In this lesson

Maps show you where on a wafer dies fail. Charts show you which wafers, bins and tests are responsible. This lesson walks through the charts that answer "where is the yield going?"

• Find which wafers, bins and tests lose yield
• Use the buttons and menus every chart shares
• Compare groups of wafers, such as process splits

From maps to charts

The Maps | Insights switch 1 swaps the wafer view for charts of the same dies. Overview 2 is the first tab. Its tiles state the population, so you know which dies the charts describe.

The Overview

Read it from the top. Worth a look comes first when something stands out, then What divides yield, then the standing cards: yield, bins, pass rates and regions. Scroll down: every card is a chart you can click.

Cards about groups of wafers

A card about groups of wafers has no Show on map, because it has no place on the map. It is a bar of yield for each group, such as each split. When Insights is already grouped by that field, the card is left out, because the yield card shows the same bars.

Reading the ring grid

Look for the shape of the dark cells.

• A dark row across every column is a process pattern: that ring loses yield on every wafer
• A dark cell in one column is one wafer
• Click a cell for the dies in that ring on that wafer

Reading yield by site

• With three or more sites, the median is drawn as a line to compare each site against
• A site that is low on every wafer points at that site's probe card, not at the wafers

The ⋯ menu

Every chart has the same ⋯ menu, so once you know it you know them all. Save as PNG saves the picture. Save data as CSV saves the numbers behind it, and Copy data puts them on the clipboard as tab-separated text to paste into a spreadsheet. Edit as plot opens the chart as a plot of your own (lesson 10), and About this chart adds a line saying what it shows.

About this chart

When you meet a chart you do not know, turn on About this chart in its ⋯ menu. A line then explains what the chart shows and how to read it.

Expand

The Expand button, to the left of the ⋯ button, opens a chart in its own large window. Save as PNG there gives a full-size image, headed with its settings and named for the lot and chart.

Every mark opens something

Charts are not dead ends. Click a mark to go deeper.

• A wafer's bar, box or point opens that wafer's map
• A bin, a histogram bar or a ring opens the menu of charts and tables for those dies (lesson 08)
• A capability box or a correlation cell moves the other charts to that test

What divides yield

Yield differs between wafers. This card checks whether it differs along a line you know about: each wafer field, such as the split or the tester, is compared for yield and the top fail bins (four or more wafers). Each field is one row: two bars, the worst and the best group's mean yield, a sentence giving the gap in points, and a Confidence of Clear, Possible or Not clear. Clear means a low q: the groups differ by more than noise, so it is where to look, not why. A button under a field opens its other measures. Fields that change together, such as the tester, node and probe card here, divide the wafers identically, so the card shows them as one row, since the wafers cannot tell them apart.

Allowing for another field

Wafers differ in several fields at once, and a strong one can hide another. So each field is read after taking out the strongest other field. A row that says allowing for Split shows what is left once the split's effect is removed. Its bars and the gap in points are still the plain difference between the groups.

A field can also rank because it goes with a group that has an effect of its own. In this lot the operator follows the sessions, so its row shows what the sessions did, not what the operator did. Read a row as where to look, not as a cause.

Part average testing

Some dies pass every limit and still look wrong, because they sit far from the rest of their wafer. Part average testing (PAT) finds them. The card counts them per wafer; Show all 25 wafers lists every wafer.

Map the verdicts

Map the verdicts returns to the maps coloured by PAT verdict: within the limits, outlier or fails its bin or limits. You can also choose PAT verdict in the Plot mode menu on the map toolbar.

How PAT limits are set

• Per wafer and test: the passing dies' median ± 6 robust sigma, never wider than the limits
• A wafer needs 30 passing values on a test
• It shows what PAT would flag; screening belongs in the test program

More in the wafer map guide

Group by

Group by lets you compare groups of wafers instead of single wafers. When a wafer field varies across the lot, Group by offers it. None shows each wafer; a field such as Split (5) pools wafers into groups. The number is how many groups the field makes. One wafer has nothing to group, so it has no Group by.

Grouped by Split

Every card now reports per split instead of per wafer. SS is lowest at 43.6%, TT highest at 86.5%, so the process corners account for the differences in yield. Click a split's bar to see its wafers; Back returns.

Columns of cards

Beside Group by and Show, Columns sets how many columns of cards every section of Insights uses. Auto fits as many as the width holds; a number divides the width that many ways. The keys 1 to 5 do the same, and 0 returns to Auto, as in the gallery (lesson 02). The choice stays while you move between the tabs.

Recap

• Overview shows where yield is lost: by wafer, by bin and by test
• Every chart has a ⋯ menu, and every mark opens something
• What divides yield and Part average testing point at causes; Group by compares groups

You now know where the yield is going. The next lesson looks at how test values behave, and at the tables behind every chart.

More in the wafer map guide