation are discussed in addition to the reliability-related passivation properties. 

Finally, the passivation properties of polyimide are superior to phosphoslllcate glass under conditions of severe PTHB testing. 

Conditions have been defined for applying polyimide coatings onto the silicon wafer as passivation and/or dielectric. Processing variables studied included the critical areas of adhesion, cure cycle and thermostability. Aminosilane was shown to be effective adhesion promoter. The rate of imidization was followed by F.T.I.R. employing time lapse technique. 

The increasing importance of multilevel interconnection systems and surface passivation in integrated circuit fabrication has stimulated interest in polyimide films for application in silicon device processing both as multilevel insulators and overcoat layers. The ability of polyimide films to planarize stepped device geometries, as well as their thermal and chemical inertness have been previously reported, as have various physical and electrical parameters related to circuit stability and reliability in use (1, 3). This paper focuses on three aspects of the electrical conductivity of polyimide (PI) films prepared from Hitachi and DuPont resins, indicating implications of each conductivity component for device reliability. The three forms of polyimide conductivity considered here are bulk electronic ionic, associated with intentional sodium contamination and surface or interface conductance. 

The particles continue to fly into the sampling chamber through an orifice between the reaction and the sampling chamber. The pressure of the sampling chamber is 9.5 X 10-4 torr. The particles are collected in a form that is convenient for characterization or application. For example, the particles are collected on a microgrid for transmission electron microscope (TEM) observation and on a polyimid-film for MOssbauer and x-ray diffraction studies. A standard passivation treatment, namely, slow introduction of O2 gas followed by the introduction of dry air to the chamber, is made. 

Fig. 10.23 shows a cross-sectional view of a typical circuit for a scanner, and shows the p-i-n sensor and the pass transistor. This particular circuit takes nine mask levels. The metals used are chromium and aluminum, the former for contacts to the TFTs and sensors and the latter for the interconnecting lines. The transparent conducting contact to the sensor is made with ITO and polyimide is used for passivation and isolation of the devices. 

Polyimides are Hnding increased use in microelectronics [1-6]. There are four primary areas of application 1) as fabrication aids 2) as passivants and interlevel insulators 3) as adhesives and 4) as components of the substrate or circuit board. In each plication, the requirements on properties and performance differ. This paper addresses primarily the first two applications. 

Polyimides for microelectronics use are of two basic types. The most commonly used commercial materials (for example, from Dupont and Hitachi) are condensation polyimides, formed from imidization of a spin-cast film of soluble polyamic acid precursor to create an intractable solid film. Fully imidized thermoplastic polyimides are also available for use as adhesives (for example, the LARC-TPI material), and when thermally or photo-crosslink able, also as passivants and interlevel insulators, and as matrix resins for fiber-reinforced-composites, such as in circuit boards. Flexible circuits are made from Kapton polyimide film laminated with copper. The diversity of materials is very large readers seeking additional information are referred to the cited review articles [1-3,6] and to the proceedings of the two International Conferences on Polyimides [4,5]. 

We now examine how applications and properties interact, by examining the uses of polyimides as fabrication aids and as passivants and interlevel insulators. 

In the case of a photoresist, the ultimate definable feature size together with the ability of the material to withstand either chemical etchants or plasma environments determines the domain of utility. The feature size is in turn determined by the wavelength required for exposure, the sensitivity and contrast of the resist, and the dimensional stability of the material during exposure, development, and subsequent processing. Adhesion of the resist to the substrate is critical both for patterning and use, and adhesion can be affected by surface preparations, and by residual stresses developed during deposition and cure. While photo-imagable polyimides have been introduced, their principal intended application is as a component of the finished part, either as passivant or interlevel dielectric (see below). 

In order for a polyimide to be useful as an interlevel dielectric or protective overcoat (passivant), additional demanding property requirements must be met In the case of the passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. It has been demonstrated that polyimide filrns can be excellent bulk barriers to contaminant ion motion (such as sodium) [10], but polyimides do absorb moisture [11,12], and if the absorbed moisture affects adhesion to the substrate, then reliability problems can result at sites where adhesion fails. However, in the absence of adhesion failure, the bulk electrical resistance of the polyimide at ordinary device operating temperatures and voltages appears to be high enough to prevent electrochemical corrosion [13]. 

Although the surface of most IC chips has been passivated with a layer of inorganic dielectric material such as silicon dioxide or silicon nitride (polyimides have also been used as final passivating layers), the protection provided by such layers is not sufficient to ensure reliable operation throughout the lifetime of the device. The three basic methods of protection are... 

Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant, and high electrical resistivity. Polyimides have been used as passivation coatings, (1) interlayer dielectrics, (2) die attach adhesives, (3) flexible circuitry substrates, (4) and more recently as the interlevel dielectric in high speed IC interconnections. (5) High speed applications require materials with a combination of low dielectric constant, flat dielectric response versus frequency and low water absorption. 

Some of the problems seen with the commercially available polyimides such as limited shelf life,gelation and high ionic contamination are traceable to the raw materials themselves. A zone refining technique has been perfected for use with organic materials and these precursors have been used to synthesize ultrapure polyamic acids for IC device applications. The key feature of the synthesis is the use of solid ingots of the dianhydrides. Materials prepared by this technique show low metallic impurities and have been shown to be excellent film formers for a variety of applications. In particular a polyimide derived from PMDA-ODA has been used to passivate magnetic bubble devices. IR techniques coupled with electrical measurements have been used to optimize the cure conditions and a simple resist process has been defined to passivate these devices. Device performance compares well with conventional inorganic insulators. 

Initial experiments showed that the PMDA/ODA material can be successfully cured by a ot plate bake at 280° C for 3 hours. Electrical.Loss tangent data showed that imidization was complete under these conditions. As noted before,the passivation process for bubble devices had to be optimized by using a lower temperature ( to preserve the magnetic properties of the permalloy structures).without losing the electrical insulation characteristics of the final polyimide. IR technique was used as the method of detecting complete cure. A Perkin Elmer 283 spectrometer was used for the IR analyses routinely,and in a few cases the results were checked with a Digilab FTIR instrument.Routine resolutions were 2cm with the Perkin Elmer instrument. Jh s g-dies of imide formation by IR is a well known technique ... 

Specifically for the passivation of temperature sensitive bubble memory devices,these ultrapure materials proved to be of great value. A cure process was optimized to obtain a reliable low temperature cure without affecting the magnetic coercivities of the bubble memory devices. A positive resist process, using a simple development step to pattern via holes in devices has been optimized and successfully used to fabricate devices. The devices fabricated using the the polyimide process have been compared with conventional SiC offers reliable passivations with thinner stress free films for passivations. The fabrications involve simple inexpensive process steps and are compatible with conventional resist processes. The reliability of the imide passivated devices can be considerably enhanced by the use of ultrapure starting materials to preclude harmful ionic mobilities through passivated layers. 

Thermal tempering of the photosensitive or cross-linked polymer gives the polyimide siloxane which has been previously shown to be an excellent candidate as an insulating polymer in electronics. The use of such a directly patternable polyimide for dielectric and passivation applications, particularly in microelectronics, should become increasingly important as polyimides become more widely accepted in the industry. 

The AO reaction efficiency of the Kapton H reference sample was used to calculate the Kapton equivalent fluence and erosion yields of each sample exposure. For various exposures, the step heights (or etch depths) of POSS-PI films were plotted as a function of the step height of the Kapton H film [18, 26]. The derivative functions indicated that the 3.5 and 7.0 wt % SigOn POSS polyimide films reached erosions rates of 3.7 and 0.98%, respectively, of the erosion rate for Kapton H after 395,000 beam pulses (8.47 x 1020 atoms cm-2) [9, 10]. 8.75 wt% Si80 MC-POSS-PI samples had an erosion rate that was 0.3 percent of the erosion rate for Kapton H , and 1/3 of 7.0 wt % POSS-PI at a fluence of 8.5xl020 atoms cm 2. These results support the formation of a passivating silica layer that is a result of the nano-dispersed POSS moieties reacting with AO. 

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