2.0 Evaluation Background and Experimental Setup

2.1 Evaluation Background

The purpose of the evaluation circuit was to stress antifuses and antifuse-metal contacts to induce failure. Ideally, a pure current source to accelerate the thermoelectric stress across the antifuse is most desirable. For simplicity, CMOS drivers as a programmable current source were used. We designed an evaluation circuit using the Pico Systems technology as the MCM substrate.

For this evaluation, three different load circuits appear on each multichip module providing 3 different antifuse loads:

a) 20 mA
b) 100 mA
c) 300 mA

These values were chosen based upon the drive capability of the CMOS driver and the number of antifuses that could be programmed in series. Six MCMs were used in the evaluation divided into 3 sets; set 1 was tested at +25 C, set 2 at +85 C, and set 3 at +125 C. The antifuse stress achieved given the load conditions previously described is detailed in the next section.

2.1.1 Antifuse Characteristics

Pico Systems Technical Bulletin No. 3 (section 5) provides a detailed a description of the characteristics of their antifuse, the various states of the antifuse states and its responses to applied power. In general the antifuse can exist in three states: off-state, on-state, and an off-on transition state. In the off-state, the antifuse consists of its original non-conducting amorphous (glass like - non-crystalline) state sandwiched between top and bottom metal electrodes. Application of the programming pulse across the metal electrodes leads to a transitional off-on state in which the amorphous silicon becomes a liquid and forms a complex metal-silicon composition. In the final on-state condition, the antifuse has become a conductive polycrystalline silicon-metal alloy with a low resistance. After programming, the process is irreversible.

There are 2 major factors stressing the antifuses in this evaluation: thermal stress and current stress. These factors are described below.

2.1.2 Antifuse Thermal Stress

The load conditions used in this evaluation will induce thermal stresses on the antifuses. Antifuse thermal stress is governed by the equation:


Tantifuse = Operating Temperature of antifuse
Ta = Ambient Temperature
T0 = Filament formation temperature above substrate temperature ~1400 K
I = Operating Current
R = Antifuse resistance ~ 1 Ohm
P0 = Power dissipated at the end of the filament formation process

Table 1 summarizes the thermal stresses experienced by the antifuses under various operating current, I. T0 is the melting point of the filament (the conductive path between the electrodes of the antifuse). P0 is the electrical power supplied to the filament in the liquid state with a programming current of 400 mA and fuse resistance of 1 Ohm. It is also the power which is conducted away from the filament and into the substrate. At the beginning of filament formation, R is large (1 G Ohm) and the electrical power across the antifuse is greater than the thermal power dissipated into the substrate. The cylindrical shaped antifuse grows in volume until these two power levels are in equilibrium. This leads to the consequence that the final on-resistance of the programmed antifuse is a function of the programming current.

Table 1 relates actual measured current levels during the evaluation to the calculated antifuse temperatures. As a matter of practice, antifuse stresses applied in the circuit should not approach the filament formation temperature. If it were possible to apply sufficient thermoelectric stress to achieve the filament formation temperature on a programmed antifuse (without destroying the metallization or breaking down the silicon dielectric), the antifuse formation process would be forced in the forward direction.

Table 1: Antifuse Current Stress vs. Temperature
Ta (C) I (mA) Tantifuse(C) % of Tantifuse
25 90 96 7
25 240 529 38
25 300 813 58
25 100 113 8
25 120 151 11
25 290 761 54
85 80 141 10
85 90 156 11
85 240 589 42
85 300 873 62
85 100 173 12
85 120 211 15
85 290 821 59
125 70 168 12
125 75 174 12

Figure 2 is a photo of the actual MCMs hereafter referred to as the Power Evaluation Circuit (PEC). The substrate is 30 mm2. It is packaged in a standard 96 pin Kovar quad flat pack. Six devices were used in the evaluation. The PEC uses 54ACT540 as current sources to drive a string of antifuses. By varying the number of antifuses programmed in series, different load conditions are applied to the antifuses under test (and interconnecting metallization and wire bonds). Figures 3a and 3b detail the routing resources and antifuses used in the circuit. Figures 4a, b and c are the schematics for each load circuit.



Figure 3a: 20 mA and Antifuse Routing


The 100 mA routing is similar to the 20 mA circuit except that 4 drivers are used in parallel. The actual number of antifuses in each circuit is shown in figures 4a, b and c. The circuits are routed such that the current load is shared at the point closest to the ACT540 drivers to minimize the maximum current level at the driver.



Figure 3b: 300 mA Antifuse Routing





Figure 4a: 20 mA Load Circuit





Figure 4b: 100 mA Load Circuit





Figure 4c: 300 mA Load Circuit




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